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-ZOOLOGY. 


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J.   ARTHUR   THOMSON,   MA.,   F.R.S.E. 

LECTURER   ON   ZOOLOGY  AND   BIOLOGY   IN    THE  SCHOOL  OF   MEDICINE, 

EDINBURGH;  JOINT-AUTHOR  OF  "THE  EVOLUTION  OF  SEX"; 
AUTHOR  OF  "THE  STUDY  OF  ANIMAL  LIFE." 


SECOND  EDITION,  REVISED  AND  ENLARGED 
WITH  266  ILLUSTRATIONS. 

p*   I  $     ;  *,  ;        ;*',•»" 

.  . 


NEW    YORK: 

D.    APPLETON    &    COY. 

1895. 


vi  PREFACE  TO  THE  SECOND  EDITION. 

also  written  the  chapter  on  Comparative  Physiology.  To 
my  assistant,  Mr.  R.  A.  Staig,  I  am  indebted  for  the  index. 
I  wish  to  express  my  thanks  to  my  artist  friend,  Mr. 
William  Smith,  for  the  carefulness  with  which  he  has 
executed  many  of  the  illustrations  ;  and  I  am  indebted 
to  Dr.  Traquair  for  allowing  me  to  figure  some  of  the 
specimens  in  the  Edinburgh  Museum  of  Science  and 
Art.  In  regard  to  these  illustrations,  I  may  say  that  in 
almost  every  case  they  have  either  been  derived  from 
original  memoirs  and  works  of  reference,  or  drawn  from 
specimens.  Of  course,  no  one  who  has  worked  with  such 
excellent  practical  books  as  that  by  Marshall  and  Hurst 
or  Parker's  Zootomy,  can  help  being  assisted  by  them  in 
preparing  analogous  diagrams  ;  but  I  have  refrained  from 
incurring  any  but  an  absolutely  necessary  debt  to  such 
books,  except  in  the  case  of  Figure  215,  which  Messrs. 
Macmillan  have  kindly  permitted  me  to  make  use  of. 

J.  A.  T. 

SCHOOL  OF  MEDICINE, 

EDINBURGH,  March  1895. 


CONTENTS. 


GENERAL. 

PAGE 

CHAPTER  I. 
GENERAL  SURVEY  OF  THE  ANIMAL  KINGDOM,         .  .  i 

CHAPTER  II. 
PHYSIOLOGY,        ...  .  14 

CHAPTER  III. 
MORPHOLOGY,     ...  ...          30 

CHAPTER  IV. 
EMBRYOLOGY,      .....  49 

CHAPTER  V. 
PAL/EONTOLOGY,  ....  .74 

CHAPTER  VI. 
DOCTRINE  OF  DESCENT,  .  .  .  .  .81 


x  CONTENTS. 

GENERAL. 

PAGE 

CHAPTER  XXVII. 
COMPARATIVE  PHYSIOLOGY,      .....        731 

CHAPTER  XXVIII. 
DISTRIBUTION,     .......        753 

CHAPTER  XXIX. 
ETIOLOGY,  .......        765 

APPENDIX  ON  BOOKS,        .  .        773 

INDEX,  ...  -779 


LIST   OF   ILLUSTRATIONS. 


1 .  Diagrammatic  expression  of  classification  in  a  genealogical  tree,  1 1 

2.  Structure  of  the  cell.     (CARNOY),      ....  44 

3.  Fertilised  ovum  of  Ascaris.     (BovERi),         ...  44 

4.  Diagram  of  cell  division.     (BovERi),             ...  45 

5.  Karyokinesis.     (FLEMMING),             ....  47 

6.  Diagrammatic  expression  of  alternation  of  generations,         .  55 

7.  Diagram  of  ovum,  showing  diffuse  yolk  granules,      .             .  57 

8.  Forms  of  spermatozoa,            .....  5^ 

9.  Diagram  of  maturation    and    fertilisation.      (GEDDES  and 

THOMSON),            ......  59 

10.  Spermatogenesis     and      polar     bodies.       (HERTWIG     and 

WEISMANN),         ......  61 

11.  Fertilisation  in  Ascaris  megalocephala.     (BovERi),  .             .  62 

12.  Modes  of  segmentation,          .....  64 

13.  Life  history  of  a  coral,  Monoxenia  Darwinii.     (fLECKEL),  66 

14.  Embryos  ( I )  of  bird  ;  (2)  of  man.     (His).     ...  69 

15.  Gradual  transitions  between  Paludina  Neumayri  and  Palu- 

dina  Hcernesi.     (NEUMAYR),        .             .            .            •  77 

16.  Life  history  of  A mceba,            .....  87 

17.  End-to-end  union  of  Gregarines.     (FRENZEL),          .             .  88 

18.  Life  history  of  Gregarina.     (BtJTSCHLi),      ...  89 

19.  Life  history  of  Monocystis.     (BiJTSCHLl),      ...  90 

20.  Paramcecium.     (BuTSCHLi),  .  .  .  .91 

21.  Conjugation      of      Paramctciiun      aurelia  —  four      stages. 

(MAUPAS),             ......  92 

22.  Diagrammatic  expression  of  process  of  conjugation  in  Para- 

mcecittm  aurelia.     ( MAUPAS),        ....  93 

23.  Vorticella.     (BiJTSCHLT),        .....  95 

24.  Volvox  globator.     (COHN),      .              ,              .              .              .  96 

25.  Diagram  of  Protomyxa  aurantiaca.     (H^CKEL),      .             .  99 

26.  Formation  of  shell  in  a  simple  Foraminifer.     (DREYER),      .  100 


xii  LIST  OF  ILLUSTRATIONS. 

FIG.  PAGE 

27.  Polystomella.     (SCHULTZE),  .....         101 

28.  A  pelagic  Foraminifer — Hastigerina  ( Globigerina)  Murrayi. 

(BRADY),   .......         102 

29.  Optical  section  of  a  Radiolarian  (Actinomma].     (H^CKEL),          103 

30.  A  colonial    flagellate  infusorian — Proterospongia  Hceckelii. 

(SAVILLE  KENT),  .....  104 

31.  Simple  sponge — Ascetta  primordialis.     (ILECKEL),  .  117 

32.  Section  of  a  sponge.     (F.  E.  SCHULZE),       .  .  118 

33.  Diagram    showing    types     of    canal     system     of    sponge. 

(KORSCHELT  and  HEIDER),         .  .  .  .119 

34    Development  of  Sycandra  raphamis.     (F.  E.  SCHULZE),     .         123 

35.  Diagrammatic  representation  of  development  of   Oscarella 

lobularis.     (HEIDER),        .....         124 

36.  A.  Young  Dicyema.      B.   Female  Orthonectid  (Rhopalura 

Giardii}.     (WHITMAN  and  JULIN),           ,             .  .         127 

37.  Salinella.     (FRENZEL),           .             .             .            .  .128 

38.  Diagram  of  Coelenterate  structure,     .             .             .  .132 

39.  Hydra,  hanging  from  water- weed.     (GREENE),         .  .          134 

40.  Minute  structure  of  Hydra.     (PARKER  and  JICKELI),  .         137 

41.  Development  of  Hydra.     (BRAUER),             .            .  .         139 

42.  Surface  view  of  A tirelia.     (ROMANES),         .             .  .         144 

43.  Vertical  section  of  A urelia.     (CLAUS),          .             .  .         145 

44.  Diagram  of  life  history  of  A  urelia.     (H^CKEL),       .  .         146 

45.  Lucernaria.     (KOROTNEFF),  ....          148 

46.  Structure  of  sea  anemone.     (ANDRES),          .             .  .150 

47.  Section  through  sea  anemone.     (ANDRES),    .              .  .         151 

48.  Diagrammatic   sections    of    Zoantharian    and   Alcyonarian. 

(CHUN),      .......  153 

49.  Diagram  of  a  Gymnoblastic  Hydromedusa.     (ALLMAN),      .  156 

50.  Diagrammatic  figure  of  a  simple  Turbellarian,  .  .  163 

51.  Diagrammatic  expression  of  part  of  the  structure  of  a  simple 

Turbellarian,           .             .             .             .             .  .163 

52.  Structure  of  liver  fluke.     (SOMMER),              .             .  .          165 

53.  Reproductive  organs  of  liver  fluke.     (SOMMER),       ,  .         166 

54.  Life-history  of  liver  fluke.     (THOMAS),          .             .  .168 

55.  Diagram     of     reproductive      organs     in      Cestode  joint. 

(LEUCKART),        .  .  .  .  .  .172 

56.  Life  history  of  Tania  solium.     (LEUCKART),  .  .          174 

57.  Transverse  section  of  the  Nemertean  Drepanophorus  latus. 

(BURGER),  ...  .177 

58.  Transverse  section  of  a  Nemertean  (Carinella}.     (BURGER),         178 

59.  Diagram  of  structure  of  a  Nematode  (Oxyziris}.     (CALEB),  182 


LIST  OF  ILL USTRA  TIONS.  xiii 

FIG.  PAGE 

60.  Anterior  region  of  earthworm.     (HERING),                .             .  190 

6 1.  Transverse  section  of  earthworm.     (CLAPAR£DE),    .             .  193 

62.  Reproductive  organs  of  earthworm.     (HERING),       .             .  195 

63.  Stages  in  the  development  of  earthworm.     (WlLSON),           .  198 

64.  Arenicola  piscatorum.     (CUNNINGHAM  and  RAMAGE),        .  202 

65.  Anterior  part  of  nervous  system  in  Arenicola.     (VoGT  and 

YUNG),      .......  203 

66.  Dissection  of  anterior  region  of  A renicola.     (CosMOVici),  .  204 

67.  Cross  section  of  A  renicola.     (CosMOVici),    .             .             .  205 

68.  Development  of  Polygordius.     (FRAIPONT),               .             .  207 

69.  Parapodium  of  a  marine  Polychsete,  Heteronereis.    (QuATRE- 

FAGES),        .......  210 

70.  Transverse  section  of  leech.     (A.  G.  BOURNE),        .             .  217 

71.  Dissection  of  leech,     .  .  .  .  .  .219 

72.  Development  of  Sagitta.     (O.  HERTWIG),    .             .             .  222 

73.  Interior  of  Brachiopod  shell,  showing  calcareous  support  for 

the  "arms."     (DAVIDSON),           ....  226 

74.  Pluteus  larva  with  rudiment  of  adult.     (JOHANNES  MULLER),  228 

75.  Alimentary  system  of  starfish.     (MULLER  and  TROSCHEL),  231 

76.  Diagrammatic  cross  section  of  starfish  arm.     (LuDWio),       .  233 

77.  Ventral  half  of  sea  urchin.     (TiEDEMANN),                .             .  238 

78.  Dissection  of  Holothurian.     ( HUNTER),        .             .             .  242 

79.  Diagrammatic  vertical  section  through  disc  and  base  of  one 

of  the  arms  of  Antedon  rosaceus.     (MiLNES  MARSHALL),  245 

80.  Stages  in  development  of  Echinoderms.     (SELENKA),          .  247 

81.  Forms  of  Echinoderm  larva.     (MULLER),      .             .             .  249 

82.  Appendages  of  Norway  lobster,          ....  258 

83.  A    single    eye    element    or    ommatidium    of   the    lobster. 

(PARKER),             .....  260 

84.  Longitudinal  section  of  lobster,  showing  some  of  the  organs,  262 

85.  Female  reproductive  organs  of  crayfish.     (SucKOw),             .  265 

86.  Section  through  the  egg  of  Astacus  after  the  completion  of 

segmentation.     (REICHENBACH),              .            .            .  266 

87.  Longitudinal  section  of  later  embryo  of  Astacus.     (REICH- 

ENBACH),               ......  267 

88.  Embryo  of  crayfish,  flattened  out,  with  removal  of  yolk. 

(REICHENBACH),  ......  268 

89.  Acorn  shell  (Balanus  tintinnabuluni}.     (DARWIN),              .  273 

90.  Development  of  Sacculina.     (DELAGE),        .             .             .  274 

91.  Zoaea  of  common  shore  crab  ( Carcinus  mcsnas}.     (FAXON),  281 

92.  External  form  of  Peripatus.     (BALFOUR),     .             .             .  286 

93.  Dissection  of  Peripatus  capensis.     (BALFOUR),          .             .  288 


xvi  LIST  OF  ILLUSTRATIONS. 

FIG.  PAGE 

159.  Spiral  valve  of  skate.     (T.J.PARKER),      .  .  .487 

160.  Upper  part  of  the  dorsal  aorta  in  the  skate.     (MONRO),    .  488 

161.  Heart  and  adjacent  vessels  of  skate.     (MoNRO),    .             .  489 

162.  Urinogenital  organs  of  male  skate,               .             .             .  491 

163.  Urinogenital  organs  of  female  skate.     (MoNRO),    .             .  492 

164.  Elasmobranch  development.     (BALFOUR),              .             .  494 

165.  External  characters  of  a  Teleostean — a  carp.     (LEUNis),  .  496 

1 66.  Caudal  vertebra  of  haddock,             ....  497 

167.  Disarticulated  skull  of  cod,              '.             .             .             .  498 

1 68.  Pectoral  girdle  and  fin  of  cod,          ....  499 

169.  Diagram  of  Teleostean  circulation.     (NuHN),         .             .  501 

170.  Young  skate.     (BEARD),      .....  506 

171.  Outline  of  Acanthodes  sulcatus.     (TRAQUAIR),       .             .  507 

172.  Pterichthys  milleri.     (TRAQUAIR),               .             .             .  509 

173.  Skeleton  of  Ceratodus  fin.     (GEGENBAUR),             .             .  513 

174.  Head  region  of  Protopterus.     (W.N.PARKER),     .             .  514 

175.  Vertebral  column  and  pelvic  girdle  of  bull  frog,      .             .  532 

176.  Skull  of  frog — upper  and  lower  surface.    (W.  K.  PARKER),  533 

177.  Pectoral  girdle  of  frog.     (EcKER),    ....  535 

178.  Side  view  of  frog's  pelvis.     (ECKER),          .             .             .  536 

179.  Brain  of  frog.     (ECKER),     .....  537 

180.  Nervous  system  of  frog.     (ECKER),              .             .             .  538 

1 8 1.  Arterial  system  of  frog.     (EcKER),               .             .             .  542 

182.  Venous  system  of  frog.     (ECKER),  ....  544 

183.  Urinogenital  system  of  male  frog.     ( ECKER),         .             .  548 

184.  Urinogenital  system  of  female  frog.     (ECKER),       .             .  548 

185.  Division  of  frog's  ovum.     (EcKER),              .             .             .  550 

186.  Gastrula  stage  of  newt.     (HERTWIG),         .             .             .  551 

187.  Dissection  of  tadpole.     (MILNES  MARSHALL  and  BLES),  .  552 

188.  Life  history  of  frog.     (BREHM),      ....  554 

189.  Gecilian  (Ichthyophis]  with  eggs.     (SARASIN),        .             .  557 

190.  External  appearance  of  tortoise,       .                           .             .  563 

191.  Carapace  of  tortoise,              .             .                          .             .  564 

192.  Internal  view  of  skeleton  of  turtle,               .             .             .  565 

193.  Dissection  of  Chelonian  heart.     (HUXLEY),            .             .  566 

194.  Heart  and  associated  vessels  of  tortoise.     (NuHN),              .  567 

195.  Hatteria  or  Sphenodon.     (HAYEK),             .             .             .  568 

196.  Side  view  of  skull  of  Lacerta.     (W.K.PARKER),               .  572 

197.  Heart  and  associated  vessels  of  lizard.     (NuHN),                 .  574 

198.  Lung  of  Chamaleo  vulgaris,  showing  air  sacs.     (WlEDER- 

SHEIM),     .......  576 

199.  Snake's  head.     (NuHN),      .....  579 


LIST  OF  ILLUSTRATIONS.  xvii 

FIG.  PAGE 

200.  Skull  of  grass  snake.     (W.  K.  PARKER),    .            .            .  581 

201.  Lower  surface  of  skull  of  a  young  crocodile,            .             .  584 

202.  Crocodile's  skull  from  dorsal  surface,            .             .             .  585 

203.  Half  of  the  pelvic  girdle  of  a  young  crocodile,         .             .  586 

204.  Origin  of  amnion  and  allantois.     (BALFOUR),         .             .  590 

205.  Comparison  of  pelvic  girdles  of  cassowary,  Iguanodon,  and 

crocodile,   .             .             .             .             .             .  593 

206.  Position  of  organs  in  a  bird.     (SELENKA),              .            .  597 

207.  Diagrammatic  section  of  young  bird.     (GADOW),  .      •"  598 

208.  Disarticulation  of  bird's  skull.     (GADOW),               .             .  602 

209.  Under  surface  of  gull's  skull,            ....  603 

210.  Wing  of  dove,           ......  604 

211.  Side  view  of  pelvis  of  cassowary,     ....  605 

212.  Bones  of  hind  leg  of  eagle,               ....  606 

213.  Brain  of  pigeon.     (BRONN),             ....  607 

214.  Diagrammatic  section  of  cloaca  of  male  bird.     (GADOW),  .  608 

215.  Circulation  of  pigeon.     (PARKER),             ..           .            .  609 

216.  Urinogenital  organs  of  pigeon,        ....  612 

217.  Diagrammatic  section  of  egg.     (ALLEN  THOMSON),           .  613 

218.  Stages  in  development  of  chick.     (MARSHALL),      .             .  615 

219.  Diagrammatic  section  of  embryo  within  egg.     (KENNEL),  616 

220.  Pectoral  girdle  and  sternum  of  swan,           .             .             .  622 

221.  Position    of    wings    in    pigeon    at    maximum    elevation. 

(MAREY),  .......  623 

222.  Wings  coming  down.     (MAREY),   ....  624 

223.  Wings  completely  depressed.     (MAREY),   .            .             .  624 

224.  Segmentation  of  rabbit's  ovum.     (VAN  BENEDEN),            .  648 

225.  Development  of  hedgehog.  Three  early  stages.  (HUBRECHT),     649 

226.  Two  stages  in  segmented  ovum  of  hedgehog.     (HUBRECHT),  650 

227.  Development  of  foetal  membranes.     (HERTWIG),   .             .  652 

228.  Diagram  of  foetal  membranes.     (TURNER),             .             .  653 

229.  View   of  embryo   of  rabbit,    with   its   foetal   membranes. 

(KENNEL),            ......  657 

230.  Side  view  of  rabbit's  skull,  ,             ...             .             .  662 

231.  Dorsal  view  of  rabbit's  skull,  .  .  .  .663 

232.  Under  surface  of  rabbit's  skull,        ....  664 

233.  Rabbit's  fore  leg,      ......  665 

234.  Rabbit's  hind  leg,     ......  665 

235.  Dorsal  view  of  rabbit's  brain.     (KRAUSE),               .             .  667 

236.  Under  surface  of  rabbit's  brain.     (KRAUSE),  .         .            .  667 

237.  Diagram  of  caecum  in  rabbit,            .             .                          .  670 

238.  Duodenum  of  rabbit.     (CLAUDE  BERNARD),                      .  671 

b 


xviii  LIST  OF  ILLUSTRATIONS. 

FIG.  PAGE 

239.  Circulatory  system  of  rabbit.     (PARKER  and  KRAUSE),       .  672 

240.  Vertical  section  through  rabbit's  head,         .             .             .  675 

241.  Urinogenital  organs  of  male  rabbit,             .             .             .  677 

242.  Urinogenital  organs  of  female  rabbit,           .             .             .  677 

243.  Pectoral  girdle  of  Echidna,               ....  681 

244.  Pelvis  of  Echidna,  ......  682 

245.  Urinogenital  organs  of  male  duckmole.     (OWEN),              .  683 

246.  Urinogenital  organs  of  female  duckmole.     (OwEN),           .  683 

247.  Lower  jaw  of  kangaroo,       .....  684 

248.  Foot  of  young  kangaroo,      .....  688 

249.  Foot  of  ox,                 ......  694 

250.  Fore  leg  of  pig,         ......  694 

251.  Side  view  of  sheep's  skull,  with  roots  of  back  teeth  exposed,  695 

252.  Stomach  of  sheep.     (LEUNIS),         ....  696 

253.  Side  view  of  calf's  fore  leg,              ....  697 

254.  Side  view  of  lower  part  of  pony's  fore  leg,               .             .  699 

255.  Side  view  of  ankle  and  foot  of  horse,           .             .             .  699 

256.  Side  view  of  horse's  skull,  roots  of  back  teeth  exposed,      .  700 

257.  Feet  of  horse  and  its  progenitors.     (NEUMAYR),     .             .  701 

258.  Fore  limb  of  Balcsnoptera,                ....  705 

259.  Fore  limb  of  whale  (Megaptera  longimand].     (STRUTHERS),  706 

260.  Pelvis  and  hind  limb  of  Greenland  whale.     (STRUTHERS),  707 

261.  Vertebra,  rib,  and  sternum  of  Balanoptera,              .             .  708 

262.  Lower  surface  of  dog's  skull,  .  .  .  .712 

263.  Skeleton  of  fox  bat,               .             .             .             .             .  720 

264.  Skeleton  of  male  gorilla,      .  .  .  .  .723 

265.  Skull  of  orang-utan,              .....  726 

266.  Skull  of  gorilla,         ......  727 


OUTLINES  OF  ZOOLOGY. 


BIRDS. 

Placentals. 

VERTEBRATES. 

H 

0 

8 

0 

MAMMALS.     Marsupials. 
Flying  Birds.    Running  Birds.                                    Monotremes. 

r 

h 

Snakes.     Lizards.             REPTILES.     Crocodiles.    Tortoises. 

Dipnoi. 

FISHES.    B°ny  Fish£S- 
Ganoids. 

Elasmobranchs. 

AMPHIBIANS. 

Newt.                             Frog. 

CYCLOSTOMATA. 

Lamprey.                  Hagfish. 

LANCELET. 

TUNICATES. 

Insects.  Arachnids. 

BALANOGLOSSUS. 

Cuttlefish. 
Gasteropods. 

ANNELIDS. 

Myriopods. 
Peripatus. 

MOLLUSCS. 

c 

c/5 
W 

ARTHROPODS. 

Crustaceans. 

Bivalves. 

"WORMS." 

Feather  stars. 

H 

Brittle  stars. 

<J 

Starfish. 

tf 

pq 

ECHINODERMS. 

W 

H 

UNSEGMENTED 

Sea  urchins. 

P4 

WORMS. 

Sea  cucumbers. 

W 

> 
£ 

HH 

Ctenophores.        Jellyfish. 

Sea  Anemones.         Corals. 

COELENTERA. 

Medusoids  and  Hydroids. 

Infusorians.             Rhizc 
SIMPLEST 

SPONGES. 

>pods.             Gregarines. 
ANIMALS. 

N 

o 

OUTLINES    OF    ZOOLOGY. 


CHAPTER    I. 

GENERAL    SURVEY    OF    THE    ANIMAL    KINGDOM. 

AT  the  outset  of  our  study  of  Zoology  it  is  useful  to  take 
a  general  survey  of  the  "  animal  kingdom."  Without  some 
such  bird's  eye  view— necessarily  very  superficial — one  is 
apt  not  to  see  the  wood  for  the  trees. 

Mammals. 

We  naturally  begin  a  survey  with  the  animals  which  are 
most  like  man — the  monkeys.  But  neither  we  nor  the 
monkeys  are  separated  by  any  structural  gulf  from  the 
other  four  limbed,  hair  bearing  animals,  to  which  Lamarck 
gave  the  name  of  Mammals.  For  although  there  are  many 
different  types  of  Mammals — such  as  monkeys  and  men ; 
horses,  cattle,  and  other  hoofed  quadrupeds  ;  cats,  dogs, 
and  bears ;  rats,  mice,  and  other  rodents ;  hedgehogs, 
shrews,  and  moles,  and  so  on — the  common  possession  of 
certain  characters  unites  them  all  in  one  class,  readily  dis- 
tinguishable from  Birds  or  Reptiles. 

Among  these  characters  we  rank  the  milk  giving  of  the 
mother  mammals,  the  growth  of  hair  on  the  skin,  the 
general  presence  of  convolutions  on  the  front  part  of  the 

1 


2       GENERAL  SURVEY  OF  THE  ANIMAL  KINGDOM. 

brain,  the  occurrence  of  a  muscular  partition  or  diaphragm 
between  the  chest  and  the  abdomen,  and  so  on,  as  we  shall 
afterwards  notice  in  detail.  Most  mammals  are  suited  for 
life  on  land,  but  diverse  types  such  as  seals,  whales,  and 
sea  cows  have  taken  to  the  water,  while  the  bats  are  as 
markedly  suited  for  aerial  life. 

Among  the  mammalian  characteristics  of  great  import- 
ance .are  those  which  relate  to  the  bearing  of  young,  and 
even  a  brief  consideration  of  these  enables  us  to  see  that 
some  mammals  are  distinguished  from  others  by  differences 
deeper  than  those  which  separate  whales  from  carnivores, 
or  rodents  from  bats.  These  deep  differences  may  be 
stated  briefly  as  follows  : — (a)  Before  birth  most  young 
mammals  are  very  closely  united  (by  a  complex  structure 
called  the  placenta)  to  the  mothers  who  bear  them.  (b)  But 
this  close  connection  between  mother  and  unborn  young  is 
only  hinted  at  in  the  kangaroos  and  other  pouched  animals 
or  Marsupials,  which  bring  forth  their  young  in  a  peculiarly 
helpless  condition,  as  it  were  prematurely,  and  in  most 
cases  place  them  in  an  external  pouch,  within  which  they 
are  sheltered  and  nourished,  (c)  In  the  Australian  duck- 
mole  and  its  two  relatives,  the  placental  connection  is  quite 
absent,  for  these  animals  lay  eggs  as  birds  and  most 
reptiles  do.  Besides  these  differences  in  the  bearing  of 
young,  there  are  others  relating  to  structure  which  are  of 
great  importance,  and  which  seem  to  warrant  the  division  of 
Mammals  into  three  sub-classes  : — 

1.  Prototheria,     Ornithodelphia,    or    Monotremes — the    egg    laying 

duckmole  (Omithorkytukus],  Echidna  and  Proechidna. 

2.  Metatheria,  Didelphia,  or  Marsupials — the  prematurely  bearing, 

usually  pouch  possessing  kangaroos  and  their  relatives. 

3.  Eutheria,  Monodelphia,  or  Placentals — those  in  which  there  is  a 

close    (placental)   union   between    the   unborn   embryo   and   its 
mother,  e.g..  Ungulates,  Carnivores,  Monkeys. 

Birds. 

There  can  be  no  hesitation  as  to  the  class  which  we 
should  rank  next  to  Mammals.  For  Birds  are  in  most 
respects  as  highly  developed  as  Mammals,  though  in  a 
divergent  direction.  They  are  characterised  by  their  feathers 
and  wings,  and  many  other  adaptations  for  flight,  by  their 
high  temperature,  by  the  frequent  sponginess  and  hollow- 


VERTEBRATES.  3 

ness  of  their  bones,  by  the  tendency  to  fusion  in  many 
parts  of  the  skeleton,  e.g.,  backbone  and  ankle,  by  the 
absence  of  teeth  in  modern  forms,  by  the  fixedness  of  the 
lungs  and  their  association  with  numerous  air  sacs,  and 
so  on. 

But  here  again  different  grades  must  be  distinguished — 
(i)  there  is  the  vast  majority — the  flying  birds,  who  have  a 
breast-bone  keel  or  carina,  to  which  the  muscles  used  in 
flight  are  in  part  attached  (Carinatae) ;  (2)  there  is  the 
small  minority  of  running  birds  (ostriches,  emu,  cassowary, 
and  kiwi),  whose  wings  are  incapable  of  flight,  and  who 
have  no  keel  (Ratitae) ;  and  (3)  there  is  an  extinct  type, 
Archceopteryx,  with  markedly  reptilian  affinities. 

Reptiles. 

There  are  no  close  relationships  between  Birds  and 
Mammals,  but  the  old-fashioned  Monotremes  have  some 
markedly  reptilian  features,  and  so  have  some  aberrant 
living  birds,  such  as  the  Hoatzin  and  the  Tinamou.  More- 
over, when  we  consider  the  extinct  Mammals  and  Birds, 
we  perceive  other  resemblances  linking  the  two  highest 
classes  of  animals  to  Reptiles. 

Reptiles  do  not  form  a  compact  class,  but  rather  an 
assemblage  of  classes.  In  other  words,  the  types  of  Reptile 
differ  much  more  widely  from  one  another  than  do  the 
types  of  Bird  or  Mammal.  Nowadays,  there  are  five  distinct 
types  : — the  crocodilians,  the  unique  New  Zealand  "lizard" 
(Hatterid),  the  lizards  proper,  the  snakes,  and  the  tortoises. 
But  the  number  of  types  is  greatly  increased  when  we  take 
account  of  the  entirely  extinct  saurians  who  had  their  golden 
age  in  the  inconceivably  distant  past. 

The  Reptiles  which  we  know  nowadays  are  scaly-skinned 
animals,  they  resemble  Birds  and  Mammals  in  having  a 
practically  or  really  four  chambered  heart,  in  never  having 
gills,  and  in  having  during  embryonic  life  two  important 
"fcetal  membranes,"  known  as  the  amnion  and  the  allantois. 

Amphibians. 

The  Amphibians,  such  as  frogs  and  newts,  were  once 
regarded — e.g.,  by  Cuvier — as  naked  Reptiles,  but  a  more 
accurate  classification  has  linked  them  rather  to  the  Fishes. 


4       GENERAL  SURVEY  OF  THE  ANIMAL  KINGDOM. 

Thus  Professor  Huxley  grouped  Birds  and  Reptiles  together 
as  Sauropsida;  Amphibians  and  Fishes  together  as  Ich- 
thyopsida — for  reasons  which  shall  be  afterwards  stated. 
Amphibians  mark  the  transition  from  aquatic  life,  habitual 
among  Fishes,  to  terrestrial  life,  habitual  among  Reptiles,  for 
while  almost  all  Amphibians  have  gills — in  their  youth  at 
least — all  the  adults  have  lungs,  and  some  retain  the  gills  as 
well.  In  having  limbs  which  are  fingered  and  toed,  and 
thus  very  different  from  fins,  they  resemble  Reptiles.  But 
the  two  foetal  membranes  characteristic  of  the  embryonic  life 
of  higher  Vertebrates  are  not  present  in  Amphibian  embryos, 
and  the  general  absence  of  an  exoskeleton  in  modern  forms 
is  noteworthy. 

Fishes. 

The  members  of  this  class  are  as  markedly  adapted  to 
life  in  the  water  as  birds  to  life  in  the  air.  The  tail  usually 
forms  the  locomotor  organ,  and  the  limbs  are  fins.  There 
are  also  unpaired  median  fins  supported  by  fin  rays.  All 
have  permanent  gills  borne  by  bony  or  gristly  arches. 
There  is  an  exoskeleton  of  scales,  and  the  skin  also  bears 
numerous  glandular  cells  and  sensory  structures. 

In  many  ways  Fishes  are  allied  to  Amphibians,  especially 
if  we  include  among  Fishes  three  peculiar  forms,  known  as 
Dipnoi,  which  show  hints  of  a  three  chambered  heart,  and 
have  a  lung  as  well  as  gills.  Other  Fishes  have  a  two 
chambered  heart,  containing  only  impure  blood,  which  is 
driven  to  the  gills,  whence,  purified,  it  passes  directly  to  the 
body. 

Apart  from  the  divergent  Dipnoi,  there  are  three  orders 
of  Fishes  : — the  cartilaginous  Elasmobranchs,  such  as  shark 
and  skate ;  the  Ganoids,  such  as  sturgeon  and  bony  pike ; 
and  the  Teleosteans  or  bony  fishes,  such  as  cod,  herring, 
salmon,  eel,  and  sole. 

Primitive  (?)  Vertebrates. 

Under  this  title  we  include  (i)  the  class  of  Round- 
mouths  or  Cyclostomata ;  (2)  the  class  of  which  the  lancelet 
or  Amphioxus  is  the  only  adequately  known  type;  (3)  the 
class  of  Tunicates,  some  of  which  are  called  sea-squirts ; 
and  (4),  with  much  hesitation,  several  strange  formSj 


VER  TEBRA  TES.  5 

especially  Balanoglossus,  which  exhibit  structures  suggestive 
of  affinity  with  Vertebrates. 

The  Cyclostomata,  represented  by  the  lamprey  (Petro- 
myzon)  and  the  hag  (Myxine),  and  some  other  forms, 
probably  including  an  interesting  fossil  known  as  Palceo- 
spondylus,  are  sometimes  ranked  with  Fishes  under  the  title 
Marsipobranchii.  But  they  have  no  definitely  developed 
jaws,  no  paired  fins,  no  scales,  and  are  in  other  ways  more 
primitive. 

The  lancelet  (Amphioxus),  for  which  the  class  Cephalo- 
chorda  has  been  erected,  is  even  simpler  in  its  general 
structure.  Thus  there  is  an  absence  of  limbs,  skull,  jaws, 
well-defined  brain,  heart,  and  some  other  structures.  The 
vertebral  column  is  represented  by  an  unsegmented  (or  un- 
vertebrated)  rod,  called  the  notochord,  which  in  higher 
animals  (except  Cyclostomata  and  some  fishes)  is  a  transitory 
organ  replaced  by  a  backbone. 

The  Tunicata  or  Urochorda  form  a  class  of  remarkable 
forms,  the  majority  of  which  degenerate  after  larval  life. 
In  the  larvae  of  all,  and  in  the  few  adults  which  are  neither 
peculiarly  specialised  nor  degenerate,  we  recognise  some  of 
the  fundamental  characters  of  Vertebrates.  Thus  there  is  a 
dorsal  supporting  axis  (a  notochord)  in  the  tail  region,  a 
dorsal  nervous  system,  gill  clefts  opening  from  the  pharynx 
to  the  exterior,  a  simple  ventral  heart,  and  so  on. 

Of  Balanoglossus  and  its  allies,  for  which  the  class  Hemi- 
chorda  or  Enteropneusta  has  been  established,  it  is  still 
difficult  to  speak  with  confidence.  The  possession  of  gill 
clefts,  the  dorsal  position  of  an  important  part  of  the 
nervous  system,  the  occurrence  of  a  short  supporting  struc- 
ture on  the  anterior  dorsal  surface  and  other  features,  have 
led  some  to  place  them  at  the  base  of  the  Vertebrate  series. 

At  this  stage,  having  reached  the  base  of  the  Vertebrate  series,  we 
may  seek  to  define  a  Vertebrate  animal,  and  to  contrast  it  with  Inverte- 
brate forms. 

The  distinction  is  a  very  old  one,  for  even  Aristotle  distinguished 
mammals,  birds,  reptiles,  amphibians,  and  fishes  as  "blood  holding," 
from  cuttlefish,  shell  bearing  animals,  crustaceans,  insects,  &c.,  which 
he  regarded  as  "  bloodless."  He  was,  indeed,  mistaken  about  the 
bloodlessness,  but  the  distinctiveness  of  the  higher  animals  first  men- 
tioned has  been  recognised  by  all  subsequent  naturalists,  though  it  was 
first  precisely  expressed  in  1797  by  Lamarck. 


6       GENERAL  SURVEY  OF  THE  ANIMAL  KINGDOM. 

Yet  it  is  no  longer  possible  to  draw  a  boundary  line  between  Verte- 
brates and  Invertebrates  with  that  firmness  of  hand  which  characterised 
the  early  or,  indeed,  the  pre-Darwinian  classifications.  For  we  now 
know — ( I )  that  Fishes  and  Cyclostomata  do  not  form  the  base  of  the 
Vertebrate  series,  for  the  lancelet  and  the  Tunicates  must  also  be  in- 
cluded in  the  Vertebrate  alliance  ;  (2)  that  Balanoglossus  and  Cep halo- 
discus  have  several  Vertebrate-like  characteristics  ;  (3)  that  some  of  the 
Invertebrates,  especially  Chaetopods  and  Nemerteans,  show  some  hints 
of  affinities  with  Vertebrates.  The  limits  of  the  Vertebrate  alliance 
have  been  widened,  and  though  the  recognition  of  their  characteristics 
has  become  more  definite,  not  less  so,  the  apartness  of  the  sub-kingdom 
has  disappeared. 

It  does  not  matter  much  whether  we  retain  the  familiar  title  Verte- 
brata,  or  adopt  that  of  Chordata,  provided  that  we  recognise — (i)  that 
it  is  among  Fishes  first  that  separate  vertebral  bodies  appear  in  the 
supporting  dorsal  axis  of  the  body;  (2)  that,  as  a  characteristic,  the  back- 
bone is  less  important  than  the  notochord,  which  precedes  it  in  the 
history  alike  of  the  race  and  of  the  individual.  Nor  need  we  object  to 
the  popular  title  backboned,  provided  we  recognise  that  the  adjective 
"bony"  is  first  applicable  among  Fishes,  and  not  even  to  all  of  them. 

The  essential  characters  of  Vertebrates  may  be  summed  up  in  the 
following  table,  where  they  are  contrasted,  somewhat  negatively,  with 
what  is  true  of  Invertebrates  : — 


"  BACKBONKLES§,"  INVERTEBRATE 
OR  NON-CHORDATE. 


The  greater  part  of  the  nervous  system  is 

on  the  ventral  surface. 
No  corresponding  structure  is  known. 


No  corresponding  structures  are  known 
with  any  certainty. 


The  eye  is  usually  derived  directly  from 

the  skin. 
The  heart,  if  present,  is  dorsal. 


"  BACKBONED,"  VERTEBRATE 
OR  CHORDATE. 


The  central  nervous  system — brain  and 
spinal  cord — is  dorsal,  and  t^tbular. 

There  is  a  dorsal  supporting  axis  or  noto- 
chord, which  is  in  most  cases  replaced 
by  a  backbone. 

Gill-slits  or  visceral  clefts  open  from  the 
sides  of  the  pharynx  to  the  exterior. 
In  fishes,  and  at  least  young  amphi- 
bians, they  are  associated  with  gills 
and  are  useful  in  respiration  ;  in 
higher  forms  they  are  transitory  and 
functionless,  except  when  modified 
into  other  structures. 

The  essential  parts  of  the  eye  are  formed 
by  an  outgrowth  from  the  brain. 

The  heart  is  ventral. 


INVERTEBRATES. 

Molluscs. 

This  series  of  forms  includes  Bivalves,  such  as  cockle  and 
mussel,  oyster  and  clam  ;  Gasteropods,  such  as  snail  and 
slug,  periwinkle  and  buckie  ;  Cephalopods,  such  as  octopus 


INVER  TEBRA  TES.  7 

and  pearly  nautilus.  They  may  be  placed  highest  among 
Invertebrates  since  many  of  them  exhibit  a  concentration 
of  the  nervous  system  greater  than  occurs  elsewhere. 

Unlike  Vertebrates,  and  such  Invertebrates  as  Insects  and 
Crustaceans,  Molluscs  are  without  segments  and  without 
appendages.  A  muscular  protrusion  of  the  ventral  surface, 
known  as  the  "  foot,"  serves  in  the  majority  as  an  organ  of 
locomotion.  In  most  cases,  a  single  or  double  fold  of 
-skin,  called  the  "mantle,"  makes  a  protective  shell.  The 
nervous  system  has  three  chief  pairs  of  nerve  centres  or 
ganglia.  In  many  cases,  the  larval  stages  are  very  char- 
acteristic. 

Arthropods. 

This  large  series  includes  Crustaceans,  Myriopods,  Insects, 
Spiders,  and  other  forms,  which  have  segmented  bilaterally 
symmetrical  bodies  and  jointed  appendages.  The  skin 
produces  an  external  cuticle,  the  organic  part  of  which  con- 
sists of  a  substance  called  chitin,  associated  in  Crustaceans 
with  carbonate  of  lime.  The  nervous  system  consists  of  a 
dorsal  brain,  connected,  by  a  nerve  ring  around  the  gullet, 
with  a  ventral  chain  of  ganglia. 

Echinoderms. 

This  is  a  well-defined  series,  including  starfishes,  brittle 
stars,  sea  urchins,  sea  cucumbers,  and  feather  stars.     The 
r  symmetry  of  the  adult  is  usually  radial,  though  that  of  the 
i  larva  is  bilateral.      A  peculiar  system,  known  as  the  water-; 
vascular  system,  is  characteristic,  and  is  turned  to  various 
uses,  as  in  locomotion  and  respiration.     There  is  a  marked 
tendency  to  deposition  of  lime  in  the  tissues.     The  develop- 
ment is  strangely  circuitous  or  "  indirect." 

Segmented  "  Worms" 

It  is  hopeless  at  present  to  arrange  with  any  definiteness 
those  heterogeneous  forms  to  which  the  title  "  worm  "  is 
given.  For  this  title  is  little  more  than  a  name  for  a  shape, 
assumed  by  animals  of  varied  nature  who  began  to  move 
head  foremost  and  to  acquire  sides.  There  is  no  class  of 
"  worms,"  but  an  assemblage — a  mob — not  yet  reduced  to 
order.  It  seems  useful,  however,  to  separate  those  which 


8       GENERAL  SURVEY  OF  THE  ANIMAL  KINGDOM. 


are  ringed  or  segmented,  from  those  which  are  unsegmented. 
The  former  are  often  called  Annelids,  and  include  : — 

Chaetopoda,  or  Bristle-footed  worms,  e.g.,  earthworm 
and  lobworm  ;  and 

Hirudinea,  or  Leeches  ;  and  some  smaller  classes. 

Unsegmented  "  Worms" 

These  differ  from  the  higher  "  worms  "  in  the  absence  of 
true  segments  and  appendages,  and  resemble  them  in  their 
bilateral  symmetry.  The  series  includes  Turbellarians  or 
Planarians ;  the  parasitic  Trematodes  or  Flukes ;  the  para- 
sitic Cestodes  or  Tapeworms ;  the  Nemerteans  or  Ribbon- 
worms  ;  the  frequently  parasitic  Nematodes  or  Thread- 
worms ;  and  several  smaller  classes. 

As  to  certain  other  forms,  such  as  the  sea  mats  (Polyzoa 
or  Bryozoa),  the  lamp  shells  (Brachiopoda),  and  the  worm- 
like  Sipunculids,  it  seems  best,  at  this  stage,  to  confess  that 
they  are  incertce  sedis. 

But  the  general  fact  is  not  without  interest  that  in  the 
midst  of  the  well-defined  classes  of  Invertebrates  there  lies, 
as  it  were,  a  pool  from  which  many  streams  of  life  flow,  for 
among  the  heterogeneous  "  worms  "  we  detect  affinities  with 
Arthropods,  Molluscs,  Echinoderms,  and  even  Vertebrates. 

At  this  stage  we  may  notice  that  in  all  the  above  forms  the  typical 
symmetry  is  bilateral  (in  Echinoderms,  the  radial  symmetry  belongs  only 
to  the  adults) ;  that  in  most  types  a  body  cavity  or  coelome  is  developed  ; 
that  the  embryo  consists  of  three  germinal  layers  (external  ectoderm  or 
epiblast,  internal  endoderm  or  hypoblast  lining  the  gut,  and  a  median 
mesoderm  or  mesoblast  lining  the  body  cavity).  In  the  next  two  classes 
(Coelentera  and  Sponges)  the  conditions  are  different,  as  may  be  expressed 
in  the  following  table,  though  it  is  open  to  question  whether  the  contrast 
is  quite  so  great  as  it  seems  : — 


SPONGES  AND  COELENTERA. 


HIGHER  ANIMALS  (CCELOMATA). 


There  is  no  body  cavity.      There  is  but 
one  cavity,  that  of  the  food  canal. 


There  is  no  definite  middle  layer  of  cells 
(mesoderm),  but  rather  a  middle  jelly 
(mesogloea). 

The  radial  symmetry  of  the  gastrula  em- 
bryo is  retained  in  the  adult,  and  the 
longitudinal  (oral-aboral)  axis  of  the 
adult  corresponds  to  the  long  axis  of 
the  gastrula. 


There  is  a  body  cavity  or  coelome  between 
the  food  canal  and  the  walls  of  the 
body.  But  this  is  often  incipient,  or 
degenerate. 

There  is  a  distinct  middle  layer  of  cells 
(mesoderm)  between  the  external 
ectoderm  and  the  gut  lining  endo- 
derm. 

The  longitudinal  axis  of  the  adult  does 
not  correspond  to  the  long  axis  of  the 
gastrula  embryo. 


INVERTEBRA  TES. 


Cozlentera. 

This  series  includes  jelly  fish,  sea  anemones,  corals,  zoo- 
phytes, and  the  like,  most  of  which  are  equipped  with 
stinging  cells,  by  means  of  which  they  paralyse  their  prey. 
.  All  but  four  or  five  are  marine.  The  body  may  be  a 
tubular  polype,  or  a  more  or  less  bell-like  "medusoid,"  and 
in  some  cases  the  two  forms  are  included  in  one  life  cycle. 
Budding  is  very  common,  and  many  of  the  sedentary  forms 
— "  corals  " — have  shells  of  lime. 

Porifera. 

Sponges,  or  Porifera,  are  the  simplest  many  celled  animals. 
In  the  simplest  forms,  the  body  is  a  tubular,  two  layered 
sac,  with  numerous  inhalent  pores  perforating  the  walls,  with 
a  central  cavity  lined  by  cells  bearing  lashes  or  flagella,  and 
with  an  exhalent  aperture.  But  budding,  folding,  and  other 
complications  arise,  and  there  is  almost  always  a  skeleton, 
calcareous,  siliceous,  or  "horny,"  or  both  siliceous  and 
horny  at  once.  Water  passes  in  by  the  small  inhalent  pores 
and  out  by  the  exhalent  aperture.  With  few  exceptions 
they  are  marine. 

All  the  animals  hitherto  mentioned  have  bodies  built  up  of  many 
cells  or  unit  masses  of  living  matter,  but  there  are  other  animals,  each 
of  which  consists  of  a  single  cell.  These  simplest  animals  are  called 
Protozoa. 

/      Every  animal  hitherto  mentioned,  from  mammal  or  bird  to  sponge, 

[  develops,  when  reproduction  takes  its  usual  course,  from  a  fertilised 

^  egg  cell.     This  egg  cell  or  ovum  divides  and  redivides,  and  the  daughter 

cells  are  arranged  in  various  ways  to  form  a  "body."     But  the  Protozoa 

form  no  "body,"  they  remain  single  cells,  and  when  they  divide,  the 

daughter  cells  almost  invariably  go  apart  as  independent  organisms. 

Here,  then,  is  the  greatest  gulf  which  we  have  hitherto  noticed 
—  that  between  multicellular  organisms  (Metazoa)  and  unicellular 
organisms  (Protozoa).  But  the  gulf  was  bridged,  and  traces  of  the 
bridge  remain.  For  (a)  there  are  a  few  Protozoa  which  form  loose 
colonies  of  cells,  and  (b]  there  are  multicellular  organisms  of  great 
simplicity. 

Protozoa. 

The  Protozoa  remain  single  cells,  with  few  exceptions. 
Thus  they  form  no  "  body ; "  and  necessarily  therefore  they 


io     GENERAL  SURVEY  OF  THE  ANIMAL  KINGDOM. 

have  no  organs,  nor  sexual  reproduction   in   the  ordinary 

sense  of  the  phrase.     The  series  includes — 

.    (a)    Infusorians,  with   actively    moving   lashes    of  living 

matter ; 
(fr)    Rhizopods,  with  outflowing  threads  or  processes  of 

living  matter ; 

(c)    Gregarines,  parasitic  forms,  without  either  lashes  or 
outflowing  processes. 

Note  on  Classification. 

We  naturally  group  together  in  the  mind  those  impressions  which  are 
like  one  another.  In  this  lies  the  beginning  of  all  classification,  whether 
that  of  the  child,  the  savage,  or  the  zoologist.  For  there  are  many 
possible  classifications,  varying  according  to  their  purpose,  according  to 
the  points  of  similarity  which  have  been  selected  as  important.  Thus 
we  may  classify  animals  according  to  their  habitats  or  their  diet  without 
taking  any  thought  of  their  structure. 

But  a  strictly  zoological  classification  is  one  which  seeks  to  show  the 
natural  relationships  of  animals,  to  group  together  those  which  resemble 
one  another  in  their  real  nature  or  structure.  It  must  therefore  be 
based  on  the  results  of  comparative  anatomy,  technically  speaking,  on 
"  homologies,"  or  real  resemblances  of  structure.  Whales  must  not  be 
ranked  with  fishes,  nor  bats  with  birds. 

To  a  classification  based  on  structural  resemblances,  two  corrobora- 
tions  are  necessary,  from  embryology  and  from  palaeontology.  On  the 
one  hand,  the  development  of  the  forms  in  question  must  be  studied  ; 
thus  no  one  dreamed  that  a  Tunicate  was  a  Vertebrate  until  its  life- 
history  was  worked  out ;  on  the  other  hand,  the  past  history  must  be 
inquired  into,  thus  the  affinity  between  Birds  and  Reptiles  is  confirmed 
by  a  knowledge  of  the  extinct  forms. 

In  classification  it  is  convenient  to  recognise  certain  grades  or  degrees 
of  resemblance,  which  are  spoken  of  as  species,  genera,  families,  orders, 
classes,  and  so  on. 

To  give  an  illustration,  all  the  tigers  are  said  to  form  the  species 
Felis  tigris,  of  the  genus  Felis,  in  the  family  Felidae,  in  the  order 
Carnivora,  within  the  class  Mammalia.  The  resemblances  of  all  tigers 
are  exceedingly  close ;  well-marked,  but  not  so  close,  are  the  resem- 
blances between  tigers,  lions,  jaguars,  pumas,  cats,  etc.,  which  form  the 
genus  Felis  ;  broader  still  are  the  resemblances  between  all  members  of 
the  cat  family  Felidse  ;  still  wider  those  between  cats,  dogs,  bears,  and 
seals,  which  form  the  order  Carnivora ;  and  lastly,  there  are  the  general 
resemblances  of  structure  which  bind  Mammals  together  in  contrast  to 
Birds  or  Reptiles. 

It  must  be  understood  that  the  real  things  are  the  individual  animals, 
and  that  a  species  is  a  subjective  conception  within  which  we  include  all 
those  individuals  who  resemble  one  another  so  closely  that  we  feel  we 
need  a  specific  name  applicable  to  them  all.  And  as  resemblances 
which  seem  important  to  one  naturalist  may  seem  trivial  to  others,  there 


CLASSIFICA  TION. 


II 


are  often  wide  differences  of  opinion  as  to  the  number  of  species  which 
a  genus  contains.  In  a  handful  of  small  shells  the  "splitters"  may 
recognise  20  species  where  the  "  slumpers  "  see  only  3.  Thus  Hseckel 
says  of  calcareous  sponges  that,  as  the  naturalist  likes  to  look  at  the 
problem,  there  are  3  species,  or  21,  or  289,  or  591  ! 

But  while  no  rigid  definition  can  be  given  of  a  species,  seeing  that 


FIG.  i. — Diagrammatic  expression  of  classification  in  a 
genealogical  tree.  B  indicates  possible  position  of  Balano- 
glossus,  D  of  Dipnoi,  S  of  Sphenodon  or  Hatteria. 

the  conception  is  one  of  practical  convenience  and  purely  relative,  there 
are  certain  common-sense  considerations  to  be  borne  in  mind — 

I.    No   naturalist   now   believes,   as    Linnoeus   did,   in   the   fixity   of 


12     GENERAL  SURVEY  OF  THE  ANIMAL  KINGDOM. 

species  ;  we  believe,  on  the  contrary,  that  one  form  has  given  rise  to 
another.  At  the  same  time,  the  common  characteristic  on  the  strength 
of  which  we  deem  it  warrantable  to  give  a  name  to  a  group  of  individuals 
must  not  be  markedly  fluctuating.  The  specific  character  should  exhibit 
a  certain  degree  of  constancy  from  one  generation  to  another. 

2.  Sometimes  a  minute  character,  such  as  the  shape  of  a  tooth  or 
the  marking  of  a  scale,  is  so  constantly  characteristic  of  a  group  of 
individuals  that  it  may  be  safely  used  as  the  index  of  more  important 
characters.     On  the  other  hand,  the  distinction  between  one  species  and 
another  should  always  be  greater  than  any  difference  between  the  members 
of  a  family  (using  the  word  family  here  to  mean  the  progeny  of  a  pair). 
For  no  one  would  divide  mankind  into  species  according  to  the  colour 
of  eyes  or  hair,  as  this  would  lead  to  the  absurd  conclusion  that  two 
brothers  belonged  to  different  species.     Thus  it  is  often  doubly  un- 
satisfactory when  a  species  is  established  on  the  strength  of  a  single 
specimen,  (a)  because  the  constancy  of  the  specific  character  is  undeter- 
mined ;  (b]  because  the  variations  within  the  limits  of  the  family  have 
not  been  observed.     Indeed,  it  has  happened  that  one  species  has  been 
made  out  of  a  male  and  another  out  of  its  mate.     But  the  characters 
of  a  single  specimen  are  sometimes  so  distinctive  that  the  zoologist  is 
safe  in  making  it  the  type  of  a  new  species,  or  even  of  a  new  genus. 

3.  While  cases  are  known  where  members  of  different  species  have 
paired  and  brought  forth  fertile  hybrids,  this  is  not  common.       The 
members  of  a  species  are  fertile  inter  se,  but  not  usually  with  members  of 
other  species.     In  fact,  the  distinctness  of  species  has  largely  depended 
on  a  restriction  of  the  range  of  fertility. 

To  sum  up,  a  species  is  but  a  relative  conception,  convenient  when 
we  wish  to  include  under  one  title  all  the  members  of  a  group  of 
individuals  who  resemble  one  another  in  certain  characters.  There  is 
no  absolute  constancy  in  these  specific  characters,  and  one  species  often 
melts  into  another  with  which  it  is  connected  by  intermediate  varieties. 
At  the  same  time,  the  characters,  on  account  of  which  the  naturalist 
gives  a  specific  name  to  a  group  of  individuals,  should  be  greater  than 
those  which  distinguish  the  members  of  any  one  family,  should  show 
a  relative  constancy  from  generation  to  generation,  and  should  be 
associated  with  reproductive  peculiarities  which  tend  to  restrict  the 
range  of  mutual  fertility  to  the  members  of  the  proposed  species. 

It  will  be  enough  now  simply  to  state  some  of  the  more  important 
grades  of  classification  : — 

Individuals. 

Varieties  among  these  individuals.    . 

Species,  e.g.,  Felis  tigris. 

Genus,  Felis. 

Family,  Felidae. 

Order,  Carnivora. 

Class,  Mammalia. 

Phylum  or  Series,  Vertebrata. 


[TABLE. 


TABULAR  SURVEY  OF  CLASSES. — (For  Future  Reference.} 

METAZOA  CHORDATA. 


MAMMALIA. 


AVES. 


REPTILIA. 


AMPHIBIA. 


PISCES. 


Eutheria.     Placentals. 

Metatheria.      Marsupials.      Non-pla- 

cental. 
Prototheria.  Monotremes.  Oviparous. 

{Carinatae.     Keeled  flying  birds. 
Ratitae.     Keel-less  running  birds. 
Extinct  reptile-like  birds. 

,-Crocodilia.    Crocodiles  and  alligators. 
I  Ophidia.     Snakes. 
J  Lacertilia.     Lizards. 
I  Rhyncocephalia.     Sphenodon. 
I  Chelonia.     Tortoises  and  turtles. 
^Extinct  Classes. 

f  Anura.     Tail-less  frogs  and  toads. 
I  Urodela.     Tailed  newts. 
•<  Gymnophiona,  e.g.,  C&cilia. 
I  Labyrinthpdpnts    and     other    extinct 
V         Amphibians. 

Dipnoi.     Mud  fishes. 

Teleostei.     Bony  fishes. 

Ganoidei,  e.g.,  Sturgeon. 

Elasmobranchii.    Cartilaginous  fishes.; 

•),      and      Lamprey 


if 
II 


/Hagfish    (Myxine), 
"  I         (Petromyzon). 


CYCLOSTOMATA.  ~\ 

CEPHAI.OCHORDATA.    A  mphioxus. 

UROCHORDATA.      Tunicates. 

HEMICHORDATA.    Balanoglossus,  Cephalodiscus. 

METAZOA   NON-CHORDATA. 


MOLLUSCA. 


T  Cephalopoda.     Cuttle  fishes. 
-f  Gasteropoda.     Snails. 
^Lamellibranchiata.     Bivalves. 


TArachnoidea.     Spiders,  scorpions,  mites. 
J  Insecta. 

ARTHROPODA.      -;  Myriopoda.     Centipedes  and  millipedes. 
I  Protracheata.     Peripatus. 
^Crustacea. 

TCrinoidea.     Feather  stars.     (Cystoids  and  Blastoids,  extinct.) 
I  Ophiuroidea.     Brittle  stars. 
\  Asteroidea.     St?-  ^oVl 
I  F.chinoidea.     Se 
v  Holothuroidea. 


ECHINODERMA.  \  Asteroidea. 


Star  fish. 
Sea  urchins. 

Sea  cucumbers. 


"  J  Chaetopoda. 
1  Discophora. 


'  WORMS." 


CCELENTERA. 


PORIFERA. 


Bristle-footed  worms. 
Leeches. 


Annelids. 


f  Brachiopoda.     Lamp  shells. 
-|  Pplyzoa,  e.g.,  Sea  mat  (Flustra). 
V.Sipunculoidea,  e.g. ,  Sipunculus. 


Nematoda. 
Nemertea. 


Thread-worms. 
Ribbon -worms. 


{Cestoda.     Tape-worms.       ^ 
Trematoda.     Flukes.  VPlathelmin 

Turbellaria.     Planarians.    J 

(  Ctenophora,  e.g. ,  Beroe. 

\  Scyphozoa.     Jellyfish  and  sea  anemones. 

VHydrozoa.     Zoophytes  and  medusoids. 

Sponges. 

PROTOZOA. 

INFUSORIA.     RHIZOPODA.     GREGARINIDA. 
Simplest  forms  of  animal  life. 


thes. 


CHAPTER    II. 

THE    FUNCTIONS    OF    ANIMALS. 

(PHYSIOLOGY.) 

MOST  animals  live  a  conscious  and  active  life,  busied  with 
the  search  for  food,  the  wooing  of  mates,  the  building  of 
homes,  and  the  tending  of  young.  These  and  other  forms 
of  activity  depend  upon  internal  changes  within  the  body. 
For  the  movements  of  all  but  the  very  simplest  animals  are 
due  to  the  activity  of  contractile  parts  known  as  muscles, 
which  are  controlled  by  nervous  centres  and  by  impulse- 
conducting  fibres. 

But  as  the  work  done  means  expenditure  of  energy, 
and  is  followed  by  muscular  and  nervous  exhaustion,  the 
necessity  for  fresh  supplies  of  energy  is  obvious.  This 
recuperation  is  obtained  from  food,  but  before  this  can 
restore  the  exhausted  parts  to  their  normal  state,  or  keep 
them  from  becoming,  in  any  marked  degree,  exhausted,  it 
must  be  rendered  soluble,  diffused  throughout  the  body, 
and  so  chemically  altered  that  it  is  readily  incorporated 
into  the  animal's  substance,  t  In  other  words,  it  has  to  be 
digested. 

We  may  say  then  that  there  are  two  master  activities  in 
the  animal  body,  those  of  muscular  and  those  of  nervous 
parts,  to  which  the  other  internal  activities  are  subsidiary 
conditions,  turning  food  into  blood  and  thus  repairing  the 
waste  of  matter  and  energy,  keeping  up  the  supply  of 
oxygen  and  the  warmth  of  the  body,  sifting  out  and 
removing  waste  products. 

Besides  the  more  or  less  constantly  recurrent  activities  or 
functions,  which  are  summed  up  under  the  general  term 
"  metabolism,"  there  are  the  processes  of  growth  and  repro- 


DIVISION  OF  LABOUR.  15 

duction.  When  income  exceeds  expenditure  in  a  young 
animal,  growth  goes  on,  and  the  inherited  qualities  of  the 
organism  are  more  and  more  perfectly  developed.  At  the 
limit  of  growth,  when  the  animal  has  reached  "  maturity," 
it  normally  reproduces,  that  is  to  say,  liberates  parts  of 
itself  which  give  rise  to  new  individuals.  /  It  is  this  power 
of  growing  and  reproducing  which  most  distinguishes  an 
organism  from  an  inanimate  thing. 

Division  of  Labour. 

All  the  ordinary  functions  of  life  are  exhibited  by  the 
simple  unicellular  animals  or  Protozoa.  Take  the  Amoeba 
for  example.  It  moves  by  contracting  its  living  substance, 
it  draws  back  sensitively  from  hurtful  influences,  it  engulfs 
and  digests  food,  it  gets  rid  of  waste,  and  it  absorbs  oxygen, 
without  which  its  living  matter  cannot  continue  active  or 
indeed  alive.  ('*  For  activity  implies,  in  part,  an  oxidation,  a 
combustion  of  material,  and  respiration  in  plants  and  animals 
alike  consists  in  absorbing  oxygen,  and  in  liberating  the  car- 
bonic acid  gas  which  is  one  of  the  waste  products  both  of 
life  and  burning.  x, 

But  all  these  activities  occur  in  the  Amoeba  within  the 
compass  of  a  unit  mass  of  living  matter, — a  single  cell, 
physiologically  complete  in  itself.  There  is  no  division  of 
labour,  there  are  as  yet  no  parts. 

In  all  other  animals,  from  Sponges  onwards,  there  is  a 
"  body  "  consisting  of  hundreds  of  unit  masses  or  cells.  It 
is  impossible  for  these  to  remain  the  same,  for  some  are 
internal  and  others  external,  nor  would  it  be  well  for  the 
organism  that  all  its  units  should  retain  the  primitive  and 
many  sided  qualities  of  Amoebae.  <' Division  of  labour,  con- 
sequent on  diversity  of  conditions,  is  thus  established  in  the 
organism.  In  some  cells  one  kind  of  activity  predominates, 
in  others  a  second,  in  others  a  third.  And  this  division  of 
labour  is  followed  by  that  complication  of  structure  which 
we  call  differentiation. 

Thus,  in  the  fresh  water  Hydra,  which  is  one  of  the 
simplest  many  celled  animals,  the  units  are  arranged  in 
two  layers,  and  form  a  tubular  body.  Those  of  the  outer 
layer  are  protective,  nervous,  and  muscular ;  those  of 


1 6         THE  FUNCTIONS  OF  ANIMALS. 

the  inner  layer  absorb  and  digest  the  food,  and  are  also 
muscular. 

In  worms  and  higher  organisms,  there  is  a  middle  layer 
in  addition  to  the  other  two,  and  this  middle  layer  becomes, 
for  instance,  predominantly  muscular.  ^Moreover,  the  units 
or  cells  are  not  only  arranged  in  strands  or  tissues,  each 
with  a  predominant  function,  but  become  compacted  into 
well-defined  parts  or  organs.  None  the  less  should  we 
remember  that  each  cell  remains  a  living  unit,  and  that,  in 
addition  to  its  principal  activity,  it  usually  retains  others  of 
a  subsidiary  character. 

History. 

Physiologists,  or  those  who  study  the  activities  of  organisms  and  of 
their  parts,  were  at  first  content  to  speak  of  these  as  the  result  of 
"  animal  and  vital  spirits,"  of  moods  and  temperaments. 

Stimulated,  however,  by  the  anatomists'  disclosure  of  organs,  the 
physiologists  soon  began  to  explain  the  organism  as  a  complex  engine 
of  many  parts.  The  muscles  were  recognised  as  the  mechanism  which 
produced  movement,  the  heart  pumped  the  blood  through  the  body,  the 
brain  was  the  seat  of  thought,  and  so  on.  This  was  an  exceedingly 
necessary  and  natural  step  in  analysis.  Nor  has  it  yet  been  thoroughly 
taken  in  every  case,  for  there  are  many  organs,  especially  in  backbone- 
less  animals,  about  whose  predominant  use  we  are  uncertain.  But  the 
physiologists  of  this  school  sometimes  finished  their  work  too  quickly. 
That  the  liver  was  an  organ  for  secreting  bile  was  deemed  a  completely 
satisfactory  statement,  until  it  began  to  be  seen  that  this  organ  is  the 
seat  of  many  other  activities.  Moreover,  some  thought  that  it  was 
possible  to  deduce  the  function  of  an  organ  from  its  visible  structure,  as 
one  might  infer  the  use  of  a  piston  from  its  shape.  To  a  certain  extent 
this  is  true,  as  when  we  show  how  an  image  is  formed  on  the  retina  of 
the  eye.  {But  we  cannot,  in  terms  of  visible  structure,  explain  another 
function  of  the  eye — that  of  distinguishing  the  "  colours  "  of  things.  In 
fact,  it  must  be  clearly  understood  that  each  organ  is  far  more  than  a 
piece  of  mechanism  in  a  living  engine, — that  it  is  a  complicated  factory 
of  living  units,  each  with  sujbtle  and  manifold  powers,  J 

In  1801,  Bichat  analysed  the  animal  body  into  its  component  tissues 
— muscular,  nervous,  glandular,  £c.,  and  being  a  physiologist  as  well  as 
an  anatomist,  sought  to  explain  the  activities  of  the  organism  in  terms 
of  the  contractile,  irritable,  secretory,  or  other  properties  of  its  tissues. 
This  was  a  further  step  in  the  analysis,  and  one  of  great  importance. 

About  forty  years  later,  however,  it  began  to  be  recognised  that  the 
body  was  a  great  city  of  cells,  each  with  a  life  of  its  own.  The  functions 
were  not  merely  the  activities  of  organs  of  various  construction,  or  of 
tissues  with  various  properties,  they  were  the  results  of  the  life  of  the 
component  units  or  cells. 

Finally,  in  thfcse  last  days,  the  physiologists  have  touched  the  bottom 


PLANTS  AND  ANIMALS.  17 

in  their  analysis,  for  they  are  endeavouring  to  discover  the  physical  and 
chemical  changes  associated  with  the  living  stuff  or  protoplasm  itself. 
These  are  obviously  at  the  foundation  of  the  whole  matter. 

Plants  and  Animals. 

Before  we  give  a  sketch  of  the  chief  functions  in  a  higher 
animal,  let  us  briefly  consider  the  resemblances  and  differ- 
ences between  plants  and  animals. 

(a.)  Resemblance  in  Function.  —  The  life  of  plants  is 
essentially  like  that  of  animals,  as  has  been  recognised  since 
Claude  Bernard  wrote  his  famous  book,  Phenomenes  de  la 
vie  communs  aux  animaux  et  aux  vegetaux.  The  beech 
tree  feeds  and  grows,  digests  and  breathes,  as  really  as  does 
the  squirrel  on  its  branches.  In  regard  to  none  of  the  main 
functions  is  there  any  essential  difference.  Many  simple 
plants  swim  about  actively ;  young  shoots  and  roots  also 
move;  and  there  are  many  cases  in  which  even  the  full-  I 
grown  parts  of  plants  exhibit  movements.  Moreover,  the  ' 
tendrils  of  climbers,  the  leaves  of  the  sensitive  plant,  the 
tentacles  of  the  sun-dew,  the  stamens  of  the  rock  rose,  the 
stigma  of  the  musk,  are  but  a  few  instances  of  the  numerous 
plant  structures  which  exhibit  marked  sensitiveness. 

(b.}  Resemblance  in  Structure. — The  simplest  plants  (Pro- 
tophyta)   like  the   simplest   animals   (Protozoa)    are   single 
cells ;  the   higher  plants   (Metaphyta)  and  higher  animals 
(Metazoa)  are  built  up  of  cells  and  of  various  modifications  A 
of  cells.    '  In  short,  all  organisms  have  a  cellular  structure,  1 
This  general  conclusion  is  known  as  the   Cell  Theory  or 
Cell  Doctrine  (see  p.  41). 

(c.}  Resemblance  in  Development. — When  we  trace  the 
beech  tree  back  to  the  beginning  of  its  life,  we  find  that  it 
arises  from  a  uniLjelement  or  egg  cell,  which  is  fertilised  by 
intimate  union  with  a  male  element  derived  from  the  pollen- 
grain.  When  we  trace  the  squirrel  back  to  the  beginning 
of  its  life,  we  find  that  it  also  arises  from  a  unit. element  or 
egg  cell,  which  is  fertilised  by  intimate  union  with  a  male 
cell  or  spermatozoon.  Thus  all  the  many  celled  plants  and 
animals  begin  as  fertilised  egg  cells,  except  in  cases  of 
virgin  birth  (parthenogenesis)  or  of  asexual  reproduction. 
From  the  egg  cell,  which  divides  and  redivides  after  fertilisa- 
tion, the  body  of  the  plant  or  animal  is  built  up  by  con- 

9 


1 8  THE  FUNCTIONS   OF  ANIMALS. 

tinued  division,  arrangement,  and  modification  of  cells. 
Thus,  plants  and  animals  resemble  one  another  in  their 
essential  functions,  in  their  cellular  structure,  and  in  their 
development. 

But  while  there  is  no  absolute  distinction  between  plants 
and  animals,  they  represent  divergent  branches  of  a  V-shaped 
tree  of  life.  It  is  easy  to  distinguish  extremes,  like  bird 
and  daisy,  less  easy  to  contrast  sponge  and  mushroom,  well 
nigh  impossible  to  decide  whether  some  very  simple  forms, 
which  Haeckel  called  "protists,"  have  a  bias  towards  plants 
or  towards  animals.  /  But  the  food  which  most  plants  absorb 
is  cruder  or  chemically  simpler  than  that  which  animals  are 
able  to  utilise.  Thus  plants  derive  the  carbon  they  require 
from  the  carbonic  acid  gas  of  the  air,  whereas  only  a  few 
(green)  animals  have  this  power.  Almost  all  animals  depend 
for  their  carbon  supplies  on  the  sugar,  starch,  and  fat  already 
made  by  other  animals,  or  by  plants.  As  regards  nitrogen, 
most  plants  derive  this  from  nitrates  and  the  like,  absorbed 
along  with  water  by  the  roots ;  whereas  animals  obtain 
their  nitrogenous  supplies  from  the  complex  proteids  formed 
within  other  organisms.  Most  plants,  therefore,  feed  at  a 
lower  chemical  level  than  do  animals,  and  it  is  characteristic 
of  them  that,  in  the  reduction  of  carbonic  acid,  and  in  the 
manufacture  of  starch  and  proteids,  the  kinetic  energy  of 
sunlight  is  transformed  by  the  living  matter  into  the 
potential  chemical  energy  of  complex  food  stuffs.  Animals, 
on  the  other  hand,  get  their  food  ready-made ;  they  take 
the  pounds  which  plants  have,  as  it  were,  accumulated  in 
pence,  and  they  spend  them.  For  it  is  characteristic  of 
animals  that  they  convert  the  potential  chemical  energy  of 
food  stuffs  into  the  kinetic  energy  of  locomotion  and  other 
activities.  In  short,  the  great  distinction — an  average  one 
at  best — is  that  most  animals  are  more  active  than  most 
plants.  Let  us  condense  in  tabular  summary  the  time- 
honoured  "  distinctions  between  plants  and  animals." 


[TABLE. 


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20  THE  FUNCTIONS   OF  ANIMALS. 


CHIEF    FUNCTIONS    OF    THE    ANIMAL    BODY. 

We  have  seen  that  there  are  two  master  activities  in 
animals,  those  of  muscular  and  of  nervous  structures,  and 
that  the  other  functions,  always  excepting  reproduction, 
are  subservient  to  these.  Let  us  now  consider  the 
various  functions,  as  they  occur  in  some  higher  organism, 
such  as  man,  reserving  comparative  treatment  for  a 
subsequent  chapter. 

Nervous  Activities. 

Life  has  been  described  as  consisting  of  action  and 
reaction  between  the  organism  and  its  environment,  and  it 
is  evident  that  an  animal  must  in  some  way  feel,  or  become 
aware  of  surrounding  influences.  In  a  higher  animal  we 
find  parts  which  are  specially  excitable.  These  are  the 
sensory  end-organs  :  the  retina  of  the  eye  for  light,  certain 
parts  of  the  ear  for  sound,  papillae  on  the  tongue  for  taste, 
part  of  the  lining  of  the  nasal  chamber  for  smell,  tactile 
corpuscles  of  the  skin  for  pressure  and  temperature. 

All  these  end-organs  are  associated  with  nerves  which  are 
stimulated  by  the  excitation  of  the  end-organ,  and  conduct 
the  stimulus  inwards  to  what  are  called  centres  or  ganglia. 

In  Vertebrate  animals  the  brain  and  spinal  cord  contain  a 
series  of  such  centres,  some  of  which  serve  for  the  per- 
ception of  the  changes  produced  in  the  end-organs  by  the 
stimulus,  while  others  preside  over  the  activities  of  the 
muscles.  I  As  we  ascend  in  the  scale  we  find  that  in  addition 
the  brain  possesses,  to  an  increasing  extent,  the  power  of  , 
correlating  present  and  past  experiences,  and  originating  or 
inhibiting  action  in  accordance  with  the  judgment  formed.  J 

Thus,  nervous  activities  involve  (a)  end-organs  or  sense 
organs  ;  (£)  centres  or  ganglia  ;  and  (c)  the  conducting  nerves, 
some  of  which  are  afferent  (or  sensory)  passing  from  end- 
organs  to  ganglia,  while  others  are  efferent  (or  motor) 
passing  from  centres  to  muscles.  And  in  whatever  part 
there  is  activity  there  is  necessarily  waste  of  complex  sub- 
stances and  some  degree  of  exhaustion. 

/       It  is  interesting  to   notice,  as  a  triumph  of  histological  technique, 
that  Hodge,  Gusfav  Mann,  and  others  have  succeeded  in  demonstrat- 


MUSCULAR  ACTIVITY.  21 

ing  in  nerve  cells  the  structural  results  (cellular  collapse,  £c.)  of  fatigue, 
and  that  in  such  diverse  types  as  bee,  frog,  bird,  and  dog. 

Muscular  Activity. 

The  movements  of  a  unicellular  animal  are  due  to  the 
contractility  of  the  living  matter,  or  of  special  parts  of  the 
cell  such  as  cilia  (see  p.  106).  In  sponges,  there  are  often 
specially  contractile  cells ;  in  most  higher  animals  such  cells 
are  aggregated  to  form  the  muscles  on  whose  activity  all 
movement  depends, 

In  many  of  the  lower  animals,  e.g.,  sea-anemones  and  sea- 
squirts,  the  contractile  strands  consist  of  long  spindle-shaped  L 
cells  which  appear  almost  homogeneous ;  these  are  called  , 
smooth  muscle  fibres.     They  occur  in  certain  parts  of  the 
body  in  higher  Vertebrates,  e.g.,  on  the  wall  of  the  urinary ) 
bladder.     A  more  specialised  kind  of  muscle,  prevailing  in 
active    animals,    consists    of   fibres   which    show   alternate 
light  and  dark  cross  bands ;  these  are  called  striped  muscle 
fibres.     The  two  kinds,  unstriped  and  striped,  may  be  seen 
to  pass  into  one  another  in  the  same  animal,  and  in  a 
general  way  one  may  think  of  the  former  as  slowly  con- 
tracting, the  latter  as  rapidly  contracting. 

A  piece  of  living  muscle  consists  of  fine  transparent  tubes 
or  fibres,  each  invested  by  a  sheath  or  sarcolemma,  and  the 
whole  muscle  is  surrounded  by  connective  tissue.  It 
usually  runs  from  one  part  of  the  skeleton  to  another  and 
is  fastened  to  the  skeleton  by  tendons  or  sinews.  It  is 
stimulated  by  motor  nerves,  and  is  richly  supplied  with 
blood. 

When  a  muscle  contracts,  usually  under  a  stimulus 
propagated  along  a  motor  nerve,  there  is  of  course  a  change 
of  shape — it  becomes  shorter  and  broader.  The  source  of"1 
the  energy  expended  in  work  done  is  the  "  chemical  ex- 
plosion "  which  occurs  in  the  fibres,  for  the  oxygen  stored 
up  (intramolecularly)  in  the  muscle  enters  into  rapid  union 
with  a  carbohydrate.  Heat,  CO2,  and  water  are  produced 
as  the  result  of  this  combustion,  and  lactic  acid  is  also 
formed  as  a  bye-product.  Besides  the  chemical  change  and 
the  change  of  shape,  there  are  also  changes  of  electrical 
potential  associated  with  each  contraction. 


22  THE  FUNCTIONS   OF  ANIMALS. 

Digestion. 

The  energy  expended  in  doing  work  or  in  growth  is 
balanced  by  the  potential  energy  of  the  food  stuffs  taken 
into  the  body.  These  consist  of  proteids,  carbohydrates, 
fats,  water,  and  salts  in  varying  proportions  according  to  the 
diet  of  the  animal.  1  Oxygen  may  also  be  regarded  as  form- 
ing part  of  the  food. 

In  some  of  the  lower  animals,  such  as  sponges,  the  food 
particles  are  directly  engulfed  by  some  of  the  cells  with 
which  they  come  in  contact.  Within  these  cells  they  are 
dissolved  ;  this  is  known  as  intracellular  digestion.  In  most 
cases,  however,  the  food  is  rendered  soluble  and  diffusible 
within  the  food-canal  by  the  action  of  certain  ferments  made 
by  the  cells  which  line  the  gut  or  form  the  associated 
glands.  The  great  peculiarity  of  these  fermenting  substances 
is  that  a  small  quantity  can  act  upon  a  large  mass  of 
material  without  itself  undergoing  any  apparent  change.  But 
however  digestion  be  effected,  it  means  making  the  food 
soluble  and  diffusible.  In  a  higher  vertebrate,  there  are 
many  steps  in  the  process. 

(a)  The  first  ferment  to  affect  the  food,  masticated  by  the  teeth  and 
moistened  by  the  saliva,  is  ihzptyalin  of  the  salivary  juice,  which  changes 
starch  into  sugar.     The  juice  is  formed  or  secreted  by  various  salivary 
glands  around  the  mouth. 

(b)  The  food  is  swallowed,  and  passes  down  the  gullet  to  the  stomach, 
where  it  is  mixed  with  the  gastric  juice  secreted  by  glands  situated  in 
the  walls.     These  walls  are  also  muscular,  and  their  contractions  churn 
the  food  and  mix  it  with  the  juice.     In  the  juice  there  is  some  free  hydro- 
chloric acid  and  a  ferment  called  pepsin  ;  these  act  together  in  turning 
proteids  into  peptones.     The  juice  has  also  a  slight  solvent  effect  on  fat, 
and  the  acid  on  the  carbohydrates. 

(c)  The  semi-digested  food,  as  it  passes  from  the  stomach  into  the 
small  intestines,  is  called  chyme,  and  on  this  other  juices  act.     Of  these 
the  most  important  is  the  secretion  of  the  pancreas,  which  contains 
various  ferments,   e.g.)  trypsin,  and  affects  all    the    different  kinds  of 
organic  food.     It  continues  the  work  of  the  stomach,  changing  proteids 
into  peptones  ;  it  continues  the  work  of  the  salivary  juice,  changing  starch 
into  sugar  ;  it  also  emulsifies  the  fat,  dividing  the  globules  into  extremely 
small  drops,  which  it  tends  to  split  into  fatty  acids  and  glycerine. 

(d)  Into  the  beginning  of  the  small  intestine,  the  bile  from  the  liver 
also  flows,  but  this  is  not  of  great  digestive  importance,  being  rather  of 
the  nature  of  a  waste  product.      It  seems  to  have  a  slight  solvent,  emul- 
sifying, and  saponifying  action  on  the  fats  ;  in  some  animals  it  has  a 
slight  power  of  converting  starch  into  sugar  ;  by  its  alkalinity  it  helps 
the  action  of  the  trypsin  of  the  pancreas  (which,  unlike  pepsin,  acts  in  a 


ABSORPTION.  23 

neutral  fluid)  ;  it  affects  cell  membranes,  so  that  they  allow  the  passage 
of  small  drops  of  fat  and  oil  ;  and  it  is  said  to  have  various  other 
qualities. 

(e)  In  addition  to  the  liver  and  the  pancreas,  there  are  on  the  walls  of 
the  small  intestine  a  great  number  of  small  glands,  which  secrete  a  juice 
which  probably  seconds  the  pancreatic  juice.  The  digested  material  is 
in  part  absorbed  into  the  blood,  and  the  mass  of  food,  still  being  digested, 
is  passed  along  the  small  intestine  by  means  of  the  muscular  contraction 
of  the  walls,  known  as  peristaltic  action.  It  reaches  the  large  intestine 
and  its  reaction  is  now  distinctly  acid  by  reason  of  the  acid  fermentation 
of  the  contents.  The  walls  of  the  large  intestine  contain  glands  similar 
to  those  of  the  small  intestine,  and  the  digestive  processes  are  completed, 
while  absorption  also  goes  on ;  so  that  by  the  time  the  mass  has  reached 
the  rectum,  it  is  semi-solid,  and  is  known  as  fseces.  These  contain  all 
the  indigestible  and  undigested  remnants  of  the  food  and  the  useless 
products  of  the  chemical  digestive  processes. 

Absorption. 

But  the  food  must  not  only  be  rendered  soluble  and 
diffusible,  it  must  be  carried  to  the  different  parts  of  the 
body,  and  there  incorporated  into  the  hungry  cells.  It  is 
carried  by  the  blood-stream,  and  in  part  also  by  what  are 
called  lymph  vessels,  which  contain  a  clear  fluid  resembling 
blood  minus  red  blood  corpuscles. 

Absorption  begins  in  the  stomach  by  direct  osmosis  into  the  capillaries 
or  fine  branches  of  blood  vessels  in  its  walls,  and  a  similar  absorption, 
especially  of  water,  takes  place  along  the  whole  of  the  digestive  tract. 
But  lining  the  intestines  there  are  special  hair-like  projections  called 
villi  ;  they  contain  capillaries  belonging  to  the  portal  system  (blood 
vessels  going  to  the  liver),  and  small  vessels  known  as  lacteals  connected 
with  lymph  spaces  in  the  wall  of  the  intestine.  The  lacteals  lead  into  a 
longitudinal  lymph  vessel  or  thoracic  duct,  which  opens  into  the  junction 
of  the  left  jugular  and  left  subclavian  veins  at  the  root  of  the  neck.  The 
contents  of  the  duct  in  a  fasting  animal  are  clear ;  after  a  meal  they 
become  milky ;  the  change  is  due  to  the  matters  discharged  into  it  by 
the  lacteals.  It  is  probable  that  nearly  all  the  fat  of  a  meal  is  absorbed 
from  the  intestines  by  the  lacteals,  but  it  is  not  certain  in  what  measure, 
if  at  all,  this  is  true  of  the  other  dissolved  food  stuffs  ;  the  greater  part 
certainly  passes  into  the  capillaries  of  the  portal  system,  which  are  con- 
tained in  each  villus.  The  peptone  or  digested  proteid,  as  it  passes 
through  the  cells  of  the  villi,  is  changed  into  other  proteids  nearly 
related  to  those  of  the  blood,  for  no  peptone  is  found  in  the  portal  vein. 

Function  of  the  Liver. 

We  now  know  the  fate  of  the  fats,  and  of  the  proteids 
of  the  food,  and  the  manner  in  which  they  pass  into  the 
blood ;  but  we  must  follow  the  starchy  material,  or  carbo- 


24  THE  FUNCTIONS   OF  ANIMALS. 

hydrates,  a  little  further.  The  starch,  we  know,  is  converted 
into  sugar,  and  this,  with  the  sugar  of  the  food,  passes  into 
the  capillaries  of  the  villi,  and  is  carried  to  the  liver.  Dur- 
ing digestion  there  is  an  increase  of  sugar  in  the  blood  vessel 
going  to  the  liver  from  the  intestine,  that  is,  in  the  portal 
vein,  but  no  increase  in  the  hepatic  veins,  the  vessels  leav- 
ing the  liver.  The  increase  must,  therefore,  be  retained  in 
that  organ,  and  we  recognise  as  one  of  the  functions  of  the 
liver,  the  regulation  of  the  amount  of  sugar  in  the  blood. 
There  is  no  special  organ  for  the  regulation  of  the  amount 
of  fat ;  the  drops  pass  through  the  capillary  walls,  and  are 
retained  in  the  connective  tissue. 

We  must  remember  that  all  the  products  of  digestion, 
except  the  fat,  pass  through  the  liver,  which  receives  every- 
thing before  it  is  allowed  to  pass  into  the  general  circula- 
tion. Thus,  many  poisons,  especially  metals,  are  arrested 
by  the  liver,  and  many  substances  which  result  from  digestive 
processes  and  would  be  harmful,  are  there  altered  into  harm- 
less compounds.  The  excess  of  sugar,  we  have  already 
noted,  is  stored  in  the  liver.  It  is  converted  there  into  a 
substance  called  glycogen,  which  can  be  readily  retrans- 
formed  into  sugar  according  to  the  needs  of  the  system. 
Glycogen  is  stored  in  the  muscles  also,  and  is  the  material 
chiefly  useful  as  the  fuel  for  the  supply  of  muscular  energy 
and  of  the  warmth  of  the  body.  Thus,  if  an  animal  be 
subjected  to  a  low  temperature,  the  glycogen  of  the  liver 
disappears  just  as  it  does  during  the  performance  of  muscular 
work. 

Another  of  the  many  functions  of  the  liver  is  that  in  it 
nitrogenous  waste  products  begin  to  be  prepared  for  their 
final  elimination  by  the  kidneys. 

Respiration. 

There  is  another  most  important  food  stuff  to  be  noticed, 
namely,  the  oxygen  which  is  absorbed  from  the  air  by  the 
lungs.  We  may  picture  a  lung  as  an  elastic  sponge-work  of 
air  chambers,  with  innumerable  blood  capillaries  in  the 
walls,  enclosed  in  an  air-tight  box,  the  chest,  the  size  of 
which  constantly  and  rhythmically  varies.  When  we  take 
in  a  breath  the  size  of  the  chest  is  increased,  the  air  pressure 
within  is  lowered,  and  the  air  from  without  rushes  down 


EXCRETION.  25 

the  windpipe  until  the  pressure  is  equalised.  The  oxygen 
of  this  air  combines  with  a  substance  called  haemoglobin, 
contained  in  the  red  corpuscles  of  the  blood,  and  is  thus 
carried  to  all  parts  of  the  body.  The  protoplasm  of  the 
tissues  having  a  stronger  affinity  for  oxygen  than  has  the 
haemoglobin,  takes  as  much  as  it  requires.  The  carbonic 
acid  gas  formed  as  a  waste  product  is  absorbed  by  the  serum 
of  the  blood,  and  so  in  time  reaches  the  lungs.  But  as  the 
partial  pressure  of  the  carbonic  acid  in  the  air  is  lower  than 
it  is  in  the  serum,  the  gas  escapes  from  the  latter  into  the 
air  chambers  of  the  lungs.  When  the  size  of  the  chest  is 
decreased,  the  pressure  is  increased,  and  the  gas  escapes  by 
the  mouth  until  the  pressure  is  equalised.  By  the  constant 
repetition  of  the  breathing  movements,  oxygen  is  constantly 
being  taken  in,  and  carried  to  the  tissues  which  are  in  a 
marvellous  way  "hungry"  for  it,  while  the  waste  carbonic 
acid  gas  is  as  constantly  being  removed. 

Excretion. 

We  have  seen  that  the  blood  carries  the  digested  food 
to  the  various  parts  of  the  body,  and  that  it  is  also  the  carrier 
of  oxygen  and  of  the  waste  carbonic  acid  gas. 

But  there  is  much  waste  resulting  from  tissue  changes, 
which  is  not  gaseous.  It  is  cast  into  the  blood  stream  by 
the  tissues,  and  has  to  be  got  rid  of  in  some  way.  This  is 
effected  by  the  kidneys,  which  are  really  filters  introduced 
into  the  blood  stream.  But  they  are  the  most  marvellous 
filters  imaginable,  and  give  us  a  good  example  of  the  in- 
tricacy of  life  processes.  For  the  kidneys  not  only  cast  out 
of  the  blood  all  the  waste  products  that  result  from  the 
metabolism  of  proteids,  and  contain  nitrogen ;  but  they 
maintain  the  composition  of  the  blood  at  its  normal,  reject- 
ing any  stuffs  that  vary  from  that  normal,  either  qualitatively 
or  quantitatively,  doing  this  work  according  to  laws  quite 
different  from  the  simple  ones  of  diffusion  or  solubility  ; 
thus,  sugar  and  urea  are  about  equally  soluble,  and  yet  the 
sugar  is  kept  in  the  body,  while  the  urea  is  cast  out.  Even 
substances  as  insoluble  as  resins  are  removed  from  the 
blood  by  the  living  cells  of  the  kidneys. 

A  considerable  quantity  of  water,  and  traces  of  salts,  fats, 
&c.,  leave  the  body  by  the  skin,  but  its  chief  use  is  to  pro- 


26  THE  FUNCTIONS   OF  ANIMALS. 

tect  and  to  regulate  the  temperature  by  variations  in  the 
size  of  its  blood  vessels. 

This  completes  our  sketch  (a)  of  the  process  by  which  the 
food  becomes  available  for  the  organism  as  fuel  for  the 
maintenance  of  its  life  energies,  and  (b]  of  the  removal  of 
the  waste  products  which  are  formed  as  the  ashes  of  life. 

There  are  indeed  some  organs  which  we  have  not  men- 
tioned, such  as  the  spleen,  which  seems  to  be  an  area  for 
the  multiplication  of  blood  corpuscles,  and  the  thyroid 
gland,  which  seems  to  have  to  do  with  keeping  the  blood  at  a 
certain  standard  of  efficiency,  but  what  we  have  said  is 
perhaps  enough  to  convey  a  general  idea  of  the  processes  of 
life  in  a  higher  animal. 

In  conclusion,  it  is  perhaps  useful  to  remark  that  when  in  the 
course  of  further  studies  the  student  meets  with  organs  which  are  called 
by  the  same  name  as  those  found  in  man  or  in  Mammals,  as,  for  example, 
the  "liver  "  of  the  Molluscs,  he  must  be  careful  not  to  suppose  that  the 
function  of  such  a  "  liver  "  is  the  same  as  in  Mammals,  for  comparatively 
little  investigation  into  the  physiology  of  the  lower  types  of  animal  life 
has  as  yet  been  made.  At  the  same  time,  he  must  clearly  recognise  that 
the  great  internal  activities  are  in  a  general  way  the  same  in  all  animals ; 
thus,  respiration,  whether  accomplished  by  skin,  or  gills,  or  air  tubes, 
or  lungs,  by  help  of  the  red  pigment  (haemoglobin)  of  the  blood,  or  of 
some  pigment  which  is. not  red,  or  occurring  without  the  presence  of  any 
/  blood  at  all,  always  means  that  oxygen  is  absorbed  almost  like  a  kind  of 
)  food  by  the  tissues,  and  that  the  carbonic  acid  gas  which  results  from 
the  oxidation  of  part  of  the  material  of  the  tissues  is  removed. 

Modern  Conception  of  Protoplasm. 

The  activities  of  animals  are  ultimately  due  to  physical 
and  chemical  changes  associated  with  the  living  matter  or 
protoplasm.  This  is  a  mere  truism.  We  do  not  know  the 
nature  of  this  living  matter ;  in  fact,  our  most  certain  know- 
ledge of  it  is  that  in  our  brains  its  activity  is  expressed  as 
thought. 

When  more  is  known  in  regard  to  the  chemistry  and 
physics  of  living  matter,  it  may  be  possible  to  bring  vital 
phenomena  more  into  a  line  with  the  changes-  which  are 
observed  in  inorganic  things.  At  present,  however,  it  is 
idle  to  deny  that  vital  phenomena  are  things  apart.  /  Not 
even  the  simplest  of  them  can  be  explained  in  terms  of 
chemistry  and  physics. >  Even  the  passage  of  digested  food 
from  the  gut  to  the  blood  vessels  is  more  than  ordinary 


MODERN  CONCEPTION  OF  PROTOPLASM.  27 

physical  osmosis ;  it  is  modified  by  the  fact  that  the  cells 
are  living. 

From  the  point  of  view  of  a  student  of  physics  Dr.  J.  Joly 
draws  the  following  contrast  between  an  animate  and  an 
inanimate  body : — "  While  the  transfer  of  energy  into  any 
inanimate  material  system  is  attended  by  effects  retardative 
of  the  transfer  and  conducive  to  dissipation,  the  transfer  of 
energy  into  any  animate  material  system  is  attended  by 
effects  conducive  to  the  transfer  and  retardative  of  dis- 
sipation." 

But  though  we  cannot  analyse  living  matter,  nor  thoroughly 
explain  the  changes  by  which  the  material  of  the  body 
breaks  down  or  is  built  up,  we  can  trace,  by  chemical 
analysis,  how  food  passes  through  various  transformations 
till  it  becomes  a  useable  part  of  the  living  body,  and  we  can 
also  catch  some  of  the  waste  products  formed  when  muscles 
or  other  parts  are  active. 

Apart  from  any  theory,  it  is  certain  that  waste  products 
are  formed  when  work  is  done,  and  that  living  animals  have 
a  marvellous  power  of  rapid  repair,  of  ceaselessly  changing, 
and  yet  remaining  more  or  less  the  same.  Theory  begins 
when  we  attempt  to  make  the  general  idea  of  waste  and 
repair  more  precise.  In  the  study  of  "protoplasm,"  both 
morphologist  and  physiologist  have  reached  their  strict 
limits.  Further  analysis  becomes  physical  and  chemical,  ! 
and  ends  in  the  confession  that  protoplasm  is  a  marvellous 
form  of  matter  in  motion  or  a  subtle  kind  of  motion  of 
which  we  can  form  only  a  very  vague  conception. 

What  is  known  in  regard  to  the  structure  of  protoplasm  does  not  help 
the  physiologist  very  much.  As  we  shall  afterwards  see,  the  micro- 
scopists  discover  an  intricate  network  which  pervades  each  unit  of  living 
matter,  but  no  physiologist  dreams  of  explaining  the  life  of  a  cell  in 
terms  of  its  microscopically  visible  structure.  Yet,  as  Burdon  Sander- 
son says,  "We  still  hold  to  the  fundamental  principle  that  living  matter 
acts  by  virtue  of  its  structure,  provided  the  term  structure  be  used  in  a 
sense  which  carries  it  beyond  the  limits  of  anatomical  investigation, 
?'.<?.,  beyond  the  knowledge  which  can  be  attained  either  by  the  scalpel 
or  the  microscope."  But,  in  the  end,  this  means  that  living  matter  acts 
in  virtue  of  its  peculiar  qualities,  its  characteristic  motion,  of  which  we 
can  form  only  a  hypothetical  conception,  and  can  give  no  scientific 
explanation. 

One  general  idea,  however,  the  study  of  structure  has  suggested, 
which  the  conclusions  of  physiologists  corroborate.  This  idea  is — that 


28  THE  FUNCTIONS   OF  ANIMALS. 

a  cell  consists  of  a  relatively  stable  living  framework,  and  of  a  change- 
ful content  enclosed  by  it. 

Now,  many  physiologists  regard  the  framework  as  the  genuine  living 
protoplasm,  and  the  contents  as  the  material  upon  which  it  acts.  "  The 
framework  is  the  acting  part,  which  lives,  and  is  stable  ;  the  content  is 
the  acted-on  part,  which  has  never  lived,  and  is  labile,  that  is, — in  a 
state  of  metabolism  or  chemical  transformation."  This  view  naturally 
leads  those  who  adopt  it  to  regard  protoplasm  as  a  sort  of  ferment  acting 
on  less  complex  material  which  is  brought  to  it,  and  which  forms  the 
really  changeful  part  of  each  cell.  You  will  remember  that  the  strange 
characteristic  of  a  ferment  is  that  it  can  act  on  other  substances  without 
being  itself  affected  by  the  changes  which  it  produces,  and  that  it  can  go 
on  doing  so  continuously  with  a  power  which  has  no  direct  relation  to 
its  amount.  In  these  respects,  therefore,  living  matter  resembles  a 
ferment. 

Somewhat  different,  however,  is  another  idea, — that  the  protoplasm 
is  itself  the  seat  of  constant  change  ;  that  it  is  constantly  being  unmade 
and  remade.  On  the  one  hand,  more  or  less  crude  food  passes  into  life 
by  an  ascending  series  of  assimilative  or  constructive  chemical  changes 
with  each  of  which  the  material  becomes  molecularly  more  complex  and 
more  unstable.  On  the  other  hand,  the  protoplasm,  as  it  becomes  active 
or  a  source  of  energy,  breaks  down  in  a  descending  series  of  disruptive 
or  destructive  chemical  changes  ending  in  waste  products. 

The  former  view,  which  considers  protoplasm  as  a  sort  of  ferment, 
restricts  the  metabolism  to  the  material  on  which  the  protoplasm  acts. 
The  second  view  regards  protoplasm  as  the  climax  or  central  term  of  the 
constructive  and  disruptive  metabolism. 

Anabolism  and  Katabolism. 

All  physiologists  are  agreed  that  in  life  there  is  a  twofold 
process  of  waste  and  repair,  of  discharge  and  restitution,  of 
activity  and  recuperative  rest.  But  there  is  no  certainty  as 
to  the  precise  nature  of  this  two-fold  process. 

In  your  future  physiological  studies  you  will  have  to  consider  the 
power  that  our  eyes  have  of  appreciating  those  different  kinds  of  light 
which  give  rise  to  sensations  of  colour.  It  was  in  studying  these  that 
Professor  Hering  was  led  to  an  interesting  theory  of  living  matter.  He 
supposes  that  there  exist  in  or  about  the  retina  three  different  "visual 
substances,"  which  we  may  call  A,  B,  C.  He  supposes  that  each  of 
these  is  continually  undergoing  one  of  two  kinds  of  metabolism.  It  is 
either  being  built  up  by  assimilation,  or  it  is  being  broken  down  in  dis- 
assimilation.  He  supposes  that  each  of  these  substances  is  affected  by 
two  kinds  of  light,  and  that  these  two  kinds  of  light^  have  opposite 
influences  on  the  metabolism  of  the  substance.  When  we  have  a  sensa- 
tion of  white,  or  of  red,  or  of  yellow,  it  is  supposed  that  in  one  of  the 
three  kinds  of  visual  substance  disassimilation  is  preponderant.  When 
we  have  a  sensation  of  black,  or  of  green,  or  of  blue,  it  is  supposed  that 
in  one  of  the  three  kinds  of  visual  substances  assimilation  is  preponderant. 
Excess  of  disassimilation  in  A  gives  us  the  sensation  of  white ;  excess 


AN  ABOLISH  AND  KATABOLISM.  29 

of  assimilation  of  A  gives  us  the  sensation  of  black  ;  and  similarly  with 
red  and  green  for  B,  with  yellow  and  blue  for  C. 

But  generalising  from  his  studies  on  colour  sensation,  Hering  was  led 
to  regard  all  life  as  an  alternation  of  two  kinds  of  activity,  both  induced 
by  stimulus,  the  one  tending  to  storage,  construction,  assimilation  of 
material,  the  other  tending  to  explosion,  disruption,  disassimilation. 
Both  processes  are,  according  to  Hering,  activities ;  both  are  dependent 
upon  stimulus  ;  they  differ,  however,  in  direction  and  results. 

In  your  future  physiological  studies  you  will  also  learn  of  the  paths  or 
channels  by  which  the  brain  sends  its  mysterious  commands  to  the 
various  parts  of  the  body.  You  will  learn  that  some  of  these  bear 
impulses  to  activity,  while  others  convey  commands  which  send  the 
affected  part  to  rest. 

It  was  in  studying  and  greatly  elucidating  these  interesting  facts,  that 
Professor  Gaskell  was  led  to  a  theory  of  vital  action  somewhat  different 
from  that  of  Hering. 

Gaskell  believes  that  life  means  an  alternation  of  two  processes,  one  of 
them  a  running  down  or  disruption  (katabolism),  the  other  a  winding 
up  or  construction  (anabolism).  The  disruptive  or  katabolic  process  in 
which  energy  is  discharged,  takes  place  occasionally  and  in  obedience  to 
stimulus  ;  the  constructive  or  anabolic  process  of  restitution  goes  on  con- 
stantly and  of  itself,  i.e.,  without  the  necessity  of  stimulus.  Thus 
Gaskell's  theory  suggests  an  alternation  of  activity  and  rest,  of  stimulated 
disruption  and  self-regulative  construction,  while  Hering's  theory 
suggests  an  alternation  of  two  antagonistic  kinds  of  activity,  assimilation 
and  disassimilation,  both  requiring  stimulus.  The  student  will  find  the 
theories,  which  I  have  briefly  noticed,  discussed  in  Professor  M.  Foster's 
article  PHYSIOLOGY,  in  the  Encyclopedia  Britannica,  and  in  an  address 
by  Professor  Burdon  Sanderson  (British  Association  Reports,  1889,  and 
also  published  in  Nature,  September  1889). 


CHAPTER  III. 

THE  ELEMENTS  OF  STRUCTURE. 
(MORPHOLOGY.) 

ANIMALS  may  be  studied  alive  or  dead,  in  regard  to  their 
activities  or  .in  regard  to  their  parts.  We  may  ask  how  they 
live,  or  what  they  are  made  of;  we  may  investigate  their 
functions  or  their  structure.  '  The  study  of  life,  activity, 
function,  is  physiology ;  the  study  of  parts,  architecture, 
structure,  is  morphology.  , 

The  first  task  of  the  morphologist  is  to  describe  structure 
(descriptive  anatomy);  the  second  is  to  compare  the  parts 
of  one  animal  with  those  of  another  (comparative  anatomy); 
the  third  is  to  generalise,  to  formulate  the  "principles  of 
morphology,"  or  the  laws  of  vital  architecture. 

But  just  as  the  physiologist  investigates  life  or  activity  at 
different  levels,  passing  from  his  study  of  the  animal  as  a 
unity  with  certain  habits,  to  consider  it  as  an  engine  of 
organs,  a  web  of  tissues,  a  city  of  cells,  and  a  whirlpool  of 
living  matter;  so  the  morphologist  has  to  investigate  the 
form  of  the  whole  animal,  then  in  succession  its  organs, 
their  component  tissues,  their  component  cells,  and  finally, 
the  structure  of  protoplasm  itself.  The  tasks  of  morphology 
and  of  physiology  are  parallel. 

Morphology  thus  includes  not  only  the  description  of  ex- 
ternal form,  not  only  the  anatomy  of  organs,  but  also  that 
minute  anatomy  of  tissues  and  cells  and  protoplasm  which 
we  call  histology.  Moreover,  there  is  no  real  difference  be- 
tween studying  fossil  animals  which  died  and  were  buried 
countless  years  ago,  and  dissecting  a  modern  frog.  The 
anatomical  palaeontologist  is  also  a  student  of  morphology. 
Finally,  as  the  greater  part  of  embryology  consists  in  study- 


EXTERNAL   FOR  At.  31 

ing  the  anatomy  and  histology  of  an  organism  at  various 
stages  of  its  development,  the  work  of  the  embryologist  is 
also  in  the  main  morphological,  though  he  has  also  to  in- 
form us,  if  he  can,  about  the  physiology  of  development. 

Morphology  has  been  defined  by  Geddes-as  "the  study 
of  all  the  statical  aspects  of  organisms,"  in  contrast  to 
physiology,  which  is  concerned  with  their  vital  dynamics. 
In  this  chapter  we  shall  follow  the  historical  development  of 
morphology,  and  work  from  the  outside  inwards  in  deeper 
and  deeper  analysis. 


I.  EXTERNAL  FORM. 

The  form  of  an  animal  is  due  to  the  interaction  of  two  variables — the 
protoplasmic  material  which  composes  the  organism,  and  the  environ- 
ment which  plays  upon  it.  In  fact,  an  animal  takes  definite  form  just 
as  a  crystal  does  ;  in  both  the  shape  is  determined  by  the  nature  of  the 
stuff  and  by  the  surrounding  influences.  Activity,  or  function,  also 
affects  form  ;  but  function  is  merely  action  and  reaction  between  the 
animal  and  its  surroundings.  Such  statements,  however,  are  platitudes  ; 
we  are  far  from  being  able  to  explain  the  conditions  of  growth  which 
lead  to  this  shape  or  that. 

As  regards  symmetry,  animals  may  be  distinguished  as: — 

(a)  Radially  Symmetrical ; 

(b)  Bilaterally  Symmetrical ;  or 

(c)  Asymmetrical. 

In  a  radially  symmetrical  animal,  such  as  a  jelly-fish,  the  body  can 
be  halved  by  a  number  of  vertical  planes — it  is  symmetrical  around  a 
median  vertical  axis.  That  is,  it  is  the  same  all  round,  and  has  no 
right  or  left  side.  In  a  bilaterally  symmetrical  body,  such  as  our  own, 
there  is  but  one  plane  through  which  the  body  can  be  halved.  In  an 
asymmetrical  animal,  such  as  a  snail,  accurate  halving  is  impossible. 

Radial  symmetry  is  illustrated  by  simple  Sponges,  most  Ccelentera, 
and  by  many  adult  Echinoderms.  As  it  is  the  rule  in  the  two  lowest 
classes  of  Metazoa,  and  as  it  is  characteristic  of  the  very  common 
embryonic  stage  known  as  the  gastrula  (an  oval  or  thimble  shaped  sac 
consisting  of  two  layers  of  cells),  it  is  probably  more  primitive  than  the 
bilateral  symmetry  characteristic  of  most  animals  above  Ccelentera. 
/  Radial  symmetry  seems  best  suited  for  sedentary  life,  or  for  aimless 
'  floating  and  driftng.  Bilateral  symmetry  probably  arose  as  it  became 
advantageous  for  animals  to  move  energetically  and  in  definite  direc- 
tions, to  pursue  their  prey  and  avoid  their  enemies.  Among  many 
celled  animals,  some  worm  type  probably  deserves  the  credit  of  begin- 
ning the  profitable  habit  of  moving  head-foremost ;  had  some  one  not 
taken  this  step,  we  should  never  have  known  our  right  hand  from  our 
left. 


32  THE  ELEMENTS   OF  STRUCTURE. 


II.  ORGANS. 

We  give  this  name  to  any  well-defined  part  of  an  animal, 
such  as  heart  or  brain.  The  word  suggests  a  piece  of 
mechanism ;  but  the  animal  is  more  than  a  complex  engine, 
and  many  organs  have  several  different  activities  to  which 
their  visible  structure  gives  little  clue. 

Differentiation  and  Integration. 

When  we  review  the  animal  series,  or  study  the  develop- 
ment of  an  individual,  we  see  that  organs  appear  gradually. 
/  The  gastrula  cavity — the  future  stomach — is  the  first  acquisi- 
tion, but   some  would  make  out  that  it  was  primitively  a 
brood  chamber.     To  begin  with,  it  is  a  simple  sac,  but  it 
soon  becomes  complicated  by  digestive  and  other  outgrowths. 
The  progress  of  the  individual,  and  of  the  race,  is  from 
simplicity  to  complexity.     When  we  think  over  the  animal 
[  series  we  also   notice  that  before  definite  nervous  organs 
|  appear  there  is  diffuse  irritability,  before  definite  muscular 
organs  appear  there  is  diffuse  contractility,  and  so  on.  f  In 
other  words,  functions  come  before  organs.  )  The  attainment 
of  organs  implies  specialisation  of  parts,  or  concentration  of 
functions  in  particular  areas  of  the  body. 

Contrast  a  frog  with  Hydra,  and  one  of  the  great  facts 
about  the  evolution  of  organs  is  illustrated.  Among  the 
living  units  which  make  up  a  frog,  there  is  much  more 
division  of  labour  than  there  is  among  those  of  Hydra.  An 
excised  representative  sample  of  Hydra  will  reproduce  the 
whole,  but  you  cannot  perform  this  experiment  with  the 
frog.  Now,  the  structural  result  of  this  physiological  division 
of  labour  is  differentiation.  The  animal,  or  part  of  it,  becomes 
more  complex,  more  heterogeneous. 

Contrast  a  bird  and  a  sponge,  and  another  great  fact 
about  the  evolution  of  organs  is  illustrated.  The  bird  is 
more  of  a  unity  than  a  sponge  ;  its  parts  are  more  closely 
knit  together  and  more  adequately  subordinated  to  the  life 
of  the  whole.  We  call  this  kind  of  progress,  integration. 
Differentiation  involves  the  acquisition  of  new  parts  and 
powers,  these  are  consolidated  and  harmonised  as  the 
animal  becomes  more  integrated. 


HOMOLOGOUS   ORGANS.  33 

Correlation  of  Organs. 

It  is  of  the  very  nature  of  an  organism  that  its  parts 
should  be  mutually  dependent.  (  The  organs  are  all 
partners  in  the  business  of  life,  and,  if  one  member  suffer, 
others  also  are  affected.  This  is  especially  true  of  certain 
organs  which  have  developed  and  evolved  together,  and  are 
knit  by  close  physiological  bonds.  Thus,  the  circulatory  and 
the  respiratory  systems,  the  muscular  and  the  skeletal  systems, 
the  brain  and  the  sense  organs,  are  very  closely  united,  and 
we  say  that  they  are  correlated.  A  variation,  for  better  or 
worse,  in  one  system  often  brings  about  a  correlated  variation 
in  another,  but  sometimes  we  cannot  trace  the  connection. 

Homologous  Organs. 

Organs  which  arise  from  the  same  primitive  layer  of 
the  embryo  (see  Chapter  IV.),  have  something  in  common. 
But  when  a  number  of  organs  arise  in  the  same  way,  from 
the  same  embryonic  material,  and  are  at  first  fashioned  on 
the  same  plan,  they  have  still  more  in  common.  Nor  will 
this  fundamental  sameness  be  affected  though  the  final 
shape  and  use  of  the  various  organs  be  very  different.  /We 
call  organs  which  are  thus  structurally  and  developmentally 
similar,  homologous.}  Thus,  the  nineteen  pairs  of  appendages 
on  a  crayfish  are  all  homologous  ;  the  three  pairs  of  "jaws" 
in  an  insect  are  homologous  with  the  insect's  legs  ;  and  it 
is  also  true  that  the  fore-leg  of  a  frog,  the  wing  of  a  bird,  the 
flipper  of  a  whale,  the  arm  of  a  man,  are  all  homologous. 
On  the  other  hand,  the  wing  of  a  bird  and  the  wing  of  an 
insect,  which  resemble  one  another  in  being  organs  of 
flight,  are  not  the  least  alike  in  structure  ;  this  is  expressed 
by  saying  that  they  are  only  analogous.  Yet  two  organs 
may  be  both  homologous  and  analogous,  e.g.,  the  wing  of  a 
bird  and  the  wing  of  a  bat,  for  both  are  fore-limbs,  and 
both  are  organs  of  flight.  Sometimes  two  organs  or  two 
organisms — deeply  different  in  structure — have  a  marked 
superficial  resemblance,  simply  because  both  have  arisen  in 
relation  to  similar  conditions  of  life.  Thus  a  burrowing 
amphibian,  a  burrowing  lizard,  and  a  burrowing  snake,  re- 
semble one  another  in  being  limbless,  but  this  "convergence" 
of  form  does  not  indicate  any  relationship  between  them. 

To  describe  such  cases  the  term  homoplastic  is  used. 


34  THE  ELEMENTS  OF  STRUCTURE. 

Change  of  Function. 

Division  of  labour  involves  restriction  of  functions  in  the 
several  parts  of  an  animal,  and  no  higher  Metazoa  could  have 
arisen  if  all  the  cells  had  remained  with  the  many-sided 
qualities  of  Amoebae.  Yet  we  must  avoid  thinking  about 
organs  as  if  they  were  necessarily  active  in  one  way  only. 
For  many  organs,  e.g.,  the  liver,  have  several  very  distinct 
functions,  and  we  know  how  wondrously  diverse  are  the 
activities  in  our  brains.  In  addition  to  the  main  function 
of  an  organ  there  are  often  secondary  functions  ;  thus,  the 
wings  of  an  insect  may  be  respiratory  as  well  as  locomotor, 
and  part  of  the  food  canal  of  Tunicates  and  Amphioxus  is 
almost  wholly  subservient  to  respiration.  Moreover,  in 
organs  which  are  not  very  highly  specialised,  it  seems  as  if 
the  component  elements  retained  a  considerable  degree  of 
individuality,  so  that  in  course  of  time  what  was  a  secondary 
function  may  become  the  primary  one.  ^Thus  Dohrn,  who 
has  especially  emphasised  this  idea  of  function  change,  says  : 
"  Every  function  is  the  resultant  of  several  components,  of 
which  one  is  the  chief  or  primary  function,  while  the  others 
are  subsidiary  or  secondary.  The  diminution  of  the  chief 
function  and  the  accession  of  a  secondary  function  changes 
the  total  function ;  the  secondary  function  becomes  gradually 
the  chief  one  ;  the  result  is  the  modification  of  the  organ." 
Notice,  in  illustration,  how  the  structure  known  as  the 
allantois  is  an  unimportant  bladder  in  the  frog,  while  in 
Birds  and  Reptiles  it  forms  a  foetal  membrane  (chiefly 
respiratory)  around  the  embryo,  and  in  most  Mammals 
forms  part  of  the  placenta  which  effects  nutritive  connection 
between  offspring  and  mother. 

"  Substitution  of  Organs." 

The  idea  of  several  changes  of  function  in  the  evolution 
of  an  organ,  suggests  another  of  not  less  importance  which 
has  been  emphasised  by  Kleinenberg.  An  illustration  will 
explain  it.  In  the  early  stages  of  all  vertebrate  embryos, 
the  supporting  axial  skeleton  is  the  notochord, — a  rod 
developed  along  the  dorsal  wall  of  the  gut.  From  Fishes 
onwards,  this  embryonic  axis  is  gradually  replaced  in 
development  by  the  vertebral  column  or  backbone ;( the 
notochord  does  not  become  the  backbone,  but  is  replaced 


RUDIMENTARY  ORGANS.  35 

by  it.j  (it  is  a  temporary  structure,  around  which  the 
vertebral  column  is  constructed,  as  a  tall  chimney  may  be 
built  around  an  internal  scaffolding  of  wood.)  Yet,  it  re- 
mains as  the  sole  axial  skeleton  in  Amphioxus,  likewise  in 
great  part  in  hag  and  lamprey,  but  becomes  less  and  less 
persistent  in  Fishes  and  higher  Vertebrates,  as  its  substitute, 
the  backbone,  develops  more  perfectly.  Now,  what  is  the 
relation  between  the  notochord  and  its  substitute  the  back- 
bone, seeing  that  the  former  does  not  become  the  latter  ? 
Kleinenberg's  suggestion  is  that  the  notochord  supplies  the 
stimulus,  the  necessary  condition,  for  the  formation  of  the 
backbone.  Of  course,  we  require  to  know  more  about  the 
way  in  which  an  old-fashioned  structure  may  stimulate  the 
growth  of  its  future  substitute,  but  the  general  idea  of  one 
organ  leading  on  to  another  is  suggestive.  It  is  consistent 
with  our  general  conception  of  development — that  each  stage ) 
supplies  the  necessary  stimulus  for  the  next  step ;  it  also 
helps  us  to  understand  more  clearly  how  new  structures, 
too  incipient  to  be  of  use,  may  persist. 

Rudimentary   Organs. 

In  many  animals  there  are  structures  which  attain  no 
complete  development,  which  are  rudimentary  in  com- 
parison with  those  of  related  forms,  and  seem  retrogressive 
when  compared  with  their  promise  in  embryonic  life.  But 
it  is  necessary  to  distinguish  various  kinds  of  rudimentary 
structures.  (a)  As  a  pathological  variation,  probably  due 
to  some  germinal  defect,  or  to  the  insufficient  nutrition  of 
the  embryo,  the  heart  of  a  mammal  is  sometimes  incom- 
pletely formed.  Other  organs  may  be  similarly  spoilt  in 
the  making.  They  illustrate  arrested  development.  (b) 
Some  animals  lose,  in  the  course  of  their  life,  some  of  the 
promiseful  characteristics  of  their  larval  life  ;  thus  parasitic 
crustaceans  at  first  free-living,  and  sessile  sea  squirts  at 
first  free-swimming,  always  undergo  degeneration.  The 
retrogression  can  be  seen  in  each  lifetime.  But  the  little 
Kiwi  of  New  Zealand,  with  mere  apologies  for  wings,  and 
many  cave  fishes  and  cave  crustaceans  with  slight  hints  of 
eyes,  illustrate  degeneration  which  has  taken  such  a  hold  of 
the  animals  that  the  young  stages  also  are  degenerate.  The 
retrogression  cannot  be  seen  in  each  lifetime,  evident  as  it 


36  THE  ELEMENTS   OF  STRUCTURE. 

is  when  we  compare  these  degenerate  forms  with  their 
ancestral  ideal,  (c)  But  among  "  rudimentary  organs  "  we 
also  include  structures  somewhat  different,  e.g.,  the  gill 
clefts  which  persist  in  embryonic  reptiles,  birds,  and 
mammals,  though  they  serve  no  obvious  purpose,  or  the 
embryonic  teeth  of  whalebone  whales.  These  are  "  vestigial 
structures,'"  traces  of  ancestral  history,  and  intelligible'  on  no 
other  theory.  The  gill  clefts  are  used  for  respiration  in  all 
vertebrates  below  reptiles ;  the  ancestors  of  whalebone 
whales  doubtless  had  functional  teeth.  In  regard  to  these 
persistent  vestigial  structures,  it  must  also  be  recognised 
that  we  are  not  warranted  in  calling  them  useless.  (  Though 
they  themselves  are  not  functional,  they  may  sometimes  be, 
as  Kleinenberg  suggests,  necessary  for  the  growth  of  other 
structures  which  are  useful. 

Classification  of  Organs. — We  may  arrange  the  various  parts  of  the 
body  physiologically,  according  to  their  share  in  the  life.  Thus,  some 
parts  have  most  to  do  with  the  external  relations  of  the  animals  ;  such 
are  locomotor,  prehensile,  food -receiving,  protective,  aggressive,  and 
copulatory  organs.  Of  internal  parts,  the  skeletal  structures  are  passive  ; 
the  nervous,  muscular,  and  glandular  parts  are  active.  The  reproductive 
organs  are  distinct  from  all  the  rest.  They  are  often  called  "  gonads," 
and  should  never  be  called  glands.  For  by  a  gland  we  mean  an  organ 
which  secretes,  an  organ  whose  cell-s  produce  and  liberate  some  definite 
chemical  substance,  such  as  a  digestive  ferment.  Whereas  the  gonads 
are  organs  in  which  certain  cells,  kept  apart  from  the  specialisation 
characteristic  of  most  of  the  "  body  cells "  or  "  somatic  "  cells,  are 
multiplied,  and  eventually  liberated. 

Another  classification  of  organs  is  embryological,  i.e.,  according  to  the 
embryonic  layer  from  which  the  various  parts  arise.  Thus,  the  outer 
layer  of  the  embryo  (the  ectoderm  or  epiblast)  forms  in  the  adult  (l) 
the  outer  skin  or  epidermis  ;  (2)  the  nervous  system  ;  (3)  much  at  least 
of  the  sense  organs  :  the  inner  layer  of  ttje  embryo  (the  endoderm  or 
hypoblast)  forms  at  least  an  important  part  (the  "mid  gut ")  of  the  food 
canal,  and  the  basis  of  outgrowths  (lungs,  liver,  pancreas,  &c.)  which 
may  arise  therefrom,  and  also  the  notochord  of  Vertebrates  :  the  middle 
layer  of  the  embryo  (the  mesoderm  or  mesoblast)  forms  skeleton,  con- 
nective swathings,  muscle,  &c. 

It  is  important  to  adopt  some  order  of  description.  It  is  obviously 
prejudicial  to  the  success  of  your  work  and  to  the  health  of  your  brains, 
to  describe  an  animal  in  any  order  that  occurs  to  you,  to  skip  from  food- 
canal  to  kidney,  or  from  heart  to  reproductive  organs.  Therefore,  in  my 
descriptions  I  shall  follow,  almost  consistently,  this  order  of  treatment  : — 
Mode  of  life,  form,  external  appendages,  skin,  skeleton,  muscle,  nervous 
system,  sense  organs,  food  canal,  body  cavity,  vascular  system,  respira- 
tory system,  excretory  system,  reproductive  system,  development. 


EPITHELIAL    TISSUE.  37 

III.  TISSUES. 

Zoological  anatomists,  of  whom  Cuvier  may  be  taken  as  a 
type,  analyse  animals  into  their  component  organs,  and  dis- 
cover the  homologies  between  one  animal  and  another.  But 
as  early  as  1801,  Bichat  had  published  his  Anatomic  Generate, 
in  which  he  carried  the  analysis  further,  showing  that  the 
organs  were  composed  of  tissues,  contractile,  nervous,  gland- 
ular, &c.  In  1838-9,  Schwann  and  Schleiden  formulated 
the  "  cell  theory,"  in  which  was  stated  the  result  of  yet 
deeper  analysis — that  all  organisms  have  a  cellular  structure 
and  origin.  The  simplest  animals  (Protozoa)  are  typically 
single  cells  or  unit  masses  of  living  matter ;  as  such  all 
animals  begin ;  but  all,  except  the  simplest,  consist  of 
hundreds  of  these  cells  united  into  more  or  less  homo- 
geneous companies  (tissues)  which  may  be  compacted,  as 
we  have  seen,  into  organs.  If  we  think  of  the  organism  as 
a  great  city  of  cells,  the  tissues  represent  streets  (like  some 
of  those  in  Leipzig)  in  each  of  which  some  one  kind  of 
function  or  industry  predominates. 

Since  Leydig  gave  a  strong  foundation  to  comparative 
histology  in  his  remarkable  Lehrbuch  der  Histologie  des 
Menschen  und  der  Thiere  (Frankfurt,  1857),  the  study  has 
been  prosecuted  with  great  energy,  and  has  been  constantly 
stimulated  by  improvements  in  microscopic  apparatus  and 
technique. 

The  student  should  read  the  introductory  chapters  in  one 
of  the  numerous  works  on  histology-,  so  as  to  gain  a  general 
idea  of  the  characters  of  the  different  tissues. 

There  are  four  great  kinds, — epithelial,  connective,  mus- 
cular, and  nervous. 

(a)  Epithelial  Tissue 

is  illustrated  by  the  external  layer  of  the  skin  (epidermis),  the  internal 
(endothelial)  lining  of  the  food  canal  and  its  outgrowths,  the  lining  of 
the  body  cavity,  &c.  ;  by  the  early  arrangements  of  cells  in  all  embryos  ; 
and  by  the  simplest  Metazoa,  such  as  Hydra,  whose  tubular  body  is 
lined  by  two  layers  of  epithelium.  Embryologically  and  historically, 
epithelium  is  the  most  primitive  kind  of  tissue.  It  may  be  single 
layered  or  stratified  ;  its  cells  may  be  columnar,  scale-like,  or  otherwise. 
The  cells  may  be  close  together,  or  separated  by  intercellular  spaces, 
and  they  are  often  connected  by  bridges  of  living  matter.  Nor  are  the 
functions  of  epithelium  less  diverse  than  its  forms,  for  it  may  be  ciliated 


38  THE  ELEMENTS   OF  STRUCTURE. 

(effecting  locomotion,  food-wafting,  &c.),  or  sensitive  (and  as  such 
forming  sense  organs),  or  glandular  (liberating  certain  products  or  even 
the  whole  contents  of  its  cells),  or  pigmented  (and  thus  associated  with 
respiration,  excretion,  and  protection),  or  covered  externally  with  a 
sweated-off  cuticle,  susceptible  of  many  modifications  (especially  of 
protective  value). 

(U]   Connective  Tissue, 

This  term  is  somewhat  like  the  title  "  worms."  It  includes  too  many 
different  kinds  of  things  to  mean  much.  It  represents  a  sort  of  histo- 
logical  lumber  room. 

The  embryologists  help  us  a  little,  for  they  have  shown  that  almost 
all  forms  of  connective  tissue  are  derived  from  the  mesoderm  or  middle 
layer  of  the  embryo.  As  this  mesoderm  usually  arises  in  the  form  of 
outgrowths  from  the  gut,  or  from  ("  mesenchyme  ")  cells  liberated  at 
an  early  stage  from  either  (?)  of  the  two  other  layers  of  the  embryo 
(ectoderm  or  endoderm),  we  may  say  that  connective  tissue  is  primarily 
derived  from  epithelium. 

The  general  function  of  "  connective  tissue  "  is  to  enswathe,  to  bind, 
and  to  support,  but  the  forms  assumed  are  very  various. 

(a)  The   cells   may    be    close    together,    without   any   intercellular 
"  mortar "  or  matrix.      They  may  contain  large  vacuoles,   and   thus 
produce  the  appearance  of  a  network,  or  they  may  be  laden  with  fat 
or  with  pigment. 

(b]  In  other  cases  the  cells  of  the  connective  tissue  lie  in  a  matrix, 
which  they  exude,  or  into  which  they  in  part  die  away.     Such  cells  are 
very  often  irregular  in  outline,  and  give  off  in  most  cases  fine  processes, 
which  traverse  the  matrix  as  a  network.     The  fibrous  tissue  of  tendons 
and  the  different  kinds  of  gristle  or  cartilage  illustrate  connective  tissue 
with  much  matrix.     Cartilage  is  sometimes  hardened  by  the  deposition 
of  lime  salts  in  its  substance,  and  then  has  a  slight  resemblance  to 
another   kind   of  "  connective  tissue  "  —  bone.      But   bone,   which  is 
restricted  to  Vertebrate  animals,  is  quite  different  from  the  cartilage 
which  it  often  succeeds  and  replaces.      It  is  made  by  strands  or  layers 
of  special  bone-forming  cells  (osteoblasts),  which  may  rest  on  a  cartilage 
foundation,  or  may  be  quite  independent.     These  osteoblasts  form  the 
bone  matrix,  and  some  of  them  are  involved  in  it,  and  become  the 
permanent  bone  cells.     These  have  numerous  radiating  branches,  and 
are  arranged  in  layers,  usually  around  a  cavity  or  a  blood  vessel.    (There 
are  no  blood  vessels  in  cartilage.)      The  matrix  becomes  very  rich  in 
lime  salts  (especially  phosphate)  ;  and  the  cartilage  foundation,  if  there 
was  one,  is  quite  destroyed  by  the  new  formation.     Here  we  may  also 
note  two  important  fluid  tissues,  the  floating  corpuscles  or  cells  of  the 
blood,  and  those  of  the  body  cavity  or  "  perivisceral  "  fluid,  which  is 
often  abundant  and  important  in  backboneless  animals. 

(c)  Muscular  Tissue. 

Origin. — The  single  celled  Amccba  moves  by  flowing  out  on  one  side 
and  drawing  in  its  substance  on  another.  It  is  diffusely  contractile,  and 
it  has  also  sensitive,  digestive,  and  other  functions. 

In  Hydra  and  some  other  Ccelentera  the  bases  of  some  of  the  epithelial 


NERVOUS   TISSUE.  39 

cells  which  form  the  outer  and  inner  layers  are  prolonged  into  contractile 
roots.  /  Here  then  we  have  cells  of  which  a  special  part  discharges  a 
contractile  or  muscular  function,  while  the  other  parts  retain  other 
powers,  j 

In  other  Coelentera  the  muscular  cells  are  still  directly  connected  with 
the  epithelium,  but  become  more  and  more  exclusively  contractile.  In 
all  other  animals  the  muscular  tissue  is  derived  from  the  mesoderm, 
which,  as  we  have  already  mentioned,  is  not  distinctly  present  in 
Coelentera.  In  the  majority,  the  muscle  cells  arise  on  the  walls  of  the 
body  cavity,  and  their  origin  may  often  at  least  be  described  as  epithelial. 
But  in  other  cases  the  muscles  arise  from  those  wandering  "mesenchyme" 
cells  to  which  we  have  already  referred. 

Structure. — A  distinction  is  usually  drawn  between  striped  and  un- 
striped  muscle  fibres,  but  the  distinction  seems  to  be  one  of  degree. 

Smooth  or  unstriped  muscle  fibres  are  elongated  contractile  cells, 
externally  homogeneous  in  appearance.  They  are  especially  abundant 
in  sluggish  animals,  e.g.,  Molluscs,  and  occur  in  the  walls  of  the  gut, 
bladder,  and  blood  vessels  of  Vertebrates.  They  are  less  perfectly 
differentiated  than  striped  muscle  fibres,  and  usually  contract  more 
slowly. 

A  striped  muscle  fibre  is  a  cell,  the  greater  part  of  which  is  modified 
into  a  set  of  parallel  longitudinal  fibrils,  with  alternating  "  clear  and 
dark  "  transverse  stripes.  Each  fibril  has  certainly  a  complex  internal 
structure,  but  according  to  Haycraft  the  "  stripes  "  are  the  optical  effects 
of  the  ampullated  or  beaded  form  of  the  fibrils,  and  can  be  seen  even  on 
a  collodion  film,  upon  which  the  muscle  fibres  are  pressed.  A  residue  of 
unmodified  cell  substance,  with  a  nucleus  or  with  many,  is  often  to  be 
observed  on  the  side  of  the  fibre,  and  a  slight  sheath  or  sarcolemma 
forms  the  "cell  wall."  Many  muscle  fibres  closely  combined,  and 
wrapped  in  a  sheath  of  connective  tissue,  form  a  muscle,  which,  as  every 
one  knows,  can  contract  with  extreme  rapidity  when  stimulated  by  a 
nervous  impulse. 

(d)  Nervous  Tissue. 

Origin, — Beginning  again  with  the  Amceba,  we  recognise  that  it  is 
diffusely  sensitive,  and  that  a  stimulus  can  pass  from  one  part  of  the  cell 
to  another. 

In  some  Coelentera  some  of  the  external  cells  seem  to  combine  con- 
tractile and  nervous  functions.  Therefore  they  are  sometimes  called 
"  neuro-muscular." 

But  in  Hydra  there  are  special  nervous  cells,  whose  basal  prolonga- 
tions are  connected  with  the  contractile  roots  already  described.  This 
is  a  neuro-muscular  apparatus  of  the  simplest  kind.  The  nerve  cells 
probably  receive  impressions  from  without,  and  transmit  them  as  stimuli 
to  the  contractile  elements. 

In  sea  anemones  and  some  other  Coelentera,  there  is  an  interesting 
complication,  withal  very  simple.  There  are  superficial  sensory  cells, 
connected  with  subjacent  nerve  or  ganglion  cells,  from  which  fibres  pass 
to  the  contractile  elements. 

In  higher  animals  the  sensory  cells  are  integrated  into  sense  organs, 
the  ganglionic  cells  into  ganglia,  while  the  delicate  fibres  which  form 


40  THE  ELEMENTS   OF  STRUCTURE. 

the  connections  between  sensory  cells  and  ganglionic  cells,  and  between 
the  latter  and  muscles,  are  represented  by  well-developed  nerves. 

So  far  as  we  know,  nervous  tissue  always  arises  from  the  outer  or 
ectodermic  layer  of  the  embryo,  as  we  would  expect  from  the  fact  that 
this  is  the  layer  which,  in  the  course  of  history,  has  been  most  directly 
subjected  to  external  stimulus. 

Structure. — Let  us  consider  first  the  ganglionic  cells  which  receive 
stimuli  and  shunt  them,  which  regulate  the  whole  life  of  the  organism, 
and  are  the  physical  conditions  of  "  spontaneous "  activity  and  in- 
telligence. The  simplest  are  prolonged  at  one  pole  into  an  outgrowth 
which  branches  into  an  afferent  and  efferent  nerve  fibre.  Most,  how- 
ever, give  off  outgrowths  from  two  poles  or  on  all  sides.  Internally 
they  consist  in  great  part  of  a  network  or  coil  of  fine  fibrils,  amid  which 
lies  the  usual  cell  kernel  or  nucleus.  Ganglionic  cells,  aggregated  to 
form  ganglia,  generally  lie  embedded  in  a  fibrous  cellular  substance  called 
neuroglia,  usually  regarded  as  an  ensheathing  and  supporting  material. 

In  all  but  a  few  of  the  simplest  Metazoa,  the  nerve  fibres  are  sur- 
rounded by  a  sheath  called  the  neurilemma,  said  to  be  formed  by  adjacent 
connective  tissue.  Several  nerve  fibres  may  combine  to  form  a  nerve, 
but  each  still  remains  ensheathed  in  its  neurilemma.  In  Vertebrate 
animals  each  nerve  fibre  usually  consists  of  an  internal  "  axis  cylinder," 
the  important  part,  and  an  external  unessential  medullary  sheath.  But 
even  in  the  higher  Vertebrates,  "  non-medullated  "  or  simply  contoured 
nerve  fibres  are  found  in  the  sympathetic  and  olfactory  nerves,  and  this 
simpler  type  alone  occurs  in  hag,  lamprey,  and  lancelet,  as  well  as  in  all 
the  Invertebrates  with  distinct  nerves.  Furthermore,  nerves  are  usually 
surrounded  by  an  enveloping  nucleated  layer  called  Schwann's  sheath, 
or  else  by  neuroglia. 

A  nerve  fibre  consists  of  numerous  fibrils  like  those  seen  within  a 
ganglion  cell.  These  are  regarded  by  some  as  the  essential  elements  in 
conducting  stimuli,  while  others  maintain  that  the  essential  part  is  the 
less  compact,  sometimes  well-nigh  fluid  stuff  between  the  fibrils,  or  that 
the  fibrils  are  but  the  walls  of  tubes  within  which  the  essentially  nervous 
stuff  lies. 

According  to  some  authorities,  the  nerve  fibres  are  extensive  pro- 
longations of  the  ganglion  cells ;  according  to  others  the  neuroglia  or 
other  ensheathing  elements  contribute  to  the  extension  of  the  nerve 
fibres,  or  rather  special  neuroblast  cells  make  both  sheath  and  fibre. 

IV.  CELLS. 

In  discussing  tissues,  it  was  necessary  to  refer  to  the 
component  cells.  Let  us  now  consider  the  chief  charac- 
teristics of  these  elements. 

A  cell  is  a  unit  mass  of  living  matter.  Most  of  the 
simplest  animals  and  plants  (Protozoa  and  Protophyta)  are 
single  cells ;  eggs  and  male  elements  are  single  cells ;  in 
multicellular  organisms  the  cells  are  combined  into  tissues 
and  organs. 


CELLS.  41 

Most  cells  are  too  small  to  be  distinguished  except 
through  lenses ;  many  Protozoa,  e.g.,  large  Amoebae,  are 
just  visible  to  our  unaided  eyes ;  the  chalk  forming  Fora- 
minifera  are  single  cells,  whose  shells  are  often  as  large  as 
pin-heads,  and  some  of  the  extinct  kinds  were  as  big  as 
half-crowns ;  the  bast  cells  of  plants  may  extend  for  several 
inches  ;  the  largest  animal  cells  are  eggs  distended  with  yolk. 

History. — The  word  "cell"  was  first  used  in  histological  description 
by  Hooke  (1665),  and  Grew  (1671-5),  but  not  in  a  very  accurate  or 
definite  way.  Malpighi  (1675)  also  described  minute  "utricles,"  some 
of  which  we  should  call  cells. 

Leeuwenhoek  (Phil.  Trans.  1674)  seems  to  have  been  the  first  to 
describe  single-celled  organisms.  In  the  eighteenth  century  the  analysis 
continued;  thus  Rosel  von  Rosenhof  described  the  "Proteus  animalcule" 
or  Amceba  in  1755,  and  Fontana,  in  1784,  discovered  the  kernel  or 
nucleus  of  the  cell. 

But  the  fact  that  Bichat,  in  his  Anatomie  Generate  (1801),  speaks  of 
tissues  only,  shows  that  the  import  of  cells  was  not  realised  at  the 
beginning  of  this  century. 

In  1835,  Robert  Brown  showed  that  a  nucleus  was  normally  present 
in  all  vegetable  cells,  and  in  the  same  year  Johannes  M tiller  definitely 
compared  the  cells  of  plants  with  those  of  the  notochord  in  animals. 

The  cellular  structure  and  origin  of  organisms  began  to  be  vaguely 
recognised  by  many.  At  length,  in  1838-9,  Schwann  and  Schleiden 
showed  that  all  but  the  simplest  plants  and  animals  are  built  up  of  cells, 
and  develop  from  cells,  thus  establishing  the  famous  "  cell  theory,"  or, 
rather  cell  doctrine  : — "There  is  one  universal  principle  of  development 
for  the  elementary  part  of  organisms  however  different,  and  this  principle 
is  the  formation  of  cells."  J 

This  doctrine  was  corroborated  in  many  ways.  Numerous  investi- 
gators, Prevost  and  Dumas  (1824),  Martin  Barry  (1838-41),  Reichert 
1840),  Henle  (1841),  Kolliker  (1843-6),  and  Remak  (1841-52),  showed 
how  the  cells  of  the  embryo  arise  from  the  division  of  the  fertilised 
egg  cell. 

Moreover,  Goodsir  in  1845,  Virchow  in  1858,  proved  that  in  all  cases, 
pathological  as  well  as  normal,  cells  arise  from  pre-existing  cells,  that 
omnis  cellula  e  cellula  is  a  general  fact  of  histology. 

There  was  a  strong  tendency,  however,  to  attach  too  much  import- 
ance to  the  cell  wall,  and  too  little  to  the  contained  cell  substance.  The 
all  important  protoplasm  was  not  adequately  appreciated. 

In  1835,  Dujardin  described  the  "sarcode"  of  Protozoa,  and  other 
animal  cells  ;  in  1839,  Purkinje  compared  the  substance  of  the  animal 

1  Those  interested  in  history  should  read  the  scholarly  history  of  cell  lore  by  Sir 
William  Turner,  "The  Cell  Theory,  Past  and  Present,"  Inaug.  Address  to  Scottish 
Microscopical  Society  (Edin.  1890,  also  in  Nature,  1890).  See  also  Professor 
M'Kendrick  On  the  Modern  Cell  Theory  (Proc.  Phil.  Soc.,  Glasgow,  1888),  also  his 
text-book  of  Physiology.  The  articles  MOKPHOLOGY  and  PROTOPLASM  in  the 
Encyc.  Brit.,  and  the  article  CELL  in  the  new  edition  of  Chambers' s  Encyc.,  should 
be  consulted. 


42  THE  ELEMENTS   OF  STRUCTURE. 

embryo  with  the  "cambium"  of  plant  cells;  in  1846  Von  Mohl  em- 
phasised the  importance  of  the  "  protoplasm  "  in  vegetable  cells  ;  Ecker 
(1849)  compared  the  contractile  substance  of  muscles  with  the  living 
matter  of  amoebae  ;  Bonders  also  referred  the  contractility  from  the  wall 
to  the  contents;  Cohn  suspected  that  the  "sarcode"  of  animals  and 
the  "protoplasm"  of  plants  must  be  "in  the  highest  degree  analogous 
substances  ;"  and  finally,  Max  Schultze  (1861),  accepted  the  growing 
belief  that  plants  and  animals  were  made  of  very  similar  living  matter, 
and  defined  the  cell  as  a  unit  mass  of  nucleated  protoplasm. 

Forms  of  Cells. — The  typical  and  primitive  form  of  cell  is 
a  sphere, — a  shape  naturally  assumed  by  a  complex  coherent 
substance  situated  in  a  medium  different  from  itself.  Most 
egg  cells  and  many  Protozoa  retain  this  primitive  form,  but 
the  internal  and  external  conditions  of  life  (such  as  nutrition 
and  pressures)  often  evolve  other  shapes, — oval,  rectangular, 
flattened,  thread-like,  stellate,  and  so  on. 

Structure  of  Cells. — In  a  cell  we  may  distinguish  : — 

(a)  The  general  cell  substance  or  cytoplasm,  which  con- 
sists partly  of  genuinely  living  stuff  or  protoplasm,  and 
partly  of  complex  materials  not  really  living  ; 

(I}}  A  specialised  kernel  or  nucleus,  with  a  complex  struc- 
ture, and  important,  but  hardly,  as  yet,  definable  functions  ; 

(c)  One    or    more    specialised    bodies    called    central 
corpuscles  or  centrosomata  which  seem  to  be  centres  of 
activity  during  cell  division ; 

(d)  A  cell  wall,  which  occurs  in  very  varied  form,  or  may 
be  entirely  absent. 

(a)  The  Cell  Substance. — When  a  simple  cell  is  examined 
in  its  living  state,  it  often  appears  approximately  homo- 
geneous. Its  substance  is  usually  slightly  fluid,  but  it  may 
be  firm  and  compact  in  passive  cells.  It  is  usually  trans- 
lucent, but  there  are  often  obscuring  granules  of  different 
kinds. 

In  thinking  of  the  cell  substance  or  cytoplasm,  we  must 
distinguish  the  genuinely  living  protoplasm,  of  whose  nature 
we  know  almost  nothing,  from  associated  substances,  such 
as  proteids,  carbohydrates,  fats,  pigments,  &c.,  whose 
chemical  composition  can  be  ascertained.  The  associated 
substances  which  often  crowd  the  protoplasm,  are  due  to 
the  chemical  ascent  of  food  material  towards  protoplasm,  and 
to  the  chemical  disruption  which  protoplasm  undergoes  or 
produces  as  it  lives. 


THE  NUCLEUS.  43 

(b)  The  Nucleus. — Almost  every  cell  contains  a  nucleus 
or  several.  It  used  to  be  said  that  some  very  simple 
animals  which  Haeckel  called  Monera  had  no  nuclei,  but  in 
several  they  have  been  recently  discovered.  In  other  cases, 
e.g.,  some  Infusorians,  the  nuclear  material  seems  to  be 
diffused  in  the  cell  substance.  The  red  blood  cells  of 
Mammals  seem  to  be  distinctly  nucleated  in  their  early 
stages,  but  there  is  no  trace  of  a  nucleus  in  those  which  are 
full  grown.  We  may  safely  say  that  cells  without  nuclei  are 
very  rare,  though  in  some  cells  the  nuclei  are  less  differen- 
tiated than  in  others. 

The  nucleus  is  a  very  important  part  of  the  cell,  but  it  is 
not  yet  possible  to  define  precisely  what  its  importance  is. 
In  fertilisation  an  essential  process  is  the  union  of  the 
nucleus  of  the  spermatozoon  or  male  cell  with  the  nucleus 
of  the  ovum  or  female  cell  (Fig.  3).  In  cell  division,  the 
nucleus  certainly  plays  an  essential  part.  Cells  bereft  of 
their  nuclei  die,  or  live  for  a  while  a  crippled  life.  Accord- 
ing to  some,  the  nucleus  is  important  in  connection  with 
the  nutrition  of  the  cell,  and  it  is  generally  believed  that 
there  are  complex  actions  and  reactions  between  the  living 
matter  of  the  nucleus  and  that  of  the  cytoplasm. 

The  nucleus  often  lies  within  a  little  nest  in  the  midst  of 
the  cell  substance,  but  it  may  shift  its  position  from  one 
part  of  the  cell  to  another.  It  has  a  definite  margin,  but 
this  may  be  lost,  e.g.,  before  cell  division  begins.  Inter- 
nally, it  is  anything  but  homogeneous ;  at  any  rate,  homo- 
geneous nuclei  are  rare.  /  Usually  there  is  a  network  of  fine 
strongly  stainable  (chromatin)  strands,  with  less  stainable 
(achromatin)  substance  in  the  meshes.  But  in  other  cells, 
or  at  another  time  in  the  same  cell,  the  nucleus  is  seen  to 
contain  a  coiled  (chromatin)  thread,  or  a  number  of  chro- 
matin loops  (Fig.  2).  Weismann  and  others  believe  that 
these  chromatin  elements  or  chromosomes  are  the  bearers  of 
hypothetical  bodies  whose  properties  are  supposed  to  deter- 
mine the  nature  of  an  organism  and  its  life.  Many  nuclei 
also  contain  one  or  more  little  round  bodies  or  nucleoli, 
apparently  of  less  importance.  The  term  is  applied  some- 
what vaguely  to  little  aggregations  of  chromatin,  and  more 
properly  to  vacuole-like  bodies,  in  which  some  believe 
that  the  waste  products  of  the  nucleus  are  collected. 


44 


THE  ELEMENTS   OF  STRUCTURE. 


FIG  2.  —  Structure  of 
the  cell.  (After 
CARNOY.) 

n.  Nucleus  with  chro- 
matic coil,  note  proto- 
plasmic reticulum. 


Within  the  nucleolus  an  "  endonucleolus  "  has  been  dis- 
covered. Though  the  nuclei  of  different  cells  differ  in 
details,  there  is  a  fundamental  sameness,  both  of  structure 
and  activity,  throughout  the  world  of  cells. 

(c)  The  Centrosomes. — When  a  cell  divides  into  two,  the 
chromatin  elements  or  chromosomes  of  the  nucleus  are  also 
divided  and  separate  to  form  the  two 

daughter  nuclei.  In  this  separation 
extremely  fine  "archoplasmic"  threads 
have  been  seen  passing  to  the  chromo- 
somes from  beside  two  minute  bodies 
in  the  cytoplasm.  These  two  bodies 
are  called  central  corpuscles  or  centro- 
somata  (Figs.  3  and  4);  they  seem  to  act 
like  two  centres  of  force.  They  also 
occur,  in  most  cases  singly,  in  resting 
cells,  and  it  seems  likely  that  they  are 
constant  parts  of  the  cell,  and  that 
they  arise  from  within  the  nucleus. 

(d)  The   Cell  Wall.— To  the  earlier 
histologists,   who  often   spoke  of  cells 

as  little  bags  or  boxes,  the  wall  seemed  of  much  moment. 

It  is,  however,  the  least  important  part  of  the  cell.  In 
plant  cells  there  is  usually  a  very 
distinct  wall,  consisting  of  cellulose. 
This  is  a  product,  not  a  part,  of  the 
protoplasm,  though  some  protoplasm 
may  be  intimately  associated  with  it 
as  long  as  its  growth  continues.  In 
animal  cells  there  is  rarely  a  very 
distinct  wall  chemically  distinguish- 

F,G.3.-Fertilisedovum  able  from  the  living  matter  itself. 
But  the  margin  is  often  different  from 
the  interior,  and  a  slight  wall  may  be 
formed  by  a  superficial  compacting 
of  the  threads  of  the  cell  network,  or 
by  a  physical  alteration  of  the  cell 
substance,  comparable  to  the  forma- 
tion of  a  skin  on  cooling  porridge. 
In  other  cases,  especially  in  cells 

which   are    not   very   active,    such    as    ova   and    encysted 


of     A  scar  is. 

BOVERl.) 


(After 


chr.  Chromatin  elements, 
two  from  ovum  nucleus 
and  two  from  sperm 
nucleus ;  cs.  centrosoma 
from  which  "archoplas- 
mic "  threads  radiate, 
partly  to  the  chromosomes. 


CELL  DIVISION. 


45 


FIG.    4.  —  Diagram   of    cell 
division.     (After  BOVERI.) 

chr.  Chromosomes  forming  an 
equatorial  plate  ;  cs.  centrosoma. 


Protozoa,  a  more  definite  sheath  is  formed  around  the 
cell  substance.  Again,  animal 
cells  may  die  off  superficially 
into  a  "cuticle,"  sometimes  of 
known  chemical  composition,  as, 
for  instance,  the  chitin  formed 
by  the  ectoderm  cells  in  Insects, 
Crustaceans,  and  other  Arthro- 
pods. 

In  animals,  as  well  as  in  plants, 
adjacent  cells  are  often  linked  by 
intercellular  bridges  of  living 
matter. 

Cell  Division. — Though  the 
division  of  cells,  by  which  all 
growth  is  affected,  is  a  subject 

with  which  the  physiologist  is  as  much   concerned  as  the 

morphologist,  it  will  be  convenient  to  discuss  it  here.     The 

following  facts  are  most  important. 

(1)  We  know  that  there  is  a  striking  unity  in  all  cases,  and 
that  the  nucleus  plays  an  essential  part  in  the  process.     In 
most  cases  the  dividing  nucleus  passes  through  a  series  of 
complex  changes    known   as  karyokinesis   or  mitosis,   and 
these  are  much  the  same  everywhere,  though  different  kinds 
of  cells  have  their  specific  peculiarities.     Occasionally,  how- 
ever, both  in  Protozoa  and  Metazoa  the  nucleus  divides  by 
simple  constriction  (direct  or  amitotic  division). 

(2)  The  eventful  changes  of  karyokinesis  are  as  follows: — 
(a)  The  resting  stage  of  the  nucleus  shows  a  network  or  complete 

coil  of  filaments  (chromatin  elements)  (Fig.  2). 

/  (b}  First  Stage. — As  division  begins,  the  membrane  separating 
the  nucleus  from  the  cell  substance  disappears,  and  the 
chromatin  elements  are  seen  as  a  tangled  or  broken  coil 
(Fig.  5,  i). 

<7  (<:)  Astroid- stage. — The  chromatin  elements  bend  into  looped 
pieces,  which  are  disposed  in  a  star,  the  free  ends  of  the 
U-shaped  loops  being  directed  outwards.  Meanwhile,  a 
centrosome  has  appeared  and  divided  into  two  separating 
halves,  between  which  a  spindle  of  fine  achromatin  threads 
is  formed.  This  seems  to  form  (at  least  part  of)  what  is 
called  the  nuclear  spindle.  The  centrosomes  separate  until 
one  lies  at  each  pole  of  the  cell,  surrounded  by  radiating 
"archoplasmic  "  threads,  which  become  attached  to  the 
chromosomes  (Fig.  5,  2). 


46  THE  ELEMENTS   OF  STRUCTURE. 

2  (d]  Division  and  separation  of  the  loops. — Each  of  the  loops 
which  make  up  the  star  divides  longitudinally  into  two,  and 
each  half  separates  from  its  neighbour.  They  lie  at  first 
near  the  equator  of  the  cell,  but  they  are  apparently  drawn, 
or  driven,  to  the  opposite  poles  (Fig.  5,  2-3). 

U  (e)  Diastroid. — The  single  star  thus  forms  two  daughter  stars, 
which  separate  further  and  further  from  one  another  towards 
the  opposite  poles  of  the  cell,  remaining  connected,  how- 
ever, by  delicate  threads  (Fig.  5,  3-5). 

*  (/)  Each  daughter  star  is  reconstituted  into  a  coil  or  network  for 
each  daughter  cell,  for  the  cell  substance  has  been  constricted 
meanwhile  at  right  angles  to  the  transverse  axis  of  the 
spindle.  The  halves  separate  in  the  case  of  Prot6zoa,  but 
in  most  other  cases,  e.g.,  growing  embryos,  they  remain 
adjacent,  with  a  slight  wall  between  them  (Fig.  5,  6). 

/  (g)  Each  daughter  nucleus  then  passes  into  the  normal  resting 
phase.  The  spindle  and  usually  the  centrosomes  also  dis- 
appear. 

Flemming  gives  the  following  summary  of  karyokinesis  : — 

MOTHER  NUCLEUS  DAUGHTER  NUCLEUS 

(progressive  changes).  (regressive  changes). 

a  Resting  stage.  Resting  stage,   g  /(\ 

6  Coil.  Coil.  / 

Y     c  Astroid.  Diastroid.  e 

7>-  d  Division  of  Astroid  and  its  loops  J>- 

(metakinesis). 

(3)  We  are  far  from  being  able  to  give  even  an  approximate  account 
of  the  "  mechanism  "  of  cell  division.  Rapidly  progressive  research  has 
disclosed  many  mysteries,  but  it  does  not  explain  them.  The  nucleus 
is  resolved  into  a  chromatin  framework  and  an  achromatin  matrix,  but 
we  know  the  nature  of  neither.  The  longitudinal  division  of  each  loop 
shows  how  thorough  is  the  partition  of  substance  and  implied  qualities. 
The  "  central  corpuscles,"  recently  discovered,  act  like  centres  of  force, 
and  the  indescribably  fine  threads,  which  pass  from  around  these  to  the 
chromatin  loops,  have  been  credited  with  motive  powers.  Siriiilarly  the 
threads  of  the  nuclear  spindle  are  believed  by  some  to  draw  or  drive  the 
chromosomes,  But  we  do  not  know.  The  whole  process  is  vital,  and 
therefore  inexplicable  in  terms  of  matter  and  motion,  so  long  at  least  as 
we  do  not  know  the  secret  of  protoplasm. 

(4)  On  the  other  hand,  Leuckart  and  Spencer  have  given 
a  general  rationale  of  cell  division.  Why  do  not  cells  grow 
much  larger,  why  do  they  almost  always  divide  at  a  definite 
limit  of  growth  ?  Their  answer  is  as  follows  : — Suppose  a 
young  cell  has  doubled  its  original  mass,  that  means  that 
there  is  twice  as  much  living  matter  to  be  kept  alive.  But 
the  living  matter  is  fed,  aerated,  purified  through  its  surface, 
which,  in  growing  spherical  cells  for  instance,  only  increases 


CELL  DIVISION. 


47 


as  the  square  of  the  radius,  while  the  mass  increases  as  the 
cube.  The  surface  growth  always  lags  behind  the  increase 
of  mass.  Therefore,  when  the  cell  has,  let  us  say,  quadrupled 
its  original  mass,  but  by  no  means  quadrupled  its  surface, 
difficulties  set  in,  waste  begins  to  gain  on  repair,  anabolism 
loses  some  of  its  ascendancy  over  katabolism.  At  the  limit 
of  growth,  then,  the  cell  divides,  halving  its  mass  and  gaining 
new  surface.  Of  course  surface  may  be  increased  by  out- 
flowing processes,  just  as  that  of  leaves  by  many  lobes ;  and 


FIG.  5. — Karyokinesis.     (After  FLEMMING.) 

1.  Coil  stage  of  nucleus  ;  cc,  central  corpuscle. 

2.  Division   of   chromatin   elements  into   U-shaped    loops,    and 
longitudinal  splitting  of  these  (astroid  stage). 

3-4.  Recession  of  chromatin  elements  from  the  equator  of  the  cell 
(diastroid). 

5.  Nuclear  spindle,  with  chromatin  elements  at  each  pole,  and 
achromatin  threads  between. 

6.  Division  of  the  cell  completed. 

division  may  occur  before  the  limit  of  growth  is  reached, 
but  as  a  general  rationale,  applicable  to  organs  and  bodies 
as  well  as  to  cells,  the  suggestion  of  Leuckart  and  Spencer 
is  very  helpful. 

(5)  Protoplasm. — Morphological  as  well  as  physiological  analysis 
passes  from  the  organism  as  a  whole  to  its  organs,  thence  to  the  tissues, 
thence  to  the  cells,  and  finally  to  the  protoplasm  itself.  But  although 


48  THE  ELEMENTS   OF  STRUCTURE. 

we  may  define  protoplasm  as  genuinely  living  matter  —  as  "  the 
physical  basis  of  life  " — we  cannot  definitely  say  how  much  or  what  part 
of  an  Amoeba,  or  an  ovum,  or  any  other  cell  is  really  protoplasm.  We 
are  able  to  make  negative  statements,  e.g.,  the  yolk  of  an  egg  is  not 
protoplasm,  but  we  cannot  make  positive  statements,  or  say,  This  is 
protoplasm  and  nought  else.  Thus,  what  is  spoken  of  as  the  structure  of 
protoplasm  is  really  the  structure  of  the  cytoplasm. 

In  regard  to  this  structure,  we  know  that  it  is  very  complex,  but  we 
are  not  sure  of  much  more.  For  different  experts  see  different  appear- 
ances, even  in  the  same  cells. 

Thus  some,  e.g.,  Frommann,  see  an  intricate  network  or  reticulum 
with  less  stable  material  in  the  meshes ;  others,  e.g.,  Flemming,  see  what 
looks  like  a  manifold  coil  of  fibrils;  and  others,  e.g.,  Biitschli,  see  a 
foam-like  or  vacuolar  structure.  It  seems  likely  that  the  structure  is 
different  at  different  times,  or  in  different  cells. 

Professor  Biitschli's  belief  that  the  cytoplasm  has  a  vacuolar  structure 
is  corroborated  by  his  interesting  experiments  on  microscopic  foams. 
Finely  powdered  potassium  carbonate  is  mixed  with  olive  oil  which  has 
been  previously  heated  to  a  temperature  of  5O°-6o°  C.,  an  acid  from  the 
oil  splits  up  the  potassium  carbonate,  liberates  carbon  dioxide,  and  forms 
an  extremely  fine  emulsion.  Drops  of  this  show  a  structure  like  that 
of  cytoplasm,  exhibit  movements  and  streamings  not  unlike  those  of 
Amoeboe,  and  are,  in  short,  mimic  cells.  Just  as  a  working  model  may 
help  us  to  understand  the  circulation,  so  these  oil  emulsions  may  help 
us  to  understand  the  living  cell, — by  bringing  the  strictly  vital  pheno- 
mena into  greater  prominence. 


CHAPTER    IV. 

THE    REPRODUCTION    AND    LIFE    HISTORY    OF 
ANIMALS. 

I.  REPRODUCTION. 

IN  the  higher  animals  the  beginnings  of  individual  life  are 
hidden,  within  the  womb  in  mammals,  within  the  egg  shell 
in  birds.  It  is  natural,  therefore,  that  early  preoccupation 
with  those  higher  forms  should  have  hindered  the  recog- 
nition of  what  seems  to  us  so  evident,  that  almost  every 
animal  arises  from  an  egg  cell  or  ovum  which  has  been 
fertilised  by  a  male  cell  or  spermatozoon.  The  exceptions 
to  this  fact  are  those  organisms  which  multiply  by  buds  or 
detached  overgrowths,  and  those  which  arise  from  an  egg 
cell  which  requires  no  fertilisation.  Thus  Hydra  may  form 
a  separable  bud,  much  as  a  rose  bush  sends  out  a  sucker ; 
thus  drone  bees  "  have  a  mother  but  no  father,"  for  they 
arise  from  parthenogenetic  eggs  which  are  not  fertilised. 
Apart  from  these  and  similar  cases,  the  "ovum  theory," 
which  Agassiz  called  "  the  greatest  discovery  in  the  natural 
sciences  in  modern  times,"  is  true, — that  each  organism 
begins  from  the  division  of  a  fertilised  egg  cell. 

History. — We  can  realise  this  discovery  better  if  we  consider  its 
history.  For  a  long  time,  on  into  the  present  century,  what  was  called 
the  doctrine  of  prefonnation  prevailed.  According  to  this  theory, 
development  was  merely  an  unfolding  ("evolution")  of  a  preformed 
miniature  which  lay  within  the  germ.  The  "  ovists  "  found  this  minia- 
ture model  of  the  future  organism  in  the  egg;  the  "  animalculists " 
found  and  even  figured  it  within  the  spermatozoon.  "There  is  no 
becoming,"  said  Haller,  "•  no  part  of  the  body  is  made  from  another,  all 
are  created  at  once."  But  this  was  not  all.  The  germ  was  more  than 
a  marvellous  bud-like  miniature  of  the  adult,  it  included  the  next 
4 


50    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 

generation,  and  the  next,  and  the  next,  and  all  future  generations. 
Germ  lay  within  germ,  preformed  in  transparency,  and  in  successively 
smaller  miniature,  after  the  fashion  of  an  infinite  juggler's  box.  We 
laugh  at  this,  but  we  need  not  laugh  too  much,  for  the  preformationists, 
though  wrong  and  crude  in  their  facts,  were  right  in  two  of  their  ideas, 
— that  the  germ  contains  the  potentiality  of  a  future  organism,  and  that 
it  has  relations,  not  only  to  the  animal  into  which  it  develops,  but  also 
to  generations  following.  (See  p.  71.) 

In  the  middle  of  the  seventeenth  century,  however,  Harvey  had 
reached  conclusions  which  might  have  saved  much  blundering.  Study- 
ing the  development  of  the  chick, — as  Greek  naturalists  had  tried  to  do 
wellnigh  two  thousand  years  before,  as  we  are  doing  still  in  our  embryo: 
logical  laboratories, — Harvey  maintained  that  every  animal  was  produced 
from  an  ovum  (OVUM  esse  primordium  commune  omnibus  animalibtts],  and 
that  organs  arose  by  new  formation  (epigenesis),  not  by  the  expansion  or 
"  evolution  "  of  some  invisible  preformation. 

But  the  great  champion  of  epigenesis  was  Caspar  Friedrich  Wolff, 
who,  in  his  doctorial  dissertation  of  1759,  traced  the  chick  back  to  a 
layer  of  organised  particles  (the  familiar  cells  of  to-day),  in  which  there 
was  no  likeness  of  the  future  embryo,  far  less  of  the  adult. 

Wolff  was  long  in  finding  successors,  but  in  1824  Prevost  and  Dumas 
described  the  division  of  the  ovum ;  in  1827  Von  Baer  discovered  the 
mammalian  ovum ;  while  Wagner,  Von  Siebold,  and  others  elucidated 
the  real  nature  of  the  spermatozoon. 

A  great  step  was  made  in  1838-9,  when  Schwann  and  Schleiden 
formulated  the  "cell  theory,"  according  to  which  every  organism  is 
made  up  of  cells,  and  starts  from  a  cell.  From  this  date  modern  em- 
bryology began. 

Sexual  Reproduction. 

There  is  apt  to  be  a  lack  of  clearness  in  regard  to  sexual 
reproduction,  because  the  process  which  we  describe  by 
that  phrase  is  a  complex  result  of  evolution.  It  involves 
two  distinct  facts : — (a)  the  liberation  of  special  germ  cells 
from  which  new  individuals  arise ;  (&}  the  occurrence  of  two 
different  kinds  of  germ  cells — ova  and  spermatozoa,  which 
come  to  nothing  unless  they  unite  (fertilisation).  Further- 
more, these  dimorphic  reproductive  cells  are  produced  by 
two  different  kinds  of  individuals  (females  and  males),  or 
from  different  organs  of  one  individual  or  at  different  times 
within  the  same  organ  (hermaphroditism). 

It  is  conceivable  that  organisms  might  have  gone  on 
multiplying  asexually,  by  detaching  overgrown  portions  of 
themselves  which  had  sufficient  vitality  to  develop  into 
complete  forms.  But  a  more  economical  method  is  the 
liberation  of  special  germ  cells,  in  which  the  qualities  of  the 


THE   LIBERATION  OF  SPECIAL    GERM  CELLS.     51 

organism  are  inherent.     This  is  the  primary  characteristic 
of  sexual,  as  opposed  to  asexual  multiplication. 

It  is  also  conceivable  that  organisms  might  have  remained 
approximately  like  one  another  in  constitution,  and  at  all 
times  very  nearly  the  same,  and  that  they  might  have 
liberated  similar  germ  cells  capable  of  immediate  develop- 
ment. Such  a  race  would  have  illustrated  the  one  charac- 
teristic of  sexual  reproduction,  the  liberation  of  special  germ 
cells,  but  it  would  have  been  without  that  other  characteristic 
of  sexual  reproduction, — the  existence  of  dimorphic  germ 
cells,  of  different  kinds  of  sexual  organs,  or  of  male  and 
female  individuals. 

The  Liberation  of  Special  Germ  Cells. 

One  must  think  of  this  as  an  economical  improvement  on 
the  method  of  starting  a  new  life  by  asexual  overgrowth  or 
by  the  liberation  of  buds.  Asexual  reproduction,  as  Spencer 
and  Haeckel  point  out,  is  a  mode  of  growth  in  which 
the  bud,  or  whatever  it  is,  becomes  discontinuous  from  the 
parent.  The  buds  of  a  sponge,  of  a  coral,  of  a  sea  mat 
(Polyzoon),  or  of  many  Tunicates,  remain  attached  to  the 
parent.  If  there  be  a  keen  struggle  for  subsistence,  this 
may  be  disadvantageous ;  but  in  some  cases,  doubtless,  the 
colonial  life  which  results  is  a  source  of  strength.  In  the 
case  of  Hydra,  however,  the  buds  are  set  adrift ;  the  same 
is  true  of  not  a  few  worms.  This  liberation  of  buds  takes 
us  nearer  the  sexual  process  of  liberating  special  germ  cells. 
But  unless  the  organism  is  in  very  favourable  nutritive 
conditions,  in  which  overgrowth  is  natural,  the  liberation  of 
buds  is  evidently  an  expensive  way  of  continuing  the  life  of 
a  species.  Not  only  so,  but  we  can  hardly  think  of  budding 
even  as  a  possibility  in  very  complex  organisms,  like  snails 
or  birds,  in  which  there  is  much  division  of  labour.  More- 
over, the  peculiarity  of  a  true  germ  cell  is,  that  it  is  un- 
specialised,  continuous  in  quality  with  the  original  germ  cell 
from  which  the  parent  arose,  and  not  very  liable  to  be 
tainted  by  the  mishaps  which  may  befall  the  "  body  "  which 
bears  it.  And,  finally,  in  the  mixture  of  two  units  of  living 
matter  which  have  had  different  histories,  the  possibility  of 
permutations  and  combinations,  in  other  words,  of  variation 


52    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 

is  evidently  supplied  (see  p.  63).  Thus  it  is  not  surprising 
to  find  that  the  asexual  method  of  liberating  buds  has  been 
replaced  in  most  animals  by  the  liberation  of  special  germ 
cells,  by  the  more  economical  and  advantageous  process  of 
sexual  reproduction. 

SUMMARY  OF  MODES  OF  REPRODUCTION. 

A.  In  single  celled  Animals  (Protozoa). 

(1)  The  almost  mechanical  rupture  of  an  amoeboid  cell,  which  has 

become  too  large  for  physiological  equilibrium  (e.g.,  Schizogenes}. 

(2)  The  discharge  of  numerous  superficial  buds  at  once  (e.g.,  Arcella 

and  Pelomyxa]. 

(3)  The  formation  of  one  bud  at  a  time  (very  common). 

(4)  The  ordinary  division   into  two  daughter  cells  at  the  limit   of 

growth. 

(5)  Repeated  divisions  within  limited  time  and  within  limited  space 

(a  cyst).  This  results  in  what  is  called  spore  formation,  "free 
cell  formation,"  "endogenous  multiplication"  (e.g.,  in  Gre- 
garines). 

B.  In  many  celled  Animals  (Metazoa). 

(Asexual.) 

(a)  The  separation  of  a  clump  of  body  cells,  e.g.,  from  the  surface  of 

some  Sponges.     (A  crude  form  of  budding.) 

(b]  The  formation  of  definite  buds  which  may  or  may  not  be  liberated  ; 

and  other  forms  of  asexual  multiplication. 

(Sexual.) 

(a)  The  liberation  of  cells  from  a  simple  Metazoon  in  which  there  is 

so  little  division  of  labour  that  the  distinction  between  body  cells 
and  reproductive  cells  is  not  marked.  (Hypothetical.) 

(b]  The  liberation  of  special  reproductive  or  germ  cells,  which  have 

not  taken  part  in  the  formation  of  the  body,  and  which  retain, 
more  or  less  unaltered,  the  inherent  qualities  of  the  original  germ 
cell  from  which  the  parent  arose.  These  special  reproductive 
cells — the  ova  and  spermatozoa — are  normally  united  in  ferti- 
lisation, but  some  animals  have  (parthenogenetic)  ova  which 
develop  without  being  fertilised. 

The  Evolution  of  Sex. 

A  further  problem  is  to  account  for  the  two  facts  (a)  that 
most  animals  are  either  males  or  females,  the  former  liberat- 
ing actively  motile  male  elements  or  spermatozoa,  the  latter 


THE  EVOLUTION  OF  SEX.  53 

forming  and  usually  liberating  more  passive  egg  cells  or 
ova ;  and  (ft)  that  these  two  different  kinds  of  reproductive 
cells  usually  come  to  nothing  unless  they  combine. 

The  problem  is  partly  solved  by  a  clear  statement  of  the 
facts.  Begin  with  those  interesting  organisms  which  are  on 
the  border  line  between  Protozoa  and  Metazoa,  the  colonial 
Infusorians  of  which  Volvox  is  a  type  (see  p.  95).  The  adults 
are  balls  of  cells,  and  the  component  units  are  connected  by 
protoplasmic  bridges.  From  such  a  ball  of  cells  repro- 
ductive units  are  sometimes  set  adrift,  and  these  divide  to 
form  other  individuals  without  more  ado.  In  other  con- 
ditions, however,  when  nutrition  is  checked,  a  less  direct 
mode  of  reproduction  occurs.  Some  of  the  cells  become 
large  well  fed  elements,  or  ova;  others,  less  successful, 
divide  into  many  minute  units  or  spermatozoa.  The  large 
cells  are  fertilised  by  the  small.  Here  we  see  the  formation 
of  dimorphic  reproductive  cells  in  different  parts  of  the 
same  organism.  But  we  may  also  find  Volvox  balls  in 
which  only  ova  are  being  made,  and  others  with  only  sper- 
matozoa. The  former  seem  to  be  more  vegetative  and 
nutritive  than  the  latter;  we  call  them  female  and  male 
organisms  respectively ;  we  are  at  the  foundation  of  the 
differences  between  the  two  sexes. 

All  through  the  animal  series,  from  active  Infusorians  and 
passive  Gregarines,  to  feverish  Birds  and  more  sluggish 
Reptiles,  we  read  antitheses  between  activity  and  passivity, 
between  lavish  expenditure  of  energy  and  a  habit  of  storing. 
The  ratio  between  disruptive  (katabolic)  processes  and  con- 
structive (anabolic)  processes  in  the  protoplasmic  metabolism 
varies  from  type  to  type.  We  believe  that  the  contrast 
between  the  sexes  is  another  expression  of  this  fundamental 
alternative  of  variation. 

This  theory  may  be  confirmed  in  many  ways,  e.g.,  by 
contrasting  the  characteristic  products  of  female  life, — 
passive  ova,  with  the  characteristic  products  of  male  life, — 
active  spermatozoa ;  or  by  comparing  the  complex  con- 
ditions (such  as  abundant  food,  favourable  temperature) 
which  tend  to  produce  female  offspring,  with  the  opposite 
conditions  which  tend  to  produce  males ;  or  by  contrasting 
the  secondary  sexual  characters  of  males  (e.g.,  bright  colours 
and  smaller  size),  with  the  opposite  characteristics  of  females. 


54  REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 


Stages  in  the  History  of  Fertilisation. 

While  it  is  not  difficult  to  see  the  advantage  of  fertilisation  as  a  pro- 
cess which  helps  to  sustain  the  standard  or  average  of  a  species  and  as 
a  source  of  new  variations,  we  can  at  present  do  little  more  than  indicate 
various  forms  in  which  the  process  occurs. 

(a)  Formation  of  Plasmodia,  the  flowing  together  of  numerous  feeble 

cells,  as  seen  in  the  life  history  of  those  very  simple  Protozoa 
called  Proteomyxa,  e.g.,  Protomyxa,  and  Mycetozoa,  e.g.,  flowers 
of  tan  (ALthalium  septicuui). 

(b)  Multiple  Conjugation,  in  which  more  than  two  cells  unite  and  fuse 

together,  as  in  some  Gregarines  and  in  the  sun  animalcule 
(Actinosphtzrmm). 

(c)  Ordinary  Conjugation,  in  which  two  similar  cells  fuse  together, 

observed  in  Gregarines  and  Rhizopods.  In  ciliated  Infusorians, 
the  conjugation  may  be  merely  a  temporary  union,  during  which 
nuclear  elements  are  interchanged. 

(d)  Dimorphic  Conjugation,  in  which  two  cells  different  from  one 

another  fuse  into  one,  a  process  well  illustrated  in  Vorticella  and 
related  Infusorians,  where  a  small,  active,  free  swimming  (we 
may  say,  male)  cell  unites  with  a  fixed  individual  of  normal  size, 
which  may  fairly  be  called  female  (see  Fig.  23,  p.  94). 

(e)  Fertilisation,  in  which  a  spermatozoon  liberated  from  a  Metazoon 

unites  intimately  with  an  ovum  liberated  from  another  individual 
normally  of  the  same  species. 

Divergent  Modes  of  Sexual  Reproduction. 

(a)  Hermaphroditism  is  the  combination  of  male  and 
female  sexual  functions  in  varying  degrees  within  one 
organism.  It  may  be  demonstrable  in  early  life  only,  and 
disappear  as  maleness  or  femaleness  predominates  in  the 
adult.  It  may  occur  as  a  casualty  or  as  a  reversion ;  or  it 
may  be  normal  in  the  adult,  e.g.,  in  some  Sponges  and 
Ccelentera,  in  many  "  worms,"  e.g.,  earthworm  and  leech,  in 
barnacles  and  acorn  shells,  in  one  species  of  oyster,  in  the 
snail,  and  in  many  other  Bivalves  and  Gastropods,  in  Tuni- 
cates  and  in  the  hagfish.  In  most  cases,  though  these 
animals  are  bisexual,  they  produce  ova  at  one  period  and 
spermatozoa  at  another  (dichogamy).  It  rarely  occurs  (e.g., 
in  some  parasitic  worms)  that  the  ova  of  a  hermaphrodite  are 
fertilised  by  the  spermatozoa  of  the  same  animal.  Certain 
facts,  such  as  the  occurrence  of  hermaphrodite  organs  as  a 
transitory  stage  in  the  development  of  the  embryos  of  many 
unisexual  animals  (e.g.,  frog  and  bird),  make  it  likely  that 
hermaphroditism  is  the  primitive  condition,  and  that  the 


ALTERNATION  OF  GENERATIONS.  55 

unisexual  condition  of  permanent  maleness  or  femaleness  is 
a  secondary  differentiation.  The  cases  which  we  have  cited 
above  may  be  interpreted  as  due  to  persistence  of  the  primi- 
tive condition,  or  as  reversions  to  it. 

(b)  Parthenogenesis,  as  we  know  it,  is  a  degenerate  form 
of  sexual  reproduction,  in  which  ova  produced  by  female 
organisms  develop  without  being  fertilised  by  male  elements. 
It  is  well  illustrated  by  Rotifers,  in  which  fertilisation  is  not 
known  to  occur,  while  in  some  genera  males  have  never 
been  found ;  by  many  small  Crustaceans  whose  males  are 
absent  for  a  season ;  by  aphides,  from  among  which  males 
may  be  absent  for  the  summer  (or  in  artificial  conditions  for 
several  years)  without  affecting  the  rapid  succession  of  female 


FIG.  6. —  Diagrammatic  expression  of  alternation  of 
generations. 

I.  Hydromedusae. 

ov.  Fertilised  ovum  gives  rise  to  asexual  form  A ,  which,  by  budding,, 
produces  sexual  form  or  forms  6" ;  in  case  of  Hydromedusae  A  is  re- 
presented by  hydroid  (//)  and  .9  by  medusoid  (M). 

II.  Liver  Fluke. 

ov.  Fertilised  ovum  gives  rise  to  asexual  stages  (A)  which,  from 
special  spore-like  cells  (Tv),  produce  eventually  the  sexual  fluke  (S). 

generations ;  by  the  production  of  drones  in  the  bee  hive, — 
for  the  eggs  which  give  rise  to  drones  are  unfertilised  (see 
p.  60). 

(c)  Alternation  of  Generations. — A  fixed  asexual  hydroid 
or  zoophyte  (campanularian  or  tubularian)  often  buds  off 
and  liberates  sexual  medusoids  or  swimming  bells,  whose 
fertilised  ova  develop  into  embryos  which  become  fixed  and 
grow  into  hydroids  (Fig.  49,  p.  156).  This  is  the  simplest 
illustration  of  alternation  of  generations,  which  may  be 


56    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 

defined  as  the  alternate  occurrence  in  one  life  cycle  of  two 
(or  more)  different  forms  differently  produced. 

The  liver  fluke  (Distoma  hepaticum}  of  the  sheep  produces 
eggs  which  when  fertilised  grow  into  embryos.  Within  the 
latter,  certain  cells  (which  can  hardly  be  called  eggs)  grow 
into  numerous  other  larvae  of  a  different  form.  Within  these 
the  same  process  is  repeated,  and  finally  the  larvae  thus 
produced  grow  (in  certain  conditions)  into  sexual  flukes 
(Fig.  54,  p.  1 68).  In  this  case,  reproduction  by  special  cells 
like  undifferentiated  precocious  ova,  alternates  with  reproduc- 
tion by  ordinary  fertilised  egg  cells.  So,  too,  the  vegetative 
sexless  "  fern  plant "  gives  rise  to  special  spore  cells,  which 
develop  into  an  inconspicuous  bisexual  "  prothallus,"  from 
the  fertilised  egg  cell  of  which  a  "fern  plant"  springs. 

Various  kinds  of  alternation  are  seen  in  the  life  cycle  of 
the  fresh  water  sponge,  in  the  stages  of  the  jelly  fish  Aurelia, 
in  the  history  of  some  "  worms  "  and  Tunicates.  They 
illustrate  a  rhythm  between  asexual  and  sexual  multiplica- 
tion, between  parthenogenetic  and  normally  sexual  reproduc- 
tion, between  vegetative  and  animal  life,  between  a  relatively 
"  anabolic  "  and  a  relatively  "  katabolic  "  preponderance. 

II.  EMBRYOLOGY. 

The  Egg  Cell  or  Ovum. — Apart  from  cases  of  asexual  re- 
production and  parthenogenesis  every  multicellular  organism 
begins  life  as  an  egg  cell  with  which  a  male  cell  or  sperma- 
tozoon has  entered  into  intimate  union. 

The  most  important  characteristic  of  the  reproductive 
cells,  whether  male  or  female,  is  that  they  retain  the  essen- 
tial qualities  of  the  fertilised  ovum  from  which  the  parent 
animal  was  developed. 

The  ovum  has  the  usual  characters  of  a  cell;  its  sub- 
stance is  traversed  by  a  fine  protoplasmic  network  ;  its 
nucleus  or  germinal  vesicle  contains  the  usual  chromatin 
elements ;  it  has  often  a  distinct  sheath  representing  a  cell 
wall. 

In  Sponges,  the  ova  are  well  nourished  cells  in  the  middle 
stratum  of  the  body ;  in  Coelentera  they  seem  to  arise  in 
connection  with  either  outer  or  inner  layer  (ectoderm  or 
endoderm) ;  in  all  other  animals,  they  arise  in  connection 


THE  EGG   CELL    OR   OVUM.  57 

with  the  middle  layer  or  mesoderm,  usually  on  an  area  of 
the  epithelium  lining  the  body  cavity.  In  lower  animals  they 
often  arise  somewhat  diffusely ;  in  higher  animals  their  for- 
mation is  restricted  to  distinct  regions,  and  usually  to  definite 
organs — the  ovaries. 

The  young  ovum  is  often  amoeboid,  and  that  of  Hydra 
retains  this  character  for  some  time  (Fig.  41,  p.  139).  The 
ovum  grows  at  the  expense  of  adjacent  cells,  or  by  absorbing 
material  which  is  contributed  by  special  yolk  glands  or  sup- 
plied by  the  vascular  fluid  of  the  body. 

The  yolk  or  nutritive  capital  may  be  small  in  amount, 


FIG.  7. — Diagram  of  ovum,  showing  diffuse  yolk  granules. 
g.v.  Germinal  vesicle  or  nucleus  ;  chr.  chromatin  elements. 

and  distributed  uniformly  in  the  cell  as  in  the  ova  of 
Mammals,  earthworm,  starfish,  and  sponge ;  or  it  may  be 
more  abundant,  sinking  towards  one  pole  as  in  the  egg  of 
the  frog,  or  accumulated  in  the  centre  as  in  the  eggs  of 
Insects  and  Crustaceans ;  or  it  may  be  very  copious,  dwarf- 
ing the  formative  protoplasm,  as  in  the  eggs  of  Birds,  Rep- 
tiles, and  most  Fishes. 

Round  the  egg  there  are  often  sheaths  or  envelopes  of 
various  kinds,  (a)  made  by  the  ovum  itself,  and  then  very 
delicate  (e.g.,  the  vitelline  membrane) ;  (b)  formed  by  adja- 
cent cells  (e.g.,  the  follicular  envelope)  :  or  (c)  formed  by 


58     REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 

special  glands  or  glandular  cells  in  the  walls  of  the  oviducts 
(e.g.,  the  "shells"  of  many  eggs).  The  envelope  is  often 
firm,  as  in  the  chitinous  coat  around  the  eggs  of  many 
Insects,  and  in  these  cases  there  is  often  a  little  aperture 
(micropyle)  through  which  alone  the  spermatozoon  can  enter. 
The  hard  calcareous  shells  round  the  eggs  of  Birds  and 
Tortoises,  or  the  mermaid's  purse  enclosing  the  egg  of  a  skate 
are  of  course  formed  after  fertilisation.  Egg  shells  must  be 
distinguished  from  egg  capsules  or  cocoons,  e.g.,  of  the 
earthworm,  in  which  several  eggs  are  wrapped  up  together. 

The  Male  Cell  or  Spermatozoon  is  a  much  smaller  and 
usually  a  much   more  active  cell  than   the  ovum.     In  its 


FIG.  8. — Forms  of  Spermatozoa  (not  drawn  to  scale). 

i  and  2.  Immature  and  mature  spermatozoa  of  snail ;  3.  of  bird  ; 
4.  of  man  (,&,  head;  /;z,  middle  portion  ;  t,  tail);  5.  of  salamander, 
with  vibratile  fringe  (_/") ;  6.  of  Ascaris,  slightly  amoeboid  with  cap 
(c);  7.*  of  crayfish. 

minute  size,  locomotor  energy,  and  persistent  vitality,  it 
resembles  a  flagellate  monad,  while  the  ovum  is  comparable 
to  an  amoeba  or  to  one  of  the  more  encysted  Protozoa. 

A  spermatozoon  has  usually  three  distinct  parts :  the 
essential  "  head,"  consisting  mainly  of  nucleus,  and  the 
mobile  "  tail "  which  is  often  fibrillated,  and  a  small  middle 
portion  between  head  and  tail,  which  is  regarded  by  some  as 
the  centrosome.  The  spermatozoa  of  Threadworms  and 
Crustaceans  are  sluggish,  and  inclined  to  be  amoeboid 
(Fig.  8  (6,  7) ). 

Both  ova  and  spermatozoa  are  true  cells,  and  they  are 


MATURATION  OF  THE   OVUM. 


59 


complementary,  but  the  spermatozoon  has  a  longer  history 
behind  it.  The  homologue  of  the  ovum  is  the  mother 
sperm  cell  or  spermatogonium.  This  segments  much  as  the 
ovum  does,  but  the  cells  into  which  it  divides  have  little 
coherence.  They  go  apart,  and  become  spermatozoa.  There 
is  a  striking  resemblance  between  the  different  ways  in  which 
a  mother  sperm  cell  divides  and  the  various  kinds  of  segmen- 
tation in  ova.  In  most  cases  the  spermatogonium  divides  into 
spermatocytes,  which  usually  divide  again  into  spermatides 
or  young  spermatozoa. 

Maturation  of  the  Ovum. — When  the  egg  cell  attains  its 
definite  size  or  limit  of  growth,  it  bursts  from  the  ovary  or 


C  D 


FIG.  9. — Diagram  of  maturation  and  fertilisation. 
(From  "  Evolution  of  Sex.") 

A.  Primitive  sex. cell,  supposed  to  be  amoeboid. 

B.  Ovum  ;  C.  formation  of  first  polar  body  (i. /.£.);  D.  formation 
of  second  polar  body  (2.  /£.). 

Bi.  Mother  sperm  cell;  Ci.  the  same  divided  (sperm-morula  or 
polyplast,  or  spermatogonium). 

Di.  Ball  of  immature  spermatozoa  or  spermatides  ;  sp.  liberated 
spermatozoa. 

E.  Process  of  fertilisation  ;  F.  approach  of  male  and  female  nuclei 
within  the  ovum. 

from  its  place  of  formation,  and  in  favourable  conditions 
meets  either  within  or  outside  the  body  with  a  spermatozoon 
from  another  animal.  Before  this  union  between  ovum  and 
spermatozoon  is  effected,  generally  indeed  before  it  has 
begun,  the  nucleus  or  germinal  vesicle  of  the  ovum  moves 
to  the  periphery  and  divides  twice.  This  division  results  in 
the  formation  and  extrusion  of  two  minute  cells  or  polar 


60    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 

bodies,  the  first  containing  half,  the  second  necessarily  a 
quarter  of  the  nuclear  material  which  composed  the  germinal 
vesicle.  The  nucleus  is  thus  reduced  to  a  quarter  of  its 
original  chromatin  content.  It  is  noteworthy  that  the 
second  division  follows  close  on  the  first  without  the  inter- 
vention of  the  "resting  stage,"  which  usually  succeeds  a 
nuclear  division.  Moreover,  there  is  this  important  differ- 
ence between  the  formation  of  polar  bodies  and  ordinary 
cell  division,  that  the  number  of  nuclear  rods  or  chromosomes 
suffers  reduction,  whereas  in  ordinary  karyokinesis  the 
daughter  nuclei  have  as  many  nuclear  rods  as  the  original 
cell.  The  extruded  polar  bodies  come  to  nothing,  though 
they  may  linger  for  a  time  in  the  precincts  of  the  ovum,  and 
may  even  divide.  The  extrusion  of  polar  globules  from 
mature  ova  seems  to  be  almost  universal;  but  observations 
are  lacking  in  regard  to  Birds  and  Reptiles.  Moreover, 
Weismann  and  Ischikawa  have  shown  that  in  all  partheno- 
genetic  ova  which  they  have  examined,  only  one  polar  body 
is  formed.  It  is  said,  however,  that  in  the  parthenogenetic 
eggs  which  become  drones  (Blochmann),  and  in  those  of  a 
moth  called  Liparis  (Platner),  two  polar  bodies  are  formed. 
But  in  neither  of  these  two  exceptional  cases  is  the  partheno- 
genesis habitual ;  thus  many  of  the  eggs  which  the  queen 
bee  lays  are  fertilised,  and  give  rise  to  queens  and  workers. 

One  of  the  most  important  results  of  recent  investigations  as  to  polar 
bodies  is  due  to  O.  Hertvvig  and  others.  It  may  be  briefly  stated,  with 
particular  reference  to  the  ova  of  Ascaris  megalocephala — the  thread- 
worm of  the  horse.  In  one  variety  of  this  worm  (var.  bivalens]  the 
germinal  vesicle  of  the  ovum  contains  four  nuclear  rods,  chromosomes, 
or  idants.  By  doubling  these  increase  to  eight  (Fig.  10,  B) ;  the  first  polar 
body  goes  off  with  four  (Fig.  10,  C),  and  the  second  with  two  (Fig.  10,  D) ; 
leaving  two.  Two  "reducing  divisions"  have  thus  occurred. 
Similarly,  the  homologue  of  the  ovum,  the  sperm  mother  cell  contains 
four  chromosomes  in  its  nucleus  (Fig.  10,  A1).  By  doubling  these 
increase  to  eight  (Fig.  10,  B1),  and  by  division  the  cell  forms  four 
spermatozoa,  each  with  two.  When  fertilisation  takes  place,  the  nucleus 
of  the  spermatozoon,  with  two  chromosomes,  unites  with  the  reduced 
nucleus  of  the  ovum,  also  with  two  chromosomes  ;  and  the  number  is 
thus  raised  to  four,  which  is  the  normal  number  in  the  cells  of  this 
variety  of  Ascaris  megalocephala.  There  is  thus  a  striking  parallelism  in 
the  history  of  the  two  nuclei  which  unite  in  fertilisation  ;  both  have  been 
subjected  to  reducing  divisions.  If  this  did  not  occur,  each  fertilisation 
would  involve  a  doubling  of  the  number  of  chromosomes.  Weismann 
interprets  the  whole  process  as  an  arrangement  by  which  the  corn- 


PER  TILISA  TION.  6 1 

binations  and  permutations  of  nuclear  rods  and  their  vital  qualities  are 
increased  so  as  to  give  rise  to  new  variations. 

There  are,  indeed,  other  interpretations,  and  the  facts  are  difficult 
to  understand  on  any  theory.  Thus  Minot,  Balfour,  Van  Beneden,  and 
others  have  suggested  that  the  polar  bodies  are  extrusions  of  male 
substance  from  the  ovum.  Biitschli,  Giard,  and  others  interpret  the 
premature  division  of  the  ovum  as  the  survival  of  an  ancient  habit,  and 
regard  the  polar  bodies  as  rudimentary  or  abortive  ova. 

It  may  be  possible  to  combine  various  interpretations  :  (l)  the  ovum 
divides,  like  any  other  cell,  like  the  Protozoon  ancestors,  at  its  limit  of 
growth  ;  (2)  the  extrusion  does  in  some  way  differentiate  the  ovum  and 
renders  fertilisation  possible  or  more  profitable ;  (3)  the  peculiar  reduction 
involved  in  the  process  makes  the  origin  of  new  variations  more  certain. 

Fertilisation. — In  the  seventeenth  and  eighteenth  cen- 
turies, some  naturalists,  nicknamed  "  ovists,"  believed  that 


A1  B' 


FIG.  10. — Spermatogenesis  and  Polar  bodies.     (After 
HERTWIG  and  WEISMANN.) 

Ai.  Primitive  germ  cell  of  A  scan's  megalocephala  var.  bivalens 
(4  chromosomes). 

Bi.  Sperm  mother  cell  (8  chromosomes). 

Ci.  Two  spermatocytes  formed,  each  with  4  chromosomes  (first 
reducing  division). 

Di.  Four  spermatozoa  formed,  each  with  2  chromosomes  (second 
reducing  division). 

A.  Primitive  germ  cell  (4  chromosomes). 

B.  Fully  developed  ovum  (8  chromosomes). 

C.  Formation  of  first  polar  body  (/3.i)  (first  reducing  division). 

D.  Formation   of   second    polar   body  (/£.2)    (second    reducing 
division).     First  polar  body  may  divide  into  two. 

the  ovum  was  all-important,  only  needing  the  sperm's 
awakening  touch  to  begin  unfolding  the  miniature  model 
which  it  contained.  Others,  nicknamed  u  animalculists," 
were  equally  confident  that  the  sperm  was  essential,  though 
it  required  to  be  fed  by  the  ovum.  Even  after  it  was 


62    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 

recognised  that  both  kinds  of  reproductive  elements  were 
essential,  many  thought  that  their  actual  contact  was  un- 
necessary, that  fertilisation  might  be  affected  by  an  aura 
seminalis.  Though  spermatozoa  were  distinctly  seen  by 
Hamm  and  Leeuwenhoek  in  1677,  their  actual  union  with 
ova  was  not  observed  till  1843,  when  Martin  Barry  detected 
it  in  the  rabbit. 

Of  the  many  facts  which  we  now  know  about  fertilisation, 
the  following  are  the  most  important  : — 

(i.)  Apart  from  the  occurrence  of  parthenogenesis  in  a 


FIG.   n. — Fertilisation  in  Ascaris  megalocephala. 
(After  BOVERI.) 

1.  Spermatozoon   (s£.)   entering   ovum,  which   contains   reduced 
nucleus  (A^),  having  given  off  two  polar  bodies  (/.£.  i  and  2). 

2.  Sperm  nucleus  (n),  and   ovum   nucleus  (Ar),  each   with   two 
chromatin  elements  or  idants,  with  centrosomes  (c.s.). 

3.  Centrosomes  (c.s.)  with  "  archoplasmic  "  threads  radiating  out- 
wards, in  part  to  the  chromosomes  of  the  two  approximated  nuclei. 

4.  Segmentation  spindle  before  first  cleavage. 

few  of  the  lower  animals,  an  ovum  begins  to  divide  only 
after  a  spermatozoon  has  united  with  it.  After  one  sper- 
matozoon has  entered  the  ovum,  the  latter  ceases  to  be 
receptive,  and  other  spermatozoa  are  excluded.  If,  as  rarely 
happens,  several  spermatozoa  effect  an  entrance  into  the 
ovum,  the  result  is  usually  pathological.  It  is  said,  however, 


SEGMENTATION.  63 

that  the  entrance  of  numerous  spermatozoa  (polyspermy)  is 
frequent  in  insects  and  Elasmobranch  fishes. 

(2.)  The  union  of  spermatozoon  and  ovum  is  very  intimate; 
the  nucleus  of  the  spermatozoon  and  the  reduced  nucleus  of 
the  ovum  approach  one  another,  combining  to  form  a  single 
nucleus. 

(3.)  When  this  combined  or  segmentation  nucleus  begins 
the  process  of  development  by  dividing,  each  of  the  two 
daughter  nuclei  which  result  consists  partly  of  material 
derived  from  the  sperm  nucleus,  partly  of  material  derived 
from  the  ovum  nucleus.  In  other  words,  the  union  is 
orderly  as  well  as  intimate,  and  the  subsequent  division  is 
so  exact,  that  the  qualities  so  marvellously  inherent  in  the 
sperm  nucleus  (those  of  the  male  parent),  and  in  the  ovum 
nucleus  (those  of  the  mother  animal),  are  diffused  through- 
out the  body  of  the  offspring,  and  persist  in  its  reproductive 
cells. 

As  to  the  interpretation  of  these  facts,  Weismann  maintains  the 
importance  of  the  quantitative  addition  which  the  sperm  nucleus  makes 
to  the  diminished  nucleus  of  the  ovum.  At  the  same  time,  he  finds 
an  important  source  of  transmissible  variations  in  the  mingling  of 
the  two  nuclear  substances  (amphimixis).  Others  believe  that  the 
mingling  diminishes  the  risk  of  unfavourable  idiosyncrasies  being  trans- 
mitted from  parents  to  offspring.  Others  emphasise  the  idea  that  the 
sperm  supplies  a  vital  stimulus  to  the  ovum,  and  this  seems  to  be 
corroborated  by  the  fact  well  known  to  breeders  that  impregnation  by 
a  male  with  certain  marked  characteristics  influences  the  constitution  of 
the  female,  and  may  have  an  effect  on  the  progeny  of  subsequent  years 
and  by  different  males  ("telegony  "). 

Segmentation. — The  different  modes  of  division  exhibited 
by  fertilised  egg  cells  depend  in  great  measure  on  the 
quantity  and  disposition  of  the  passive  and  nutritive  yolk 
material,  which  is  often  called  deutoplasm  in  contrast  to 
the  active  and  formative  protoplasm.  The  pole  of  the  ovum 
at  which  the  formative  protoplasm  lies,  and  at  which  the 
spermatozoon  enters,  is  often  called  the  animal  pole  ;  the 
other,  towards  which  the  heavier  yolk  tends  to  sink,  is  called 
the  vegetative  pole. 

In  contrasting  the  chief  modes  of  segmentation,  it  should 
be  recognised  that  they  are  all  connected  by  gradations. 


64    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 


FIG.  12. — Modes  of  segmentation. 

1.  Ovum  with   little  yolk   segments   totally   and  equally  into  a 
blastosphere,  e.g.,  Hydra. 

2.  Ovum  with  considerable  yolk  (y)  at  lower  pole  segments  wholly 
but  unequally,  e.g.,  frog  ;  (y.s.)  larger  yolk,  laden  cells. 

3.  Ovum  with  much  yolk  segments  partially  and   discordally, 
forming  blastoderm  (bl.),  e.g.,  bird. 

4.  Ovum  with  central  yolk  (y)  segments  partially  and  peripherally, 
e.g.,  crayfish. 


BLASTOSPHERE  AND   MORULA.  65 


A.  COMPLETE  DIVISION — Holoblastic  Segmentation. 

I.  Eggs  with  little  and  diffuse  yolk  material  divide  completely  into 

approximately  equal  cells, 

[or,  Ova  which  are  alecithal  (i.e.,  without  yolk)  undergo  approxi- 
mately equal  holoblastic  segmentation]. 

This  is  illustrated  in  most  Sponges,  most  Ccelentera,  some 
"worms,"  most  Echinoderms,  some  Molluscs,  all  Tuni- 
cates,  Amphioxus.  and  most  Mammals. 

II.  Eggs  with  a  little  yolk  material  accumulated  towards  one  pole, 

divide  completely,  but  into  unequal  cells, 

[or,  Ova  without  very  abundant  deutoplasm,  but  with  what  they 
have  lying  towards  one  pole  (telolecithal),  undergo  unequal 
holoblastic  segmentation]. 

This  is  illustrated  in  some  Sponges,  some  Ccelentera  (e.g., 
Ctenophora),  some  "  worms,"  many  Molluscs,  the 
lamprey,  Ganoid  Fishes,  Ceratodus,  Amphibians. 

B.  PARTIAL  DIVISION— Meroblastic  Segmentation. 

III.  Eggs  with  a  large  quantity  of  yolk,   on   which   the  formative 

protoplasm  lies  as  a  small  disc  at   one  pole,   divide  partially, 
and  in  discoidal  fashion, 

[or,   Ova  which  are  telolecithal,   and    have   a  large   quantity  of 
deutoplasm,  undergo  meroblastic  and  discoidal  segmentation]. 
This  is  illustrated  in  all  Cuttle  fishes,  all  Elasmobranch  and 
Teleostean  fishes,  all  Reptiles  and  Birds,  and  also  in  the 
Monotremes  or  lowest  Mammals. 

IV.  Eggs  with  a  considerable  quantity  of  yolk,   accumulated  in  a 

central   core,    and   surrounded    by   the   formative    protoplasm, 
divide  partially,  and  superficially  or  peripherally, 
[or,  Ova  which  are  centrolecithal  undergo  meroblastic  and  super- 
ficial segmentation]. 

This  is  illustrated  by  almost  all  Arthropods,  and  by  them 
alone. 

Summarising  the  above,  we  have  : — 

C          I      T^V.,,,-,1 

A.  Complete  Division.  jr* 

B.  Partial  Division. 


/   III.  Discoidal. 
\     IV.   Peripheral. 


Blastosphere  and  Morula. — The  result  of  the  division  is 
usually  a  ball  of  cells.  But  when  the  yolk  is  very  abundant 
(III.)  a  disc  of  cells — a  discoidal  blastoderm — is  formed  at 
one  pole  of  the  mass  of  nutritive  material  which  it  gradually 
surrounds. 

As  the  cells  divide  and  redivide,  they  often  leave  a  large 
5 


66    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 

central  cavity — the  segmentation  cavity — and  a  hollow  ball 
of  cells — a  blastosphere  or  blastula — results. 


FIG.  13. — Life  history  of  a  coral,  Monoxenia  Darivinii. 
(From  H/ECKKL.) 

A,  B,  Ovum.  C,  Division  into  two.  D,  Four  cell  stage.  E, 
Blastula.  F,  Free  swimming  blastula  with  cilia.  G,  Section  of 
blastula.  H,  Beginning  of  invagination.  I,  Section  of  completed 
gastrula  showing  ectoderm,  endoderm,  and  archenteron.  K,  Free 
swimming  ciliated  gastrula. 


MESODERM.  67 

But  if  the  so-called  "  segmentation  cavity  "  be  very  small 
or  absent,  a  solid  ball  of  cells  or  morula,  like  the  fruit  of 
bramble  or  mulberry,  results. 

Gastrula. — The  next  great  step  in  development  is  the 
establishment  of  the  two  primary  germinal  layers,  the  outer 
ectoderm  and  the  inner  endoderm.  or  the  epiblast  and  the 
hypoblast. 

One  hemisphere  of  the  hollow  ball  of  cells  may  be  appar- 
ently dimpled  into  the  other,  as  we  might  dimple  an  india- 
rubber  ball  which  had  a  hole  in  it.  Thus,  out  of  a  hollow 
ball  of  cells,  a  two  layered  sac  is  formed — a  gastrula  formed 
by  invagination  or  embole.  The  mouth  of  the  gastrula  is 
called  the  blastopore,  its  cavity  the  archenteron. 

But  where  the  ball  of  cells  is  practically  a  solid  morula, 
the  apparent  in-dimpling  cannot  occur  in  the  fashion 
described  above.  Yet  in  these  cases  the  two  layered  gastrula 
is  still  formed.  The  smaller,  less  yolk  laden  cells,  towards 
the  animal  pole,  gradually  grow  round  the  larger  yolk 
containing  cells,  and  a  gastrula  is  formed  by  overgrowth  or 
epibole. 

In  the  course  of  our  studies,  we  shall  have  opportunity  to 
discuss  various  forms  of  gastrulation,  and  some  other  pro- 
cesses by  which  two  layers  are  established,  such  as  that 
called  delaminatien. 

Mesoderm. — We  are  not  yet  able  to  make  general  state- 
ments of  much  value  in  regard  to  the  origin  of  the  middle 
germinal  layer — the  mesoderm  or  mesoblast.  In  Sponges 
and  Ccelentera  it  is  less  distinct  than  in  higher  forms, 
and  is  usually  represented  by  a  gelatinous  material  (meso- 
gloea)  which  appears  -between  ectoderm  and  endoderm,  and 
into  which  cells  wander  from  these  two  layers.  In  the 
other  Metazoa,  the  middle  layer  may  arise  from  a  few 
primary  mesoblasts  or  cells  which  appear  at  an  early  stage 
between  the  ectoderm  and  endoderm  (e.g.,  in  the  earth- 
worm's development) ;  or  from  numerous  "  mesenchyme  " 
immigrant  cells,  which  are  separated  from  the  walls  of  the 
blastula  or  gastrula  (e.g.,  in  the  development  of  Echino- 
derms) ;  or  as  ccelome  pouches — outgrowths  from  the  endo- 
dermic  lining  of  the  gastrula  cavity  (e.g.,  in  Sagitta,  Balano- 
glossus,  Amphioxus)  •  or  by  combinations  of  these  and  other 
modes  of  origin.  The  mesoderm  lies  or  comes  to  lie  be- 


68    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 

tween  ectoderm  and  endoderm,  and  it  lines  the  body 
cavity,  one  layer  of  mesoderm  (parietal  or  somatic) 
clinging  to  the  ectodermic  external  wall,  the  other  (visceral 
or  splanchnic)  cleaving  to  the  endodermic  gut  and  its 
outgrowths. 

Origin  of  Organs. — From  the  outer  ectoderm  and  inner 
endoderm,  those  organs  arise  which  are  consonant  with  the 
position  of  these  two  layers,  thus  nervous  system  from  the 
ectoderm,  digestive  gut  from  the  endoderm.  The  middle 
layer,  which  begins  to  be  developed  in  "  worms,"  assumes 
some  of  the  functions,  e.g.,  contractility,  which  in  Sponges 
and  Ccelentera  are  possessed  by  ectoderm  and  endoderm, 
the  only  two  layers  distinctly  represented  in  these 
classes. 

In  a  backboned  animal  the  embryological  origin  of  the 
organs  is  as  follows  : — 

(a)  From  the  Ectoderm  or  Epiblast  arise  the  epidermis 
and  epidermic  outgrowths,  the  nervous  system,  the 
most  essential  parts  of  the  sense  organs,  infoldings 
at  either  end  of  the  gut  (fore  gut  or  stomatodasum 
and  hind  gut  or  proctodaeum),  and  perhaps  the 
segmental  or  primary  excretory  duct. 
(U)  From  the  Endoderm  or  Hypoblast  arise  the  mid  gut 
(mesenteron)  and  the  foundations  of  its  outgrowths 
(e.g.,  the  lungs,  liver,  allantois,  &c.,  of  higher  Verte- 
brates), also  the  axial  rod  or  notochord.  According 
to  some  authorities,  the  blood  and  the  vascular 
system  of  Vertebrates  is  in  the  main  endodermic  in 
origin. 

(c)  From  the  Mesoderm  or  Mesoblast  arise  all  other  struc- 
tures, e.g.,  dermis,  muscles,  connective  tissue,  bony 
skeleton,  the  lining  of  the  body  cavity,  and  perhaps 
the  vascular  system.  This  layer  aids  in  the  forma- 
tion of  organs  originated  by  the  other  two.  With  it 
the  reproductive  organs  are  associated. 

Physiological  Embryology. — Of  the  physiological  conditions  of  develop- 
ment, we  know  relatively  little.  To  investigate  them,  is  one  of  the 
tasks  of  the  future.  Why  does  an  egg  cell  form  polar  bodies,  how  is 
the  sperm  attracted  to  the  ovum,  why  does  the  fertilised  egg  cell  divide, 
how  does  the  yolk  affect  segmentation,  what  are  the  conditions  of  the 
infolding  which  forms  the  endoderm,  and  of  the  outfolding  which  makes 
the  ccelome  pouches,  and  what  do  the  numerous  larval  stages  mean  ? 


GENERALISATIONS.  69 

Generalisations — (i)  The  Ovum  Theory  or  Cell  Theory. — 
All  many  celled  animals,  produced  by  sexual  reproduction, 
begin  at'  the  beginning  again.  "The  Metazoa  begin  where 
the  Protozoa  leave  off" — as  single  cells.  Fertilisation  does 
not  make  the  egg  cell  double  ;  there  is  only  a  more  com- 
plex and  more  vital  nucleus  than  before.  All  development 
takes  place  by  the  division  of  this  fertilised  egg  cell  and  its 
descendant  cells. 

(2)  The  Gastrcea  Theory. — As  a  two  layered  gastrula  stage 
occurs,  though  sometimes  disguised  by  the  presence  of  much 
yolk,  in  the  development  of  the  majority  of  animals,  H?eckel 
concluded  that  it  represents  the  individual's  recapitulation 
of  an  ancestral  stage.  He  believes  that  the  simplest  stable, 


pi. 


FIG.  14. — Embryos  (i)  of  bird  ;  (2)  of  man.     (After  His.) 
The  latter  about  twenty-seven  days  old. 

y.s.  Yolk  sac  ;  //.  placenta. 

many  celled  animal,  was  like  a  gastrula,  and  this  hypo- 
thetical ancestor  of  all  Metazoa  he  calls  a  gastr&a.  The 
gastrula  is,  on  this  view,  the  individual  animal's  recapitula- 
tion of  the  ancestral  gastrasa.  Rival  suggestions  have  been 
made  :  perhaps  the  original  Metazoa  were  balls  of  cells  like 
Volvox,  with  a  central  cavity  in  which  reproductive  cells 
lay  ;  perhaps  they  were  like  the  planula  larvae  of  some 
Ccelenterates — two  layered,  externally  ciliated,  oval  forms 
without  a  mouth. 

(3)   The  Fact  of  Recapitulation.— It  is  a  matter  of  experi- 
ence that  we  recapitulate  in  some  measure  the  history  of 


70    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 


our  ancestors.  Embryologists  have  made  this  fact  most 
vivid,  by  showing  that  the  individual  animal  develops  along 
a  path  the  stations  of  which  correspond  to  some  extent  with 
the  steps  of  ancestral  history. 


(1)  The  simplest  animals  are  single 

cells  (Protozoa). 

(2)  The  next  simplest  are  balls  of 

cells  (e.g.)   Volvox}. 

(3)  The   next   simplest   are    two- 

layered   sacs  of  cells   (e.g., 
Hydra}. 


(1)  The  first  stage  of  development 

is    a    single    cell    (fertilised 
ovum). 

(2)  The   next    is   a   ball    of    cells 

(blastula  or  morula). 

(3)  The  next  is  a  two  layered  sac 

of  cells  (gastrula). 


Von  Baer,  one  of  the  pioneer  embryologists,  acknow- 
ledged that  with  several  very  young  embryos  of  higher 
Vertebrates  before  him,  he  could  not  tell  one  from  the 
other.  Progress  in  development,  he  said,  was  from  a  general, 
to  a  special  type.  In  its  earliest  stage,  every  organism  has 
a  great  number  of  characters  in  common  with  other 
organisms  in  their  earliest  stages ;  at  each  successive  stage 
the  series  of  embryos  which  it  resembles  is  narrowed.  The 
rabbit  begins  like  a  Protozoon  as  a  single  cell,  after  a  while 
it  may  be  compared  to  the  young  stage  of  a  very  simple 
vertebrate,  afterwards  to  the  young  stage  of  a  reptile,  after- 
wards to  the  young  stage  of  almost  any  mammal,  afterwards 
to  the  young  stage  of  almost  any  rodent,  eventually  it 
becomes  unmistakably  a  young  rabbit, 

Herbert  Spencer  expressed  the  same  idea,  by  saying  that 
the  progress  of  development  was  from  homogeneous  to 
heterogeneous,  through  steps  in  which  the  individual  history 
was  parallel  to  that  of  the  race.  But  Haeckel  has  illustrated 
the  idea  more  vividly,  and  summed  it  up  more  tersely  than 
any  other  naturalist.  His  "fundamental  biogenetic  law" 
reads,  "  Ontogeny,  or  the  development  of  the  individual,  is  a 
shortened  recapitulation  of  phylogeny,  or  the  evolution  of 
the  race." 

It  is  hardly  necessary  to  say  that  the  young  mammal  is 
never  like  a  worm,  or  a  fish,  or  a  reptile.  It  is  at  most  like 
the  embryonic  stages  of  these,  and  it  may  also  be  noticed 
that  as  our  knowledge  is  becoming  more  intimate,  the 
individual  peculiarities  of  different  embryos  are  becoming 
more  evident.  Thus  Professor  Sedgwick  has  recently  said 
that  a  blind  man  could  distinguish  the  early  stages  of 


HEREDITY  71 

Elasmobranch  and  Bird  embryos.     But  this  need  not  lead 
us  to  deny  the  general  resemblance. 

Moreover,  the  individual  life  history  is  much  shortened 
compared  with  that  of  the  race.  Not  merely  does  the  one 
take  place  in  days,  while  the  other  has  progressed  through 
ages,  but  stages  are  often  skipped,  and  short  cuts  are  dis- 
covered. And  again,  many  young  animals,  especially  those 
"larvae"  which  are  very  unlike  their  parents,  often  exhibit 
characters  which  are  secondary  adaptations  to  modes  of  life 
of  which  their  ancestors  had  probably  no  experience.  In 
short,  the  individual's  recapitulation  of  racial  history  is  gen- 
eral, but  not  precise. 

But  we  do  not  understand  how  the  recapitulation  is  sustained.  Has 
the  protoplasm  of  the  embryo  some  unconscious  memory  of  the  past  ? 
Have  the  protoplasmic  molecules,  as  Hceckel  puts  it,  learned  long  since 
some  rhythmic  dance  which  they  cannot  forget  ?  And,  to  what  extent 
must  there  be  similarity  of  external  conditions  if  the  recapitulation,  "the 
perigenesis  of  the  plastidules,"  is  to  be  sustained?  For  a  careful  state- 
ment of  the  problem,  the  student  would  do  well  to  read  the  late  Pro- 
fessor Milnes  Marshall's  British  Association  address  on  RECAPITULA- 
TION, now  published  in  his  collected  papers. 

(4)  Organic  Continuity  between  Generations. — Heredity. 
—Every  one  knows  that  like  tends  to  beget  like,  that  off- 
spring resemble  their  parents,  and  sometimes  their  ancestors 
(atavism).  Not  only  are  the  general  characteristics  trans- 
mitted, but  minute  features,  idiosyncrasies,  pathological 
conditions,  innate  or  congenital  in  the  parents,  may  be 
transmitted  to  the  offspring. 

Many  attempts  have  been  made  to  explain  this,  but  the 
first  suggestion  with  any  scientific  pretensions  was  that  the 
reproductive  cells,  which  may  become  offspring,  consist  of 
samples  accumulated  from  the  different  parts  of  the  body. 

This  was  a  very  old  idea,  but  Herbert  Spencer  and 
Charles  Darwin  gave  it  new  life.  According  to  Darwin's 
"provisional  hypothesis  of  pangenesis,"  the  reproductive 
cells  accumulate  gernmules  liberated  from  all  parts  of  the 
body.  In  development  these  gemmules  help  to  give  rise  to 
parts  like  those  from  which  they  originated.  This  hypo- 
thesis has  been  repeatedly  modified,  but,  except  in  the  gen- 
eral sense  that  the  body  may  influence  its  reproductive  cells, 
"  pangenesis  "  is  discredited  by  most  biologists. 

The  idea  which  is  now  accepted  with  general  favour  is, 


72    REPRODUCTION  AND  LIFE  HISTORY  OF  ANIMALS. 

that  the  reproductive  cells,  which  give  rise  to  the  offspring, 
are  more  or  less  directly  continuous  with  those  which  gave 
rise  to  the  parent.  This  idea,  suggested  by  Owen,  Haeckel, 
Rauber,  Galton,  Jager,  Brooks,  Nussbaum,  and  especially 
emphasised  by  Weismann,  is  fundamentally  important. 

At  an  early  stage  in  the  development  of  the  embryo  the 
future  reproductive  cells  of  the  organism  are  distinguishable 
from  those  which  are  forming  the  body.  These,  the  somatic 
cells,  develop  in  manifold  variety,  and,  as  division  of  labour 
is  established,  they  lose  their  likeness  to  the  fertilised  ovum 
of  which  they  are  the  descendants.  The  future  reproductive 
cells,  on  the  other  hand,  are  not  implicated  in  the  formation 
of  the  "  body,"  but  remaining  virtually  unchanged,  continue 
the  protoplasmic  tradition  unaltered,  and  are  thus  able  to 
start  an  offspring  which  will  resemble  the  parent,  because  it 
is  made  of  the  same  protoplasmic  material,  and  develops 
under  similar  conditions. 

A  fertilised  egg  cell  with  certain  characters  (a,  b,  c\  de- 
velops into  an  organism  in  which  these  characters  are  vari- 
ously expressed ;  but  if,  at  an  early  stage,  certain  cells  are 
set  apart,  retaining  the  characters,  a,  b,  c,  in  all  their  entirety, 
then  each  of  these  cells  will  be  on  the  same  footing  as  the 
original  fertilised  egg  cell,  able  to  give  rise  to  an  organism, 
almost  necessarily  to  a  similar  organism. 

An  early  insulation  of  reproductive  cells,  directly  con- 
tinuous and  therefore  presumably  identical  with  the  original 
ovum,  has  been  observed  in  the  development  of  some 
"worm  types" — (Sagitta,  Threadworms,  Leeches,  Polyzoa), 
and  of  some  Arthropods  (e.g.,  Moina  among  Crustaceans, 
Chironomus  among  Insects,  Phalangidae  among  Spiders),  in 
Micrometrus  aggregatus  among  Teleostean  fishes,  and  with 
less  distinctness  in  some  other  animals. 

In  many  cases,  however,  the  reproductive  cells  are  not 
recognisable  until  a  relatively  late  stage  in  development, 
after  differentiation  has  made  considerable  progress.  Weis- 
mann gets  over  this  difficulty  by  supposing  that  the  con- 
tinuity is  sustained  by  a  specific  nuclear  substance — the  germ- 
plasm — which  remains  unaltered  in  spite  of  the  differen- 
tiation in  the  body.  But  it  is  perhaps  enough  to  say  that 
as  all  the  cells  are  descendants  of  the  fertilised  ovum,  the 
reproductive  cells  are  those  which  retain  intact  the  qualities 


HEREDITY.  73 

of  that  fertilised  ovum,  and  that  this  is  the  reason  why  they 
are  able  to  develop  into  offspring  like  the  parent. 

Finally,  it  may  be  noticed  in  connection  with  heredity, 
that  there  is  great  doubt  to  what  extent  the  "body"  can 
definitely  influence  its  own  reproductive  cells.  Animals 
acquire  individual  bodily  peculiarities  in  the  course  of  their 
life,  as  the  result  of  what  they  do  or  refrain  from  doing,  or 
as  dints  from  external  forces.  The  "  body  "  is  thus  changed, 
but  there  is  much  doubt  whether  the  reproductive  cells 
within  the  "  body "  are  affected  by  such  changes.  Weis- 
mann  denies  the  transmissibility  of  any  characters  except 
those  inherent  or  congenital  in  the  fertilised  egg  cell,  and 
therefore  denies  that  the  influences  of  function  and  environ- 
ment are,  or  have  been,  of  any  importance  in  the  evolution 
of  many  celled  animals.  Such  influences  affect  the  body, 
but  do  not  reach  its  reproductive  cells,  and  are  therefore 
non-transmissible.  Many  of  the  most  authoritative  biolo- 
gists are  at  present  of  this  opinion.  On  the  other  hand, 
many  still  maintain  that  profound  changes  due  to  function 
or  environment  may  saturate  through  the  organism,  and  affect 
the  reproductive  cells,  and  thus  the  race.  The  whole 
question  remains  under  discussion. 


CHAPTER   V. 

PAST    HISTORY    OF    ANIMALS. 

PALEONTOLOGY. 

IN  the  two  preceding  chapters  we  have  noticed  two  of  the 
great  records  of  the  history  of  animal  life, — that  preserved 
in  observable  structures,  and  the  modified  recapitulation 
discernible  in  individual  development ;  in  this  we  turn  to 
the  third — the  geological  record.  From  Morphology  many 
conclusions  as  to  the  course  of  evolution  have  been  drawn  ; 
the  study  of  form  must  indeed,  by  itself,  in  time  have  led  to 
the  doctrine  of  evolution, — that  the  present  is  the  child  of 
the  past.  In  the  early  days  of  the  evolution  theory  the 
modern  science  of  Embryology  was  still  in  its  infancy,  and 
could  furnish  few  arguments,  and  it  was  the  opponents  of 
the  new  theory  rather  than  its  supporters  who  appealed  to 
Palaeontology.  They  asserted  that  the  palaeontological  facts 
refused  to  lend  the  support  which  the  theory  demanded. 
To  their  attacks  the  evolutionists  then  chiefly  sought  to 
reply  by  pointing  out  that  the  geological  record  was  very 
incomplete.  The  numerous  investigations  which  have  since 
been  carried  on  on  all  sides,  now  show  conclusively  that  it 
was  imperfection  rather  of  knowledge  than  of  the  record 
which  produced  the  negative  results.  We  must,  however, 
still  acknowledge  that,  except  in  a  relatively  few  cases,  little 
is  known  of  the  ancestors  of  living  animals,  and  seek  for 
reasons  to  explain  this. 

Reasons  for  the  "Imperfection  of  the  Geological  Record" 

If  we  remember  the  rule  of  modern  Geology  that  the  past 
is  to  be  interpreted  by  the  aid  of  the  present,  there  can  be 
no  difficulty  in  realising  that  the  chances  against  the  pre- 


PAST  HISTORY  OF  ANIMALS.  75 

servation  of  any  given  animal  are  very  great.  Many  are 
destroyed  by  other  living  creatures,  or  obliterated  by 
chemical  agencies.  Except  in  rare  instances,  only  hard  parts, 
such  as  bones,  teeth,  and  shells,  are  likely  to  be  preserved, 
and  this  at  once  greatly  limits  the  evidential  value  of  fossils. 
The  primitive  forms  of  life  would  almost  certainly  be  with- 
out hard  parts,  and  have  left  no  trace  behind  them.  A 
number  of  extremely  interesting  forms,  such  as  many  worms 
and  the  Ascidians,  are,  for  the  same  reason,  almost  unrepre- 
sented in  the  rocks.  Finally,  we  cannot  suppose  that  such 
an  external  structure  as  a  shell  can  always  be  an  exact  index 
of  the  animal  within.  Some  shells,  such  as  Nautilus  and  some 
of  the  Brachiopods,  occur  as  fossils  from  remote  Palaeozoic 
ages  onwards,  but  it  is  impossible  to  believe  that  the  animal 
within  has  never  varied  during  this  period,  though  we  cannot 
now  learn  either  the  nature  or  the  amount  of  the  variation. 

After  fossilisation  has  taken  place,  the  rock  with  its  con- 
tents may  be  entirely  destroyed  by  subsequent  denudation, 
or  so  altered  by  metamorphic  changes  that  all  trace  of  organic 
life  disappears.  Of  these  fossils  which  have  been  preserved 
only  a  small  percentage  are  available,  for  vast  areas  of  fossili- 
ferous  rocks  are  covered  over  by  later  deposits,  or  now  lie 
below  the  sea  or  in  lands  which  have  not  yet  been  explored. 

With  all  these  causes  operating  against  the  likelihood  of 
preservation,  and  of  finding  those  forms  that  may  have  been 
preserved,  it  is  little  wonder  if  the  geological  record  is 
incomplete  ;  but  such  as  it  is,  it  is  in  general  agreement  with 
what  the  other  evidence,  theoretical  and  actual,  leads  us  to 
expect  as  to  the  relative  age  of  the  great  types  of  animal 
life.  Further,  those  specially  favourable  cases  which  have 
been  completely  worked  out  have  yielded  results  which 
strongly  support  the  general  theory. 

Probabilities  of  "fossils"  in  the  various  classes  of  animals, 
But  it  will  be  useful  to  note  the  probabilities  of  a  good  representation 
of  extinct  forms  in  the  various  classes  of  animals.  Thus,  among  the 
Protozoa,  the  Infusoria  have  no  very  hard  parts,  and  have  therefore 
almost  no  chance  of  preservation,  and  the  same  may  be  said  of  forms 
like  Amoebae  ;  while  the  Foraminifera  and  the  Radiolaria,  having  hard 
structures  of  lime  or  silica,  have  been  well  preserved.  The  Sponges  are 
well  represented  by  their  spicules  and  skeletons.  Of  the  Coelenterates, 
except  an  extinct  order  known  as  Graptolites,  only  the  various  forms  of 
coral  had  any  parts  readily  capable  of  preservation,  and  remains  of  these 


76  PAST  HISTORY  OF  ANIMALS. 

are  very  abundant  in  the  rocks  of  many  ancient  seas.     But,  strange  as 
it  may  seem,  some  beautiful  remains  of  jellyfish  have  been  discovered. 

Of  the  great  series  of  "worms,"  only  the  tube  makers  have  left  actual 
remains,  the  others  are  known  only  by  their  tracks,  while  of  any  that 
may  have  lived  on  the  land  there  is  no  evidence. 

The  Echinoderms,  because  of  their  hard  parts,  are  well  represented  in 
all  their  orders  except  the  Holothurians,  where  the  calcareous  structures 
characteristic  of  the  class  are  at  a  minimum. 

The  Crustacea,  being  mostly  aquatic,  and  in  virtue  of  their  hard  skin, 
are  fossilised  in  great  numbers. 

The  Arachnida  and  the  Insects,  owing  to  their  air  breathing  habit, 
are  chiefly  represented  by  chance  individuals  that  have  been  drowned, 
or  enclosed  within  tree  stumps  and  amber. 

The  Molluscs  and  Brachiopods  are  perhaps  better  preserved  than  any 
other  animals,  since  nearly  all  of  them  are  possessed  of  a  shell  specially 
suitable  for  preservation. 

Among  the  Vertebrates,  some  of  the  lowest  are  without  scales,  teeth, 
or  bony  skeleton  ;  such  forms  have  therefore  left  almost  no  traces. 

Fishes,  which  are  usually  furnished  with  a  firm  outer  covering,  or 
with  a  bony  internal  skeleton,  or  with  both,  are  well  represented. 

The  primitive  Amphibians  were  furnished  with  an  exoskeleton  of  bony 
plates,  and  are  fairly  numerous  as  fossils.  The  bones  and  teeth  of  the 
others  have  been  fossilised,  though  more  rarely.  Those  living  in  fresh 
water  have  left  footprints  as  traces. 

The  traces  of  Reptilia  depend  upon  the  habits  of  the  various  orders, 
those  living  in  water  being  oftenest  preserved,  but  the  strange  flying 
Reptiles  have  also  left  many  skeletons  behind  them. 

Of  the  Birds,  the  wingless  ones  are  best  represented,  and  then  those 
that  lived  near  seas,  estuaries,  or  lakes. 

The  history  of  Mammals  is  very  imperfect,  for  most  of  them  were 
terrestrial.  But  the  discoveries  of  Marsh,  Cope,  and  others  show  how 
much  may  be  found  by  careful  search.  The  aquatic  Mammals  are  fairly 
well  preserved. 

' l  Palceon  tologica  I  Series. ' ' 

In  spite  of  the  imperfection  of  the  "geological  record," 
in  spite  of  the  conditions  unfavourable  to  the  preservation 
of  many  kinds  of  animals,  it  is  sometimes  possible  to  trace 
a  whole  series  of  extinct  forms  through  progressive  changes. 
Thus  a  series  of  fossilised  fresh  water  snails  (Planorlns}  has 
been  worked  out ;  the  extremes  are  very  different,  but  the 
intermediate  forms  link  them  indissolubly  by  a  marvellously 
gradual  series  of  transitions.  The  same  fact  is  well  illus- 
trated by  another  series  of  fresh  water  snails  (Paludind),  and 
not  less  strikingly  among  those  extinct  Cuttlefishes  which 
are  known  as  Ammonites,  and  have  perfectly  preserved 
shells.  Similarly,  though  less  perfectly,  the  modern  croco- 
diles are  linked  by  many  intermediate  forms  to  their  extinct 


EXTINCTION  OF  ANIMALS. 


77 


ancestors,  for  it  is  impossible  not  to  call  them  by  that  name, 
and  the  modern  horse  to  its  entirely  different  progenitors. 
In  short,  as  knowledge  increases,  the  evidence  from  Palae- 
ontology becomes  more  and  more  complete. 

In  a  general  way,  it  is  true  that  the  simpler  animals  pre- 
cede the  more  complex  in  history  as  they  do  in  structural 
rank,  but  the  fact  that  all  the  great  Invertebrate  groups  are 
represented  in  the  oldest  distinctly  stratified  and  fossiliferous 
rocks — the  Cambrian  system — shows  that  this  correspon- 
dence is  only  roughly  true.  To  account  for  this  we  must 
remember  that  the  whole  mass  of  the  oldest  rocks,  known 


FIG.  15. — Gradual  transitions  between  Paludina  Ne-umayri  (a) 
and  Paludina  Hccrnesi  (/).     (From  NEUMAYR.) 

as  Archaean  or  Pre-Cambrian,  have  been  so  profoundly 
altered  that,  as  a  rule,  only  masses  of  marble  and  carbona- 
ceous material  are  left  to  indicate  that  forms  of  life  existed 
when  these  rocks  were  laid  down.  What  these  early  forms 
of  life  were,  it  seems  impossible  for  us  to  find  out,  although 
recent  discoveries,  for  instance,  of  "  annelid  tracks  "  in  rocks 
of  possible  Pre-Cambrian  age  in  N.  W.  Scotland,  suggest 
that  patient  investigation  may  yet  do  much  towards  the 
solving  of  the  problem. 

Extinction  of  Animals > 

Some  animals,  such  as  some  of  the  lamp  shells  or 
Brachiopods,  have  persisted  from  almost  the  oldest  ages 
till  now,  and  most  fossilised  animals  have  modern 
representatives  which  we  believe  to  be  their  actual 


78  PAST  HISTORY  OF  ANIMALS. 

descendants.  That  a  species  should  disappear  need  not 
surprise  us,  if  we  believe  in  the  "  transformation "  of  one 
species  into  another.  The  disappearance  is  more  apparent 
than  real,  the  species  lives  on  in  its  modified  descendants, 
"  different  species  "  though  they  be. 

But,  on  the  other  hand,  there  are  not  a  few  fossil  animals 
which  have  become  wholly  extinct,  having  apparently  left  no 
direct  descendants.  Such  are  the  ancient  Trilobites 
(perhaps  remotely  connected  with  our  king  crab),  their 
allies  the  Eurypterids,  two  classes  of  Echinoderms  (Cystoids 
and  Blastoids),  many  giant  Reptiles,  and  some  Mammals. 

It  is  almost  certain  that  there  has  been  no  sudden 
extinction  of  any  animal  type.  There  is  no  evidence  of 
universal  cataclysm,  though  local  floods,  earthquakes,  and 
volcanic  eruptions  occurred  in  the  past,  as  they  do  still, 
with  disastrous  results  to  fauna  and  flora.  In  many  cases, 
the  waning  away  of  an  order,  or  even  of  a  class  of  animals, 
may  be  associated  with  the  appearance  of  some  formidable 
new  competitors  ;  thus  Cuttlefish  would  tend  to  exterminate 
Trilobites,  just  as  man  is  rapidly  and  often  inexcusably 
annihilating  many  kinds  of  beasts  and  birds.  Apart  from 
the  struggle  with  competitors,  it  is  conceivable  that  some 
stereotyped  animals  were  unable  to  accommodate  themselves 
to  changes  in  their  surroundings,  and  also  that  some  fell 
victims  to  their  own  constitutions,  becoming  too  large,  too 
sluggish,  too  calcareous,  in  short — too  extreme. 

Illustrations  of  the  Appearance  of  Animals  in  Time. 

Such  tables  as  those  given  here  are  apt  to  be  misleading,  in  that  they 
convey  the  impression  that  the  great  types  of  structure  have  appeared 
suddenly.  It  must  be  noted  that  any  apparent  abruptness  is  merely  due 
to  incompleteness  of  knowledge  or  inaccuracy  of  expression.  The  table 
is  a  mere  list  of  a  few  important  historical  events,  but  one  must  fully 
realise  that  they  are  not  isolated  facts,  that  the  present  lay  hidden  in  the 
past  and  has  gradually  grown  out  of  it.  Of  the  relative  length  of  the 
periods  represented  here  we  know  almost  nothing,  and  we  are  also 
ignorant  of  the  earliest  ages  in  which  life  began.  But  the  general  result 
is  clear.  We  find  that  in  the  Cambrian  rocks,  before  Pishes  appeared, 
the  great  Invertebrate  classes  were  represented,  though  as  yet  but 
feebly.  As  we  pass  upwards  they  increase  in  number  and  in  differentia- 
tion. Again,  Fishes  precede  Amphibians,  Amphibians  are  historically 
older  than  Reptiles,  and  many  types  of  Reptiles  are  much  older  than 
Birds.  In  short,  in  the  course  of  the  ages  life  has  been  slowly  creeping 
upwards. 


TABULAR  SURVEY. 


79 


ternary  or 
^t-Tertiary. 

Pliocene. 

* 

Miocene. 

g 

<u 

1 

£ 

rt 

IS 

'i 

i 

o< 

I 

rt 

E 
£ 
rt 

c 

4j     (?) 

Eocene. 

c 

-1 

Modern 
Types. 

^ 
Placentals. 

Cretaceous. 

Teleo- 
steans. 

Modern 
Types. 

Toothed  and 
Primitive 
Forms. 

Jurassic. 

Archaeop- 
teryx. 

Marsupials 
and  Mono- 
tremes  (?) 

Triassic. 

Few  primi- 
tive types. 

Permian. 

arboniferous. 

Laby- 
rintho- 
donts. 

jvonian  or  Old 
ted  Sandstone. 

Dipnoi. 

Silurian. 

Ganoids 
and 
Elasmo- 
:>  ranch  s. 

Drdovician. 

Cambrian. 

Representa- 
tives of  al 
the      chie 
classes     o 
I  n  v  e  r  t  e 
brates. 

're-Cambrian 
)r  Archaean. 

8o 


PAST  HISTORY  OF  ANIMALS. 

Coelentera.  Echinoderma.  Cephalopoda. 


Quaternary  or  Post- 
Tertiary. 

Pliocene. 

^    • 

c 
C 
^o 

£s 

,§  g      Miocene. 
IS... 

c 

s 

^ 
c 

£° 

Eocene. 

•S 

~^ 

% 

Cretaceous. 
fe 

r\'^ 

•«§  |      Jurassic. 

S  J3 

§s  .. 

IS 
*o 

c 

"C 
CJ 

>r. 

TD 

'o 

s 
£ 

/chinoids. 

TJ 

'2 
^ 

'rS 
ft 

Linmlus. 

eptostrac^ 

immonite 

>elemnites 

Jj  "*  •  •  ' 

<o 

Triassic. 

H) 

1 
'o 

Permian. 

s 

d 

Carboniferous. 

Vj 

c 

'o 

8 

§ 
•^                 Devonian,  or 
C^          Old  Red  Sandstone. 

§  

o 

u 

J4 

m 

M 

'O 

C 

?^ 
Q          Silurian. 

T3 

'o 

M 

5 

H 

^ 
Ordovician. 

iraptolite 

f*t 

U 

'C 

H 

Cambrian. 

Pre-Cambrian,  or 
Archaean. 

CHAPTER     VI. 


THE    DOCTRINE    OF    DESCENT. 

WHEN  we  ask,  as  we  are  bound  to  ask,  how  the  living  plants 
and  animals  that  we  know  have  come  to  be  what  they  are — 
very  numerous,  very  diverse,  very  beautiful,  marvellous  in 
their  adaptations,  harmonious  in  their  parts  and  qualities, 
and  approximately  stable  from  generation  to  generation, — 
we  may  possibly  receive  three  answers.  According  to  one, 
the  plants  and  animals  that  we  know  have  always  been  as 
they  are ;  but  this  is  at  once  contradicted  by  the  record  in 
the  rocks,  which  contain  the  remains  of  successive  sets  of 
plants  and  animals  very  different  from  those  which  now  live 
upon  the  earth.  According  to  another,  each  successive 
fauna  and  flora  was  destroyed  by  mundane  cataclysms,  to 
be  replaced  in  due  season  by  new  creations,  by  new  forms 
of  life  which  arose  after  a  fashion  of  which  the  human  mind 
can  form  no  conception.  Of  such  cataclysms  there  is  no 
evidence,  and  if  it  be  enough  to  postulate  one  creation,  we 
need  not  assume  a  dozen.  The  third  answer  is,  that  the 
present  is  the  child  of  the  past  in  all  things,  that  the  plants 
and  animals  now  existing  arose  by  a  natural  evolution  from 
simpler  pre-existing  forms  of  life,  these  from  still  simpler, 
and  so  on  back  to  a  simplicity  of  life  such  as  that  now 
represented  by  the  very  lowest  organisms. 

This  third  theory  is  really  an  old  one  ;  it  is  merely  man's 
application  of  his  idea  of  human  history  to  the  world  around 
him.  It  was  maintained  with  much  concreteness  and 
power  by  Buffon  (1749),  by  Erasmus  Darwin  (1794),  and 
by  Lamarck  (1801).  Yet  in  spite  of  the  labours  of  these 
thoughtful  naturalists  and  of  many  others,  the  general  idea 
of  the  natural  descent  of  organisms  from  simpler  ancestors, 

6 


82  THE  DOCTRINE   OF  DESCENT. 

was  not  received  with  favour  until  Darwin,  in  his  "  Origin 
of  Species"  (1859),  made  it  current  intellectual  coin.  By 
his  work  and  by  that  of  Spencer,  Wallace,  Haeckel,  and 
many  others,  the  doctrine  of  descent,  the  general  fact  of 
evolution,  has  been  established,  and  is  now  all  but  universally 
recognised. 

The  chief  arguments  'which  Darwin  and  others  have 
elaborated  in  support  of  the  doctrine  of  descent,  according 
to  which  organisms  have  been  naturally  evolved  from  simpler 
forms  of  life,  may  be  ranked  under  three  heads — (a)  struc- 
tural, (b)  physiological,  (c)  historical. 


EVIDENCES    OF    EVOLUTION. 

(a)  Structural. — There  are  said  to  be  over  a  million  liv- 
ing animals  of  different  species.  These  species  are  linked 
together  by  varieties,  which  make  strict  severance  often 
impossible  (Fig.  15);  they  can  be  rationally  arranged  in 
genera,  orders,  families,  and  classes,  between  which  there  are 
not  a  few  remarkable  connecting  links ;  there  is  a  gradual 
increase  of  complexity  from  the  Protozoa  upwards  along 
various  lines  of  organisation  ;  it  is  possible  to  rank  them 
all  on  a  hypothetical  genealogical  tree  (Fig.  i).  A  little 
practical  experience  makes  one  feel  that  the  facts  of  classi- 
fication favour  the  idea  of  common  descent. 

Throughout  vast  series  of  animals,  we  find  in  different 
guise  essentially  the  same  parts,  twisted  into  most  diverse 
forms  for  different  uses,  but  yet  referable  to  the  same  funda- 
mental type.  It  is  difficult  to  understand  this  "  adherence 
to  type,"  this  "  homology  "  of  organs,  except  on  the  theory 
of  natural  relationship. 

There  are  many  rudimentary  organs  in  animals,  espe- 
cially in  the  higher  animals,  which  remain  very  slightly 
developed,  and  which  often  disappear  without  having  served 
any  apparent  purpose.  Such  are  the  "  gill  slits  "  or  "  visceral 
clefts  "  in  Reptiles,  Birds,  and  Mammals,  the  teeth  of  young 
whalebone  whales,  the  pineal  body  (a  rudimentary  eye)  in 
Vertebrates.  Only  on  the  theory  that  they  are  vestiges  of 
structures  which  were  of  use  in  ancestors  are  these  rudiments 


EVIDENCES   OF  EVOLUTION.  83 

intelligible.     They  are  relics  of  past  history,  comparable,  as 
Darwin  said,  to  the  unpronounced  letters  in  many  words. 

(b)  Physiological. — Observation  shows  that  animals  are  to 
some  extent  plastic.      In  natural  conditions  they  vary  in  the 
course  of  several  generations  or  even  in  a  lifetime.     This 
is  especially  the  case  if  one  section  of  a  species  be  in  any 
way  isolated  from  the  rest,  or  if  the  animals  be  subjected 
in  the  course  of  their  wanderings  to  novel  conditions   of 
life.      Even  apart  from  markedly  changed  circumstances, 
moreover,   animals   exhibit   variations   from   generation   to 
generation. 

The  evidence  from  domesticated  animals  is  very  con- 
vincing. By  careful  interbreeding  of  varieties  which  pleased 
his  fancy  or  suited  his  purposes,  man  has  produced  numerous 
breeds  of  horses,  cattle,  sheep,  and  dogs,  which  are  often 
distinguished  from  one  another  by  structural  differences 
more  profound  than  those  which  separate  two  natural 
species.  In  great  measure,  however,  domestic  breeds  are 
fertile  with  one  another,  while  different  species  rarely  are. 
The  numerous  and  very  diverse  breeds  of  domestic  pigeons, 
which  are  all  derived  from  the  rock  dove  (Columba  livid], 
vividly  illustrate  the  plasticity  or  variability  of  organisms. 

It  sometimes  happens  that  the  offspring  of  an  animal 
resemble  not  so  much  the  parent  as  some  other  form 
believed  or  known  to  be  ancestral.  Thus  a  blue  pigeon  like 
the  ancestral  Columba  livia  may  be  hatched  in  the  dove  cot, 
a  foal  may  appear  with  zebra-like  stripes,  and  in  times  of 
famine  children  may  be  born  who  are  in  some  ways  ape-like. 
Such  atavisms  or  reversions  are  not  readily  intelligible  except 
on  the  theory  of  descent. 

(c]  Historical. — Among  the  extinct  animals  disentombed 
from   the  rocks,   many  form    series   by   which   those   now 
existing  can  be  linked  back  to  simpler  ancestors.     Thus 
the  ancient  history  of  horses,  crocodiles,  and  cuttlefish  is 
known  with  a  degree  of  completeness  which  makes  it  almost 
certain  that  the  simpler  extinct  forms  were  in  reality  the 
ancestors  of  those  which  now  live.     Moreover,  that  many 
connecting  links  have  been  discovered  in  the  rocks,  and  that 
the  higher  animals  appear  gradually  in  successive  periods  of 
the  earth's  history,  are  strong  corroborations  of  the  theory. 

It  is  less  easy  to  state  in  a  few  words  how  the  facts  of 


84  EVIDENCES  OF  EVOLUTION. 

geographical  distribution,  or  the  history  of  the  diffusion  of 
animals  from  centres  where  the  presumed  ancestral  forms 
are  or  were  most  at  home,  favour  the  doctrine  of 
descent. 

The  individual  life  history  of  an  animal — often  strangely 
circuitous  or  indirect — is  interpretable  as  a  modified  re- 
/capitulation  of  the  probable  history  of  the  race.  The 
embryo  mammal  is  at  one  stage  somewhat  like  an  embry- 
onic fish,  at  another  like  an  embryonic  reptile  ;  even  in 
details,  the  recapitulation,  if  such  we  may  term  it,  is  some- 
times faithful. 

Such,  in  merest  outline,  is  the  nature  of  the  evidence 
which  leads  us  to  conclude  that  the  various  forms  of  life 
have  descended  or  have  been  evolved  from  simpler  ancestors, 
and  these  from  still  simpler,  and  so  on,  back  to  the  mist  of 
life's  beginnings. 

In  accepting  this  conclusion  naturalists  are  practically 
unanimous  ;  but  in  regard  to  the  manner  in  which  the 
modification  of  species  or  the  general  ascent  of  life  has  been 
brought  about,  there  is  much  difference  of  opinion.  The 
fact  of  evolution  is  admitted  ;  debate  goes  on  with  regard 
to  the  factors  (see  Chap.  XXX.). 


CHAPTER     VII. 

PROTOZOA THE    SIMPLEST    ANIMALS. 

Chief  classes : — (i)  Rhizopods  ;  (2)  Gregarinida,  or  Sporozoa  ; 
(3)  Infusoria. 

THE  study  of  Protozoa  is  a  study  of  beginnings.  For  while 
we  know  nothing  directly  about  the  beginnings  of  animal  life, 
the  Protozoa  give  us  hints  of  the  original  relative  simplicity. 
They  have  remained,  almost  all  of  them,  unit  masses  of  living 
matter.  And,  in  virtue  of  their  simplicity,  they  are  in  some 
measure  exempt  from  natural  death,  which  is  "  the  price  paid 
for  a  body."  Moreover,  in  their  variety  they  exhibit,  as  it 
were,  a  natural  analysis  of  the  higher  animals,  which  are 
built  up  of  many  diverse  cells. 

GENERAL  CHARACTERS. —  The  Protozoa,  the  simplest  and 
most  primitive  animals,  are  usually  very  small  unit  masses 
of  living  matter  or  single  cells,  and  differ  from  plants 
in  their  way  of  feeding.  Most  of  them  feed  on  small 
plants  or  other  Protozoa,  or  on  debris,  and  not  a  few 
are  parasitic.  Most  of  them  live  in  water,  but  many  can 
endure  dryness  for  some  time.  In  one  set  (Rhizopods)  the 
living  matter  is  without  any  rind,  and  flows  out  in  more 
or  less  changeful  threads  and  lobes,  by  the  movements  of  ivhich 
the  animals  engulf  their  food  and  glide  along.  The  others 
have  a  definite  rind,  which  in  a  large  number  (Infusorians) 
bears  motile  cilia  orflagella,  but  in  a  minority  ( Gregarines)  is 
without  any  obvious  loco  mo  tor  structures.  But  these  three 
states  may  occur  in  the  life  of  one  form',  in  fact,  each  of  the 
three  great  classes  is  marked  by  the  predominant,  and  not  by 
the  exclusive  occurrence  of  the  Rhizopod-like,  or  the  Infusorian- 
like,  or  the  Gregarine-like  phase  of  cell  life.  Ma  ny  have  a  skeleta  I 


84  EVIDENCES   OF  EVOLUTION. 

geographical  distribution,  or  the  history  of  the  diffusion  of 
animals  from  centres  where  the  presumed  ancestral  forms 
are  or  were  most  at  home,  favour  the  doctrine  of 
descent. 

The  individual  life  history  of  an  animal — often  strangely 
circuitous  or  indirect — is  interpretable  as  a  modified  re- 
/capitulation  of  the  probable  history  of  the  race.  The 
embryo  mammal  is  at  one  stage  somewhat  like  an  embry- 
onic fish,  at  another  like  an  embryonic  reptile  ;  even  in 
details,  the  recapitulation,  if  such  we  may  term  it,  is  some- 
times faithful. 

Such,  in  merest  outline,  is  the  nature  of  the  evidence 
which  leads  us  to  conclude  that  the  various  forms  of  life 
have  descended  or  have  been  evolved  from  simpler  ancestors, 
and  these  from  still  simpler,  and  so  on,  back  to  the  mist  of 
life's  beginnings. 

In  accepting  this  conclusion  naturalists  are  practically 
unanimous  ;  but  in  regard  to  the  manner  in  which  the 
modification  of  species  or  the  general  ascent  of  life  has  been 
brought  about,  there  is  much  difference  of  opinion.  The 
fact  of  evolution  is  admitted ;  debate  goes  on  with  regard 
to  the  factors  (see  Chap.  XXX.). 


CHAPTER     VII. 

PROTOZOA THE    SIMPLEST    ANIMALS. 

Chief  classes: — (i)  Rhizopods ;  (2)  Gregarinida,  or  Sporozoa  ; 
(3)  Infusoria. 

THE  study  of  Protozoa  is  a  study  of  beginnings.  For  while 
we  know  nothing  directly  about  the  beginnings  of  animal  life, 
the  Protozoa  give  us  hints  of  the  original  relative  simplicity. 
They  have  remained,  almost  all  of  them,  unit  masses  of  living 
matter.  And,  in  virtue  of  their  simplicity,  they  are  in  some 
measure  exempt  from  natural  death,  which  is  "  the  price  paid 
for  a  body."  Moreover,  in  their  variety  they  exhibit,  as  it 
were,  a  natural  analysis  of  the  higher  animals,  which  are 
built  up  of  many  diverse  cells. 

GENERAL  CHARACTERS. —  The  Protozoa,  the  simplest  and 
most  primitive  animals,  are  usually  very  small  unit  masses 
of  living  matter  or  single  cells,  and  differ  from  plants 
in  their  way  of  feeding.  Most  of  them  feed  on  small 
plants  or  other  Protozoa,  or  on  debris,  and  not  a  few 
are  parasitic.  Most  of  them  live  in  water,  but  many  can 
endure  dry  ness  for  some  time.  In  one  set  {Rhizopods)  the 
living  matter  is  without  any  rind,  and  flows  out  in  more 
or  less  changeful  threads  and  lobes,  by  the  movements  of  which 
the  animals  engulf  their  food  and  glide  along.  The  others 
have  a  definite  rind,  which  in  a  large  number  (Infusorians) 
bears  motile  cilia  or  flagella,  but  in  a  minority  (Gregarines)  is 
without  any  obvious  loco  motor  structures.  But  these  three 
states  may  occur  in  the  life  of  one  form ;  in  fact,  each  of  the 
three  great  classes  is  marked  by  the  predominant,  and  not  by 
the  exclusive  occurrence  of  the  Rhizopod-like,  or  the  Infusorian- 
like,  or  the  Gregarine-like  phase  of  cell  life.  Many  have  a  skeletal 


86  PROTOZOA. 

framework  of  lime,  flint,  or  other  material,  while  within  the 
cell  there  is  a  special  kernel  or  nucleus,  or  there  may  be  several. 
There  are  also  other  less  constant  structures.  A  Protozoan 
multiplies  by  dividing  into  two  daughter  units,  or  into  a  large 
number ;  and  two  individuals  often  unite  temporarily  or  per- 
manently, in  conjugation,  ivhich  is  analogous  to  the  union  of 
ovum  and  spermatozoon  in  higher  animals.  A  few  types, 
instead  of  remaining  single  cells,  form  by  division  or  budding 
loose  colonies,  taking  a  step,  as  it  were,  towards  the  Metazoa. 


TYPES    OF     PROTOZOA. 

First  Type — AMCEBA. 

Amceba,  a  type  of  Rhizopods,  especially  of  those  in  which 
the  outflowing  processes  of  living  matter  are  blunt  and  finger- 
like  (Lobosa). 

Description. — Amoeba  proteus  and  some  other  species  are 
found  on  the  muddy  bottoms  of  ponds  ;  A.  terricola  occurs 
in  damp  earth.  Some  are  just  large  enough  to  be  seen  with 
the  unaided  eye.  The  diameter  is  often  about  one-hundredth 
of  an  inch.  Each  is  like  a  little  sac  of  jelly,  and  glides  over 
the  surface  of  stone  and  plant  by  protruding  and  retracting 
blunt  processes  or  pseudopodia.  As  they  move  the  shape 
constantly  changes,  whence  the  old  (1755)  popular  name  of 
"  Proteus  animalcule."  Round  the  margin,  which  sometimes 
shows  an  apparent  radial  striation,  the  cell  substance  is  firmer 
and  clearer  than  it  is  in  the  interior,  which  is  granular  and 
more  fluid.  According  to  Professor  Ray  Lankester,  the 
formation  of  pseudopodia  is  due  to  the  outflowing  of  the 
central  fluid  substance  at  places  where  the  firm  outer  pellicle 
has  been  temporarily  ruptured.  In  the  centre  of  the  cell  lies 
a  single  nucleus,  and  Amceba  princeps  has  numerous  nuclei. 
The  food  consists  of  minute  Algae,  such  as  diatoms,  or  of 
vegetable  debris.  It  is  surrounded  by  the  finger-like  pro- 
cesses, and  engulfed  along  with  drops  of  water,  which  form 
food  vacuoles  in  the  cell  substance.  After  the  digestible 
parts  of  the  food  have  been  absorbed,  the  undigested  residue 
is  got  rid  of  at  any  point  of  the  protoplasm.  One  or  more 
contractile  vacuoles  are  visible  in  the  cell  substance.  They 


AMCEBA—GREGAR1NA.  87 

have  an  excretory  function,  and  serve  to  get  rid  of  the  finer 
waste  products. 

Life  History. — In  favourable  nutritive  conditions  the 
Amoeba  grows.  At  the  limit  of  growth  it  reproduces  by 
dividing  into  two.  In  disadvantageous  conditions,  such  as 
drought,  it  may  become  globular,  and  secreting  a  cell  wall  or 
cyst,  lie  dormant  for  a  time.  With  the  return  of  favourable 
conditions  it  revives,  and,  bursting  from  the  cyst  with 
renewed  energy,  begins  anew  the  cell  cycle.  The  conjuga- 
tion of  two  Amoebae  has  been  observed,  and  it  is  said  that 


FIG.  16. — Life  history  of  Amoeba. 

1.  n.  Nucleus,  c.v.  contractile  vacuole. 

2.  Division  into  two 

3.  Encystation. 

4.  Escape  of  amoeba  from  its  cyst. 

spore  formation  occasionally  occurs ;   of  these   processes, 
however,  little  is  certainly  known. 

Second  Type — GREGARINA. 

Gregarina,  a  type  of  those  Gregarinida  or  Sporozoa  in 
which  the  cell  is  divided  into  two  regions  by  a  partition. 

Description. — Various  species  occur  in  the  intestine  of  the 
lobster,  cockroach,  and  other  Arthropods.  When  young  they 
are  intracellular  parasites,  but  later  they  become  free  in  the 
gut.  They  feed  by  absorbing  diffusible  food  stuffs,  such  as 
peptones  and  carbohydrates,  from  their  hosts,  and  store  up 
glycogen  within  themselves.  The  maximum  size  is  about 
one-tenth  of  an  inch.  There  is  a  firm  cuticle  of  "  proto- 
elastin,"  which  grows  inwards  so  as  to  divide  the  cell  into  a 
larger  nucleated  posterior  region  and  a  smaller  anterior  region, 
and  also,  in  the  young  stage,  forms  a  small  anterior  cap.  The 

r  9  •  °^L  /V^ 

^Wrfu  *Yl 

,^£A    \rO^UL  U^MU      H  &LU(  ^ 


.; 


cell  substance  is  divided  into  a  firmer  cortical  layer  and  a 
more  fluid  central  substance.  The  protoplasm  often  presents 
a  delicate  fibrillar  appearance,  suggesting  that  of  striated 
muscle.  The  nucleus  is  very  distinct,  but  there  are  no 
vacuoles.  We  may  associate  the  absence  of  locomotor  pro- 
cesses, "  mouth,"  and  contractile  vacuoles,  as  well  as  the 
thickness  of  the  cuticle  and  the  general  passivity,  with  the 
parasitic  habit  of  the  Gregarines.  It  is  not  clearly  under- 
stood how  these  and  other  intestinal  parasites  have  become 
habituated  to  resist  the  action  of  digestive  juices. 

Life  History. — The  young  Gregarine  is  parasitic  in  one  of 
the  lining  cells  of  the  gut ;  it  grows,  and  leaving 
the  cell,  remains  for  a  time  still  attached  to  it 
by  the  cap  (Fig.  18,  a.yg.} ;  later  this  is  cast  off, 
and  the  individual  becomes  free  in  the  gut, 
\  while  still  increasing  in  size.  Two  individuals 
often  attach  themselves  together  end  to  end,  but 
the  meaning  of  this  is  obscure.  Encystation 
occurs,  involving  a  single  unit  or  two  together, 
and  from  the  division  of  the  encysted  cell  or 
cells  i  spores  are  formed.  All  the  protoplasm 
is  not  always  used  up  in  forming  the  spores,  but 
a  residue  may  remain,  which  forms  a  network 
of  threads  supporting  the  spores.  The  cyst  is 
sometimes  (as  in  G.  blattaruni)  complex,  with 
"  ducts  "  serving  for  the  exit  of  the  spores,  each 
of  which  is  surrounded  by  a  firm  case.  Eventu- 
ally the  cyst  bursts,  the  spore  cases  are 
liberated,  and  from  within  each  of  these  the 
single  spore  emerges  to  become  a  cellular 
parasite.  The  spore  of  G.  gigantea  is  at  first 
non-nucleated ;  it  gives  off  two  processes,  one 
of  which  becomes  detached,  vibratile,  and 
nucleated,  while  the  other  seems  to  come  to 
nothing  (Fig.  18,  sp^).  The  adult  of  this  species 
is  sometimes  three-quarters  of  an  inch  in  length 
— enormous  for  a  Protozoon. 


T 


FIG.  17.— 
End-to-end 

union  of 
Gregarines. 

(After 
FRENZEL.) 


Third  Type — MONOCYSTIS. 

Monocystis. — A  type  of  those  Gregarinida,  or  Sporozoa,  in 
which  the  cell  is  not  divided  into  two  parts  by  a  partition. 


GREGARINA—MONOCYSTIS. 


89 


Description. — Two  species  (M.  agilis  and  M.  magna)  in- 
fest the  male  reproductive  organs  of  the  earthworm  so  con- 
stantly that  we  are  almost  always  sure  of  finding  them. 
The  full  grown  adults  are  visible  to  the  naked  eye.  They 
are  usually  cylindrical,  and  frequently  much  elongated,  cells, 
but  the  shape  alters  considerably  during  the  sluggish 
movements.  There  is  a  definite  contractile  rind,  which 
is  sometimes  fibrillated,  and  a  central  more  fluid 


FIG.  18. — Life  history  of  Gregarina.     (After  BUTSCHLJ.) 

a-yg-  Young  forms  emerging  from  intestinal  cells. 

ad.  Adult  wkh  deciduous  head  cap  and  a  cuticular  partition  divid- 
ing cell  into  two. 

con.  Two  forms  conjugating  (G.  blattaruvi). 

sp.f.  Spore  formation. 

spi.  Ripe  spore  of  G.  blattaruin. 

sp-z.  Spore  of  G.  gigantca,  with  long  vibratile  part  which  breaks  off 
and  develops  into  an  adult. 

substance,  in  which  the  large  nucleus  floats.  In  one 
species  there  is  an  anterior  projection  which  resembles  the 
cap  of  Gregarina,  otherwise  unrepresented  in  Monocystis. 
As  in  Gregarina,  and  many  parasitic  forms,  a  contractile 


9o  PROTOZOA. 

vacuole  is  absent,  but  the  significance  of  this  is  not  quite 
obvious. 

Life  History. — The  young  form  is  parasitic  within  one  of 
the  reproductive  cells  of  the  earthworm.  It  grows,  and 
becomes  free  from  the  cell.  In  the  free  stage,  two  indi- 
viduals may  unite  in  the  curious  end-to-end  manner 
observed  also  in  Gregarina.  Encystation  occurs,  involving 
either  a  single  individual  or  two  together.  Within  the 
rounded  cyst,  orderly  nuclear  division  results  in  the  forma- 
tion of  spore  forming  masses.  These  form  elliptical  spore 
cases,  or  "pseudonavicellae,"  enclosed  in  a  firm  sheath,  and 
each  spore  case  seems  to  contain  several,  usually  eight, 
spores,  lying  around  a  residual  core.  The  spores  are  con- 
siderably larger  than  those  of  Gregarina.  Eventually  the 
cyst  bursts,  the  spore  cases  are  extruded,  the  spores  emerge 
from  their  firm  chitinoid  cases.  The  young  spore  is  more 


FIG.  19. — Life  history  of  Monocystis.     (After  BUTSCHLI.) 

1.  Gregarine  lies  within  a  sperm-mother-cell  of  earthworm. 

2.  Conjugation  of  two  Gregarines  within  a  cyst. 

3.  Numerous  spore-cases  (pseudonavicellse)  within  a  cyst. 

4.  A  spore-case  with  eight  spores  (sp.)  and  a  residual  core  (rb.). 

active  than  the  adult ;  indeed,  in  some  Gregarines,  it  is  for 
a  brief  period  flagellate,  then  amoeboid,  then  like  the 
sluggish  adult.  Intracellular  parasitism  and  copious  food 
naturally  act  as  checks  to  activity. 

The  species  of  Monocystis  occur  chiefly  in  "Worms  "  and 
Tunicates ;    none   are  known   in  Arthropods,  Molluscs,  or 

~\7"£krf  pkV^ro  frf^c 


Vertebrates. 


Fourth  Type — PARAMCECIUM. 


Paramcecium. — A  type  of  Infusorians,  especially  of 
those  which  are  uniformly  covered  with  short  cilia 
(Holotricha). 

Description. — Specimens  of  Param&tium  may  be  readily 
and  abundantly  obtained,  by  leaving  fragments  of  hay  to 


PARAMCEQIUM.  91 

soak  for  a  few  days  in  a  glass  of  water.  A  few  Infusorians 
have  been  lying  dormant  about  the  plant ;  they  revive  and 
multiply  with  extraordinary  rapidity.  They  are  abundant  in 
most  stagnant  pools,  and  are  just  visible  when  a  test  tube 
containing  them  is  held  between  the  eye  and  the  light. 
Their  food  consists  of  small  vegetable  particles. 

The  form  is  a  long  oval,  the  outer  portion  of  the  cell 
substance  is  differentiated  to  form  a  definite  rind.  With 
this  we  may  associate  the  fact  that  there  is  now  a  definite 
opening,  the  so-called  mouth,  which  serves  for  the  ingestion 


con. 


div 


P^IG.  2a.—Paramotciuin.    (After  BUTSCHLI). 

ad.  Adult  form,  showing  cilia,  "mouth,"  contractile  vacuoles,  e'.c. 
div.  Transverse  division. 
con.  Conjugation. 

of  food  particles.  The  surface  is  uniformly  covered  with 
cilia,  arranged  in  regular  longitudinal  rows ;  these  serve 
both  for  locomotion  and  for  driving  food  particles  towards 
the  mouth.  Among  the  cilia  on  the  cortex  there  are  small 
cavities,  in  which  lie  fine  protrusible  threads  ("trichocysts"). 
These,  though  parts  of  a  cell,  suggest  the  thread  cells  of 
Ccelentera,  and  are  probably  of  the  nature  of  weapons. 
In  the  substance  of  the  cell  lie  two  nuclei,  the  smaller 


92  PROTOZOA. 

"micronucleus"  lying  by  the  side  of  the  larger  "macro- 
nucleus."  Food  vacuoles  occur  as  in  the  Amoeba.  There 
are  two  contractile  vacuoles,  from  which  fine  canals  radiate 


FIG.  21. — Conjugation  oiParanuxciuin  atirelia — four 
stages.     (After  MAUPAS.J 

T.  Shows  macronucleus  (JV),  and  two  micronuclei  (#)  in  each  of 
the  two  conjugates. 

2.  Shows   breaking   up  of  macronucleus,   and   multiplication   of 
micronuclei  to  eight. 

3.  Shows    the    fertilisation    in    progress ;     the    macronucleus    is 
vanishing. 

4.  Shows  a  single  (fertilised)  micronuclens  in  each  conjugate. 

into  the  surrounding  protoplasm ;  these  discharge  into  the 
vacuole,  which  then  bursts  to  the  exterior. 

Life  History. — Growth  is  followed  by  obliquely  transverse 
division  into  two  (Fig  20,  div.\  One-half  includes  the 
"mouth,"  the  other  has  to  make  one.  As  well  as  this 
simple  fission,  a  process  of  transient  conjugation  also  occurs. 


JLJU 


FIG.  22. — Diagrammatic  expression  of  process 
of  conjugation  in  Paramoecium  attrelia.  (After 
MAUPAS.) 

A.  The  two  micronuclei  enlarge. 

B.  Each  divides  into  two. 

C.  Eight  micronuclei  result. 

U.  Seven  disappear  ;  one  (darkened)  divides  into  two. 

E.  An  interchange  and  fusion  occurs,  and  the  conjugates 
separate. 

F.  The  fertilised  micronucleus  divides  into  two. 

G.  Each  conjugate  begins  to  divide,  the  micronucleus 
of  each  half  dividing  into  two,  one   of  which   becomes 
macronucleus,    while   the  others  form   the    two    normal 
micronuclei.  The  top  line  represents  four  individuals,  each 
with  a  macronucleus  and  two  micronuclei. 


Two  individuals  approach  one  another  closely,  the  two 
nuclei  of  each  break  up,  an  exchange  of  pieces  of  the  micro- 
nucleus  takes  place ;  the  two  then  separate,  each  to  recon- 
struct its  two  nuclei.  This  process  is  necessary  for  the 
continued  health  of  the  species. 


VORT2CELLA.  93 

The  details  of  the  conjugating  process  have  been  worked  out  with 
great  care  by  Maupas  and  others.  They  differ  slightly  in  different 
species  ;  what  occurs  in  P.  aurelia  is  summarised  diagrammatically  in 
Fig.  22. 

The  micronuclear  elements  are  represented  by  two  minute  bodies. 
As  conjugation  begins,  these  separate  themselves  from  the  macronucleus. 
The  macronucleus  degenerates,  and  each  micronucleus  increases  in 
size  (A).  Each  divides  into  two  (B) ;  another  division  raises  their  num- 
ber to  eight  (C) ;  seven  of  these  seem  to  be  absorbed  and  disappear,  the 
remaining  eighth  divides  again  into  what  may  be  called  the  male  and 
female  elements  (D) ;  for  mutual  fertilisation  now  occurs  (E).  After 
this  exchange  has  been  accomplished,  the  Infusorians  separate,  and 
nuclear  reconstruction  begins.  The  fertilised  micronucleus  divides  into 
two  (F),  and  each  half  divides  again  (G),  so  that  there  are  four  in  each 
cell.  Two  of  these  form  the  macronuclei  of  the  two  daughter  cells 
into  which  the  Infusorian  proceeds  to  divide  (H) ;  the  other  two  form 
the  micronuclei,  but  before  another  division  occurs  each  has  again 
divided.  Thus  each  daughter-cell  contains  a  macronucleus  and  two 
micronuclei. 

Fifth  Type — VORTICELLA. 

Vorticella,  or  the  bell  animalcule,  is  a  type  of  those  ciliated 
Infusorians  in  which  the  cilia  are  restricted  to  a  region 
round  the  mouth  (Peritricha). 

Groups  of  Vorticella,  or  of  the  compound  form  Carchesium, 
grow  on  the  stems  of  fresh  water  plants,  and  sometimes  are 
readily  visible  to  the  unaided  eye  as  white  fringes.  In 
Vorticella  each  individual  suggests  an  inverted  bell  with  a 
long  flexible  handle.  The  base  of  the  stalk  is  moored  to 
the  water  weed,  the  bell  swings  in  the  water,  now  jerking 
out  to  the  full  length  of  its  tether,  and  again  cowering  down 
with  the  stalk  contracted  into  a  close  and  delicate  spiral. 
In  Carchesium  the  stalk  is  branched,  and  each  branch 
terminates  in  a  bell.  Up  the  stalk  there  runs,  in  a  slightly 
wavy  curve,  a  contractile  filament,  which,  in  shortening, 
gives  the  non-contractile  sheath  a  spiral  form.  This  con- 
tractile filament,  under  a  high  power,  may  exhibit  a  fine  stria- 
tion.  (A  similar  striated  structure  is  seen  in  some  Amoebae, 
Gregarines,  spermatozoa,  &c.,  and  above  all,  in  striped 
muscle  fibres.  It  seems  to  be  some  structural  adaptation 
to  contractility.)  The  bell  has  a  thickened  margin,  and 
within  this  lies  a  disc-like  lid ;  in  a  depression  on  the  left 
side,  between  the  margin  and  the  disc,  there  is  an  opening, 
the  mouth,  which  leads  by  a  distinct  passage  into  the  cell. 


94 


PROTOZOA. 


On  the  side  of  this  passage  there  is  a  weak  spot,  the 
potential  anus,  by  which  useless  debris  is  passed  out.  The 
cilia  are  so  arranged  as  to  waft  food  particles  into  the  mouth 
and  down  the  passage.  There  is  a  large  and  horse- 
shoe shaped  macronucleus,  and  a  small  micronucleus. 
Food  vacuoles  and  contractile  vacuoles  are  present  as 
usual. 

Sometimes  a  Vorticella  bell  jerks  itself  off  its  stalk  and 


FIG.  23. — Vorticella.     (After  BUTSCHLI.) 

1.  Structure.      N,  Macronucleus:    «,  micronucleus;  C  V,  con- 
tractile vacuole  ;  m,  mouth  ;  fv,  food  vacuole  ;  v,  vestibule. 

2.  Encysted  individual.     3.  Division. 

4.  Separation  of  a  free  swimming  unit — the  result  of  a  division. 

5.  Formation  of  8  minute  units  (ing). 

6.  Conjugation  of  microzooid  (ing)  with  one  of  normal  size. 

swims  about ;  in  other  conditions  it  may  form  a  temporary 
cyst ;  normally,  the  cilia  are  very  active,  and  the  movements 
of  the  stalk  frequent  and  rapid.  Multiplication  may  take 


VOL  VOX.  95 

place  by  longitudinal  fission — a  bell  divides  into  similar 
halves,  one  of  these  acquires  a  basal  circlet  of  cilia  and 
goes  free,  ultimately  becoming  fixed.  Or  the  division  may 
be  unequal,  and  one,  or  as  many  as  eight,  microzooids  may  be 
set  free.  These  swim  away  by  means  of  the  posterior  girdle 
of  cilia,  and  each  may  conjugate  with  an  individual  of  normal 
size.  In  this  case  a  small  active  cell  (like  a  spermatozoon) 
fuses  intimately  with  a  larger  passive  cell,  which  may  be 
compared  to  an  ovum.  The  details  of  the  process  of 
fertilisation  are  analogous  to  those  described  in  Paramcecium. 
It  is  said  that  in  some  cases  an  encysted  Vorticella  breaks 
up  into  a  number  of  minute  spores,  but  this  is  doubtful. 

Sixth  Type— VOLVOX. 

Volvox  is  a  type  of  flagellate  Infusorians,  especially  of 
those  with  flagella  of  equal  size. 

Volvox  is  found,  not  very  commonly,  in  fresh  water  pools, 
and  is  usually  classed  by  botanists  as  a  green  Alga.  It  consists 
of  numerous  biflagellate  individuals,  connected  by  fine  pro- 
toplasmic bridges,  and  embedded  in  a  gelatinous  matrix, 
from  which  their  flagella  project,  the  whole  forming  a  hollow, 
spherical,  actively  motile  colony.  In  V.  globator  the  average 
number  of  individuals  is  about  10,000;  in  V.  aureus  or 
minor,  500-1000.  The  individual  cells  are  stellate  or 
amoeboid  in  V.  globator,  more  spherical  in  V.  aureus ;  each 
contains  a  nucleus  and  a  contractile  vacuole.  At  the 
anterior  hyaline  end,  where  the  flagella  are  inserted,  there 
is  a  pigment  spot ;  the  rest  of  the  cell  is  green,  owing  to  the 
presence  of  chlorophyll  corpuscles.  In  consequence  of  the 
presence  of  these,  Volvox  is  holophytic,  i.e.,  it  feeds  as  a  plant 
does. 

In  its  method  of  reproduction  Volvox  is  of  much  biological 
interest  and  importance.  As  Klein,  one  of  its  best  describers, 
says,  it  is  an  epitome  of  the  evolution  of  sex.  Some  of  the 
colonies  afe  asexual.  In  these  a  limited  number  of  cells 
possess  the  power  of  dividing  up  to  form  little  clusters  of 
cells,  these  clusters  escape  from  the  envelope  of  the  parent 
colony,  and  form  new  free  swimming  colonies.  In  other 
colonies  there  are  special  reproductive  cells,  which  may  be 
called  ova  and  spermatozoa. 


96 


PROTOZOA. 


In  V.  globator  the  two  kinds  of  reproductive  cells  are  usually  formed 
in  the  same  colony,  the  spermatozoa  generally  first.  Technically,  the 
colony  is  usually  a  protandrous  hermaphrodite. 

In  V.  aureus  the  colony  is  oftenest  unisexual  or  dioecious,  i.e.,  either 
male  or  female.  But  it  may  be  monoecious  or  hermaphrodite,  and  then 
generally  protogynous,  i.e.,  producing  eggs  first. 

Whether  in  a  hermaphrodite  or  in  a  unisexual  colony,  the  sex  cells 
appear  among  the  ordinary  vegetative  units  ;  the  ova  are  distinguishable 


a 


FIG.  24. — Volvox  globator.     (After  CORN.) 

a.  Balls  of  sperms  ;  b.  immature  ova  ;  c.  ripe  ova. 

by  their  larger  size,  the  "sperm  mother  cells"  divide  rapidly  and  form 
numerous  (32-10x3  or  more)  slender  spermatozoa,  each  with  two  cilia. 
In  V.  globator  their  bundles  may  break  up  within  the  parent  colony ;  or, 
as  always  in  V.  aureus,  they  may  escape  intact,  and  swim  about  in  the 
water.  In  any  case,  the  ovum  is  fertilised  by  a  spermatozoon,  and,  after 
a  period  of  encystation  and  rest,  segments  to  form  a  new  colony. 
Occasionally,  however,  this  organism,  so  remarkable  a  condensation  of 
reproductive  possibilities,  exhibits  a  parthenogenetic  development  of  ova. 
Here  then  we  have  an  organism,  on  the  border  line  between  plant 


CLASSIFICATION  OF  PROTOZOA.  97 

and  animal  life,  just  across  the  line  which  separates  the  unicellular  from 
the  multicellular,  illustrating  the  beginning  of  that  important  distinc- 
tion between  somatic  or  body  cells  and  reproductive  cells,  and  occurring 
in  asexual,  hermaphrodite,  and  unisexual  phases.  Klein  records  no  less 
than  24  different  forms  of  V.  aureus  from  the  purely  vegetative  and 
asexual  to  the  parthenogenetic,  for  there  may  be  almost  entirely  male 
colonies,  almost  entirely  female  colonies,  and  other  interesting  transi- 
tional stages.  Klein  has  also  succeeded  to  some  extent  in  showing 
that  the  occurrence  of  the  various  reproductive  types  depends  on  outside 
influences. 

General  Classification  of  Protozoa. 

Since  the  Protozoa  are  unicellular  organisms  (except 
the  few  which  form  loose  colonies),  their  classification 
should  be  harmonious  with  that  of  the  cells  in  a  higher 
animal.  This  is  so.  Thus  (a)  the  Rhizopods,  in  which 
the  living  matter  flows  out  in  changeful  threads  or 
"pseudopodia,"  as  in  the  common  Amceba,  are  compar- 
able with  the  white  blood  corpuscles  or  leucocytes,  many 
young  ova,  and  other  "anweboid"  cells  of  higher  animals; 
(b)  the  Infusorians  which  have  a  definite  rind  and  bear 
motile  lashes  (cilia  or  flagella),  e.g.,  the  common  Paramoecium, 
may  be  likened  to  the  cells  of  ciliated  epithelium,  or  to  the 
active  spermatozoa  of  higher  animals ;  (c)  the  parasitic 
Gregarines  which  have  a  rind  and  no  motile  processes  or 
outflowings,  may  be  compared  to  degenerate  muscle  cells, 
or  to  mature  ova,  or  to  "encysted"  passive  cells  in  higher 
animals. 

This  comparison  has  been  worked  out  by  Professor  Geddes,  who  also 
points  out  that  the  classification  represents  the  three  physiological 
possibilities — (a)  the  amoeboid  units,  neither  very  active  nor  very  passive, 
form  a  median  compromise  ;  (b)  the  ciliated  Infusorians,  which  are 
usually  smaller,  show  the  result  of  a  relative  predominance  of  expendi- 
ture ;  (c)  the  encysted  Gregarines  represent  an  extreme  of  sluggish 
passivity. 

But,  as  Geddes  and  others  have  shown,  the  cells  of  a  higher  animal 
often  pass  from  one  phase  to  another,  —  the  young  amoeboid  ovum 
accumulating  yolk  becomes  encysted,  the  ciliated  cells  of  the  windpipe 
may,  to  our  discomfort,  sink  into  amoeboid  forms.  The  same  is  true  of 
the  Protozoa ;  thus  in  various  conditions  the  ciliated  or  flagellate  unit 
may  become  encysted  or  amoeboid,  while  in  some  of  the  simplest  forms, 
3uch  as  Protomyxa,  there  is  a  "  cell"  cycle  "  in  which  all  the  phases  occur 
in  one  life  history. 

It  is  also  important  to  notice  Professor  Ray  Lankester's  division  of 
the  Protozoa  into  naked  and  corticate  forms  (Gymnomyxa  and  Corticata). 
The  Gymnomyxa  include  the  primitive  forms  and  the  Rhizopods ;  the 
Corticata  include  the  two  extremes — Gregarines  and  Infusorians. 

7 


PROTOZOA. 
CLASSIFICATION   OF   PROTOZOA. 


(CORTICATA.) 

Predominantly 
ciliated  and 

active. 
INFUSORIANS. 


(GYMNOMYXA.) 


Predominantly 
amceboid. 

RHIZOPODS. 


(CORTICATA.) 


Predominantly 
encysted  and 

passive. 
GREGARINIDS. 


ACINETARIA. 


RADIOLARIA. 


CILIATA. 
RHYNCHOFLAGELLATA] 

DlNOFLAGELLATA. 

FLAGELLATA. 


FORAMINIFERA. 
LABYRINTHULIDEA. 

HELIOZOA. 

LOBOSA. 


GREGARINIDA 

or 
SPOROZOA. 


PROTEOMYXA  and  MYCETOZOA. 
PRIMITIVE  FORMS. 

SYSTEMATIC   SURVEY. 

A. — Primitive  Forms. 

1.  PROTEOMYXA. — A  class  established  by  Professor  Ray  Lankester,and 
described  by  him  as  "  a  lumber  room  in  which  obscure,  lowly  developed, 
and  insufficiently  known  forms  may  be  kept  until  they  can  be  otherwise 
dealt  with."     They  are  simple  in  structure,  often  parasitic  in  habit,  and 
protean  in  their  phases.     In  some  no  nucleus  has  yet  been  detected. 
They  occur  in  fresh  water,  in  the  sea,  and  parasitically. 

Examples. — Protoniyxa,  in  four  phases  : — (a)  encysted  and  breaking 
up  into  spores,  which  (b]  are  briefly  flagellate,  (c)  sink  into  amceboid 
forms,  and  (d)  flow  together  into  a  composite  "  plasmodium."  Vam- 
pyrella,)  parasitic  on  fresh-water  Algae ;  Archerina,  with  chlorophyll,  on 
Diatoms.  Protogenes,  the  simplest  "amceba."  Protobathybius,  dredged 
up  in  masses  from  the  depths.  Schizogenes^  multiplying  by  mere 
breakage.  Monobia,  dividing  into  beautiful  colonies. 

2.  MYCETOZOA. — Protozoa  which  live  on  land  and  have  a  fungus-like 
habit  of  feeding  on  decaying  vegetable  matter.     The  plasmodial  stage 
in  the  cycle  is  predominant.     The  coated  spores  are  usually  produced  in 


SYSTEMATIC  SURVEY.  99 

little  capsules  which  arise  from  the  surface  of  the  plasmodium,  and  are 
often  elaborate  in  structure.  The  spores  may  have  a  brief  flagellate 
activity,  and  then  sink  down  into  amoeboid  forms,  or  may  be  at  once 
little  amoebae  ;  the  amoebae  grow  and  consequently  multiply  and,  after  a 
while,  collect  into  the  characteristic  fused  masses  or  plasmodia,  which 
sometimes  spread  over  several  square  inches. 


FIG.  25. — Diagram  of  Protoniyxa  atirantiaca.     (After  H^CKEL.) 

i.  Encysted  ;  2.  Dividing  into  spores ;  3.  Escape  of  spores,  at 
first  flagellate,  then  amoeboid  ;  4.  Plasmodium,  formed  from  fusion 
of  small  amoebae. 

Example. — Fuligo  or  ^Ethalium  septicum,  "  flowers  of  tan" — a  large 
spreading  mass  found  in  summer  on  the  bark  of  the  tan  yard.  This  and 
the  other  forms  are  sometimes  ranked  as  plant  organisms  allied  to  Fungi, 
and  it  is  natural  that  some  of  these  primitive  forms  should  appear  to 
hesitate  between  the  two  paths,  f  Krukenberg's  discovery  of  a  peptic 
ferment  and  an  acid  in  these  forms  is  an  interesting  illustration  of  the 
general  similarity  of  digestive  processes  in  all  organisms.') 

B. — Predominantly  Amoeboid  Protozoa — Rhizopoda. 

3.  LOBOSA,  in  which  the  living  matter  flows  out  and  in  as  protean, 
usually  blunt,  never  interlaced  processes.     A  physical  difference  between 
•outer  and  inner  portions,  one  nucleus  or  more,  bubbles  of  water  en- 
gulfed along  with  the  food,  special  contractile  vacuoles,  and  granules, 
may  generally  be  observed.     They  multiply  in  most  cases  by  dividing 
into  two,  but  in  some  cases  liberate  numerous  buds  (Arcelld],  or  may 
rarely  form  spores  (Pefatuyxa).     They  sweat  off  a  protecting  cyst  in 
unfavourable  conditions.      Two  individuals  may  unite  in  conjugation. 
Most   of  them    occur   in    fresh   water,    some   in   the   sea,   a    few    are 
parasitic. 

Examples. — (a)  Naked  forms: — Amceba,  and  the  giant  amoeba  Pelo- 
myxa ;  (b)  Shelled  forms  : — Arcella,  with  a  firm  (chitinoid)  shell ; 
secreting  gas  bubbles  which  float  it ;  and  Difflugia^  shut  in  except  at  one 
end  by  a  membrane,  with  foreign  bodies  such  as  sand  grains  glued  over 
it.  Magosphcera  (Catallacta),  a  unique  form  described  by  Heeckel — (a) 
in  an  encysted  phase  ;  (b}  as  a  free  swimming  colony  of  ciliated  cells 
(like  the  embryo  of  some  sponges)  ;  (c)  as  ciliated  units  produced  from 
the  breaking  up  of  (b)  ;  (d]  as  amoeboid  forms  resulting  from  modifica- 
tions of  the  active  units. 

4.  LABYRINTHULIDEA,  compound  forms  consisting  of  a  mass  of  proto- 
plasm spreading  out  into  a  network,  and  of  numerous  spindle  shaped 
units  which  travel  continually  up  and  down  the  threads  of  the  living 
net. 


ioo  PROTOZOA. 

Examples. — Labyrintkula  (on  Algae),  Chlamydomyxa  (on  bog-moss). 

5.  HELIOZOA,  with  stiff  processes  radiating  from  a  spherical  body. 
The  outer  protoplasm  has  usually  larger  vacuoles  than  the  internal 
portion ;  there  may  be  numerous  nuclei,  and  one  or  more  contractile 
vacuoles.  Skeletal  structures  may  be  entirely  absent  (Actinosph&rium) ; 
they  may  be  represented  by  a  jelly-like  envelope  (Heterophrys] ;  or  by 
loose  flinty  needles  (Raphidiophrys] ;  or,  more  rarely,  by  a  connected 
framework  (Clathrulina).  Multiplication  by  division  or  by  spores. 
Conjugation  occurs.  Encystation  and  spore  making,  and  in  some  young 
forms  flagellate  phases  are  known  ;  the  stiff  processes  become  more 


FIG.  26. — Formation  of  shell  in  a  simple  Foraminifer. 
(After  DREYER.) 

In  A  the  shell  has  one  chamber  ;  />',  C,  and  D  show  the  formation 
of  a  second.  Note  outflowing  pseudopodia  and  the  enclosure  of  the 
shell  by  a  thin  layer  of  protoplasm  ;  note  also  the  nucleus  in  the 
central  protoplasm. 

amoeboid  in  food  catching.     Compared  with  Lobosa,  the  Heliozoa  are 
passive.     The  majority  occur  in  fresh  water. 

.    Examples.  —  Actinosphceriuin,    Actinophrys    sol    (sun  animalcules) ; 
Raphidiophrys,  forming  colonies  ;  Clathrulina^  stalked. 

6.  FORAMINIFERA.  —  Predominantly  amoeboid  forms,  with  fine 
branching  and  interlacing  processes  issuing  from  the  main  mass,  which 
is  always  within  a  shell,  calcareous  in  the  majority,  arenaceous  or  chiti- 


SYSTEMATIC  SURVEY.  101 

nous  in  others.  A  nucleus  is  present,  and  often  multiplies,  apparently 
in  association  with  reproduction.  Vacuoles,  contractile  or  otherwise, 
seem  to  be  very  rare.  Conjugation  has  not  been  certainly  observed. 
Multiplication  may  take  place  by  division,  but  usually  by  the  repeated 
division  of  the  nucleus  and  the  formation  of  internal  bud  spores.  The 
great  majority  are  marine,  occurring  at  all  depths.  Those  from  great 
depths  have  usually  shells  of  glued  sand  ;  the  limy  forms  are  found  at 
their  best  in  the  shallow  water  of  warm  seas,  but  some  occur  in  the  open 
sea,  and  sinking  down  as  they  die  form  ooze.  They  are  common  as 
fossils  from  Silurian  strata  onwards. 

Examples. — Ground,  in  both  fresh  and  salt  water,  with  one  or  two 
openings  to  its  shell,  which  is,  however,  virtually  enclosed  in  the  over- 


FIG.  27. — Polystomella.     (After  SCHULTZE.) 

flowing  protoplasm  ;  Microgroinia  socialis,  in  fresh  water,  forming 
colonies  ;  Shepheardella^  with  an  opening  at  each  end  of  a  long  mem- 
branous case  ;  Miliolina,  with  a  chambered  cell  simply  coiled,  and  a 
single  aperture.  Such  forms  are  often  called  Imperforate,  in  contrast  to 
those  whose  shells  have  many  pores.  Lagena,  with  a  simple  flask- 
shaped  cell,  with  diffuse  holes  for  the  processes  ;  Globigerina,  a  pelagic 
limy  form,  with  many  chambers  covered  with  pores,  contributes  very 
largely  to  the  ooze  ;  Hastigerina^  a  pelagic  form,  with  bubbly  protoplasm 
abundantly  overflowing  round  the  shell  .which  comes  to  be  internal  like 
a  Radiolarian  "central  capsule"  (q  v.)  ;  Amwodiscus,  from  the  depths, 


102  PROTOZOA. 

with  a  flinty  agglutinate  shell  ;  Haliphysema,  a  form  utilising  sponge 
spicules  to  cover  itself,  once  mistaken  for  a  minute  sponge,  or  for  a  very 
simple  many  celled  animal. 

Most  kinds  of  chalk  consist  mainly  of  the  shells  of  Foraminifera, 
accumulated  on  the  floor  of  ancient  seas  ;  Nummulites  and  related  fossil 
forms  were  as  large  as  shillings  or  half-crowns. 

7.  RADIOLARIA. — Marine  Rhizopods,  divided  by  a  membrane  into  an 
inner  central  capsule  (with  one  or  more  nuclei),  and  an  outer  portion, 
giving  off  radiating  thread-like  pseudopodia.  The  protoplasm  of  the 


FIG.  28. — A  pelagic  Foraminifer — Hastigerina  (Globigerina) 
Murrayi.     (After  BRADY.  ) 

Note  central  shell,  projecting  calcareous  spines  with  a  protoplasmic 
axis  ;  also  fine  curved  pseudopodia  and  vacuolated  protoplasm. 

two  regions  is  connected  by  openings  in  the  capsule  membrane,  and 
contains  many  vacuoles.     No  contractile  vacuoles  have  been  seen. 

There  is  usually  a  skeleton,  in  most  cases  siliceous  and  of  complex 
architecture,  in  some  cases  of  a  horn-like  substance,  called  acanthin. 
The  skeleton  may  be  quite  outside  the  central  capsule,  or  may  invade  it 


SYSTEMATIC  SURVEY.  103 

also.  Most  lead  an  isolated  existence  (Monocyttaria)  ;  a  few  form 
colonies  by  fusion  (Poly cytt aria}. 

Most  Radiolarians  include  unicellular  Algae  (yellow  cells),  with  which 
they  live  in  intimate  mutual  partnership  (symbiosis).  Division  is  pro- 
bably the  commonest  mode  of  multiplication,  but  flagellate  spores — 
sometimes  of  two  sizes,  small  and  large,  as  if  male  and  female — may  be 
formed  in  the  central  capsule.  Conjugation  is  still  unknown.  Professor 
Lankester  notes  that  the  central  capsule  of  a  Radiolarian  may  be  com- 
pared with  the  enclosed  shell  of  Hastigerina,  and  that  the  character  of  the 
protoplasm,  which  in  contrast  with  that  of  Foraminifera  is  abundantly 
vacuolated,  may  be  associated  with  the  pelagic  life,  which  is  rare  in  the 
former  class.  Radiolarians  form  much  of  the  ooze  of  the  great  depths, 
and  occur  abundantly  as  fossils  from  Palseozoic  times. 

Examples.  —  Thalassicola  (no  skeleton)  ;  Acanthometra  (acanthin)  ; 
Adinomma  (flinty  skeleton,  central  capsule  with  pores  all  over)  ; 


FIG.  29. — Optical  section  of  a  Radiolarian  (Actinomma). 
(After  H^CKEL.  ) 

a.  Nucleus  ;  b.  Wall  of  central  capsule  ;  c.  Siliceous  shell  within 
nucleus  ;  cl.  Middle  shell  within  central  capsule  ;  A  Outer  shell  in 
extra-capsular  substance.  Four  radial  spicules  hold  the  three 
spherical  shells  together. 

Eucyrtidium  (flinty  skeleton,  with  one  perforate  area  in  cone  shaped 
central  capsule) ;  Atilosphcera  (flinty  skeleton,  central  capsule  with  more 
than  one  perforate  area)  ;  Collozoum  and  Spharozoum,  multicellular 
colonial  forms.  >. 

C. — Predominantly  Encysted  Protozoa — Sporozoa. 

8.  GREGARINIDA  (or  better,  perhaps,  SPOROZOA).  —  Protozoa  of 
parasitic  habit,  very  passive  in  adult  life,  clothed  by  a  definite  rind, 
almost  never  with  any  locomotor  processes.  Found  in  almost  all  kinds 


104 


PROTOZOA. 


of  animals  ;  often,  especially  when  young,  within  the  cells  of  their  host  ; 
deriving  their  food  by  absorbing  diffusible  juices.  A  single  large  nucleus  ; 
no  contractile  vacuole.  Reproduction  by  division  in  early  life,  but 
typically  by  spore  formation.  An  encysted  phase  precedes  the  division 
into  encased  spores.  The  young  forms  escaping  from  a  spore  case  may 
be  flagellate  or  amoeboid  ;  but,  except  in  a  very  few  cases,  passivity  pre- 
vails, and  the  adults  are  much  restricted  in  their  contractile  movements. 
Conjugation,  followed  by  fusion,  often  precedes  encystation  ;  and  two 
forms  often  occur  joined  together  but  not  fused. 

Examples. — Monocystis,  in  earthworm ;  Gregarina^  with  a  cross  par- 
tition, in  food  canal  of  Arthropods  ;  Eimeria,  remaining,  except  in  young 
stages,  within  a  cell  of  the  host  ;  Drepanidium,  and  other  forms,  in  blood 
corpuscles  ;  Myxiditun,  with  amoeboid  adult  ;  Sarcocystis,  in  muscle 
fibres  of  Mammals  and  some  other  Vertebrates  ;  Coccidium  ovifonne,  a 


FIG.  30. — A  Colonial  Flagellate  Infusorian — Proterospongia 
Hseckelii.     (After  SAVILLE  KENT.) 

There  are  about  40  flagellate  individuals.  «,  nucleus  ;  6,  contractile 
vacuole  ;  c,  amoeboid  unit  in  gelatinous  matrix  ;  d,  division  of  an 
amoeboid  unit ;  £,  flagellate  units  with  collars  contracted  ;  f,  hyaline 
outer  membranes  ;  g,  unit  forming  spores. 

permanent  cell  parasite,  in  many  Vertebrates,  common  in  the  liver  of 
rabbits,  &c. 

jD.— Predominantly  Active  Forms  (ciliate  and  flagellate), 
generally  called  Infusorians. 

{Occurring  in  fresh  and  sea  water  ^  abundant  in  infusions.) 

9.  FLAGELLATA,  units  with  a  definite  rind,  with  1-3  actively  undu- 
lating flagella,  often  with  a  distinct  aperture  for  the  entrance  of  food. 
Reproduction  by  division  into  two,  or  by  multiple  division  within  a 
cyst.  Conjugation  and  encystation  are  common.  Some  forms  are 
colonial,  and  suggest  the  transition  of  Metazoa. 


SYSTEMATIC  SURVEY.  105 

Examples. — Mastigamceba,  possessing  a  flagellum  and  amoeboid  pro- 
cesses ;  Euglena,  very  common  in  wayside  pools,  with  green  or  variable 
colouring  matter,  probably  feeding  for  the  most  part  like  a  plant ;  the 
colonial  Volvox  ;  Codosiga,  with  stalked  colonies,  each  individual  with 
a  collar  round  the  base  of  the  flagellum  ;  Proterospongia,  colonial,  like 
a  detached  piece  of  sponge.  Many,  e.g. ,  Monads,  live  parasitically  or  in 
putrid  liquids. 

10.  DINOFLAGELLATA,    very   successful    Protozoa,    which    combine 
activity  and  passivity,  having  two  flagella  and   generally  a  cellulose 
coat.     The  one  flagellum  projects  from  a  longitudinal  groove,  the  other 
lies  in  a  transverse  groove.     Mostly  marine. 

Examples. — Peridininm  and  Ceratium. 

11.  RHYNCHOFLAGELLATA,  large  forms,  with   firm   rind   and  very 
spongy  protoplasm,  with  two  flagella,   the  larger  one  striated  like  a 
muscle,  springing  from  a  deep  groove,  the  smaller  one  near  the  aperture 
for  the  food. 

Examples.  — The  phosphorescent  Noctiluca  ;  Leptodiscus  medusoides, 
disc-like  in  form,  swimming  like  a  miniature  medusoid. 

12.  CILIATA,  provided  with  numerous  cilia,  which  bend  and  straighten 
rapidly,  driving  the  animals  along  or  wafting  food  particles  into  the 
"  mouth."     There  is  a  definite  rind.      Beside  the  large  macronucleus 
there  is  in  most  a  micronucleus  or  "paranucleus."     There  are  usually 
two  contractile  vacuoles.    Multiplication  by  rapidly  succeeding  divisions  ; 
in  rare  cases  spores  seem  to  be  formed.     Conjugation  has  in  some  cases 
at  least  been  shown  to  be  associated  with  intimate  interchange  of  micro - 
nuclear  material.     Parasitic  forms,  some  mouthless,  are  not  uncommon. 

Examples. — (a)  Peritricha,  with  a  circle  of  cilia  at  one  end  or  at 
both,  e.g.,  Vorticella ;  Trichodina,  common  on  Hydra;  (b)  Hetero- 
tricha,  with  long  and  short  cilia,  e.g. ,  the  large  Stentor,  about  -^V^h  inch 
in  length  ;  Balantidium  coli,  in  colon  of  man.  (c)  Holotricha,  uni- 
formly ciliated,  e.g. ,  Paramcecium  ;  Opalina,  in  intestine  of  frog,  with 
numerous  nuclei,  and  no  contractile  vacuoles.  (d]  Hypotricha,  locomotor 
cilia  confined  to  under  surface,  e.g.,  Stylonichia. 

13.  ACINETARIA,  ciliated  when  young,  and  probably  derived  from  the 
Ciliata,  but  more  passive  when  adult.     They  are  fixed  in  adult  life, 
generally  stalked,  and  bear  tentacle-like  processes  often  suctorial.     The 
nucleus  is  sometimes  branched.      They  have  one  or  more  contractile 
vacuoles.     They  multiply  by  division,  or  by  the  formation  of  buds  which 
usually  remain  for  a  time  partly  enclosed  by  the  parent.     Their  food 
consists  of  other  Protozoa.     They  represent  "  an  extreme  modification 
of  the  Protozoon  series,  in  which  the  differentiation  of  parts  in  a  uni- 
cellular animal  reaches  its  highest  point"  (Lankester). 

Examples.  —  Adneta,  suctorial  ;  Dendrosoma,  forming  branched 
colonies,  suctorial  ;  Ophryodendron^  non-suctorial.  While  most  Acinetse 
seize  other  Infusorians  by  means  of  their  suckers,  there  are  others  of 
minute  size,  e.g.,  Sphcerophrya  paramtxdorum^  which  penetrate  into 
their  prey  and  become  parasites. 

History. — Of  animals  so  small  and  delicate  as  Protozoa,  we  do  not 
expect  to  find  distinct  relics  in  the  much  battered  ancient  rocks.  But 
there  are  hints  of  Foraminifer  shells  even  in  the  Cambrian  ;  more  than 
hints  in  the  Silurian  and  Devonian  ;  and  an  abundant  representation  in 


io6  PROTOZOA. 

rocks  of  the  Carboniferous  and  several  subsequent  epochs.  The 
famous  Eozoon  canadeuse  of  Cambrian  rocks  is  regarded  by  most  as  a 
purely  mineral  formation. 

There  seem  at  least  to  be  sufficient  relics  to  warrant  Neumayr's 
generalisation  in  regard  to  Foraminifera,  that  the  earliest  had  shells  of 
irregularly  agglutinated  particles  (Astrorhizidae),  that  these  were  suc- 
ceeded by  forms  with  regularly  agglutinated  shells,  exhibiting  types  of 
architecture  which  were  subsequently  expressed  in  lime. 

Relics  of  siliceous  Radiolarian  shells  are  also  known  from  Silurian 
strata  onwards,  with,  perhaps,  the  exception  of  the  Devonian.  Best 
known  are  those  which  form  the  later  Tertiary  deposits  of  Barbados 
earth,  from  which  Ehrenberg  described  no  fewer  than  278  species. 


GENERAL  NOTES  ON  THE  PROTOZOA. 

Ordinary  Functions — Movement. — The  most  obvious 
function  of  a  Protozoon  is  movement,  of  which  the  simplest 
mode  is  that  termed  amoeboid.  This  is  well  illustrated  by 
an  Amoeba.  In  ordinary  conditions  it  is  continually  chang- 
ing its  shape,  putting  forth  blunt  lobes  and  drawing  others 
in.  With  this  is  usually  associated  a  streaming  movement 
of  the  granules,  while  within  the  cell  itself  a  somewhat 
similar  streaming  is  often  seen,  as  in  many  plant  cells. 
Besides  the  local  changes  of  form  seen  in  the  Amoeba,  a 
defined  contraction,  like  that  of  a  muscle  cell,  is  illustrated 
in  the  contractile  filament  of  the  stalk  of  Vorticella  and 
similar  Infusorians  ;  and  not  less  definite  are  the  movements 
of  cilia  and  flagella,  by  means  of  which  most  Infusorians  travel 
swiftly  through  the  water.  Cilia  in  movement  are  "  bent 
and  straightened  alternately,"  while  flagella,  which  are 
usually  single  mobile  threads,  "  exhibit  lashing  movements 
to  and  fro,  and  are  thrown  into  serpentine  waves  during 
these  movements." 

Considered  generally,  the  movements  are  of  two  kinds  ;  either  (i) 
reflex,  i.e.,  responses  to  external  stimulus,  as  when  the  Protozoon  moves 
towards  a  nutritive  substance,  or  (2)  automatic,  i.e.,  such  movements  as 
appear  to  originate  from  within,  without  our  being  able  to  point  to  the 
immediate  stimulus,  e.g.,  the  rhythmical  pulsations  of  contractile 
vacuoles. 

While  all  vital  activity  or  life  must .  remain  inexplicable  in  lower 
terms  until  we  know  the  chemical  nature  of  protoplasm,  it  is  useful  to 
compare  the  movements  of  Amoebse  with  the  movements  of  drops  of 
fine  emulsion,  as  Professor  Biitschli  has  done  in  great  detail.  For  in  this 


ORDINARY  FUNCTIONS.  107 

way  the  strictly  vital  may  be  distinguished  from  what  depends  on  known 
physical  conditions. 

Dr.  Verworn  has  speculatively  suggested  that  the  substance  of  the 
amoeboid  cell  is  drawn  out  towards  oxygen  in  the  medium,  that  the 
chemically  satisfied  particles  make  way  for  their  unsatisfied  neighbour 
particles,  that  external  stimulus  provokes  a  molecular  disruption,  and 
that  the  exhausted  particles  have  then  to  retreat  to  the  nucleus  which  he 
regards  as  a  trophic  centre. 

Sensitiveness. — The  Amoeba  is  sensitive  to  external  influ- 
ences. It  shrinks  from  strong  light  and  obnoxious  materials, 
it  moves  towards  eatable  substances.  This  sensitiveness  is, 
so  far  as  we  know,  diffuse, — a  property  of  the  whole  of  the 
cell  substance,  but  the  pigment  spots  of  some  forms  are  / 
specialised  regions. 

Many  Protozoa  well  illustrate  a  strange  sensitiveness  to  (the  physical 
and  chemical  stimuli  of)  objects  or  substances  with  which  they  are  not 
in  contact.  Thus  the  simple  amoeboid  Vampyrella  will,  from  a  con- 
siderable distance,  creep  directly  towards  the  nutritive  substance  of  an 
Alga,  and  the  plasmodium  of  a  Myxomycete  will  move  towards  a 
decoction  of  dead  leaves,  and  away  from  a  solution  of  salt.  The  same 
sensitiveness,  technically  termed  Chemotaxis,  is  seen  when  micro- 
organisms move  towards  nutritive  media  or  away  from  others,  when  the 
spermatozoon  (of  plant  or  animal)  seeks  the  ovum,  or  when  the  phago- 
cytes (wandering  amoeboid  cells)  of  a  Metazoon  crowd  towards  an  intrud- 
ing parasite  or  some  irritant  particle. 

Nutrition. — The  energy  which  the  Amoeba  expends  in 
movement,  it  makes  up  for  by  eating  and  digesting  food 
particles.  Most  of  the  free  Protozoa  live  in  this  manner 
upon  solid  food  particles,  whether  plant  or  animal ;  a  few 
such  as  Volvox,  in  virtue  of  their  chlorophyll,  live  entirely 
as  do  plants  ;  the  parasitic  forms  usually  absorb  soluble  and 
diffusible  substances  from  their  hosts. 

Respiration. — Like  all  living  creatures  the  Amoeba  respires, 
that  is,  its  complex  substance  is  continually  undergoing  a 
process  of  oxidation,  carbon  dioxide  being  produced  as  a 
waste  product.  Without  oxygen  none  of  the  activities  can 
be  efficiently  performed,  and  if  it  is  long  withheld  death 
ensues.  In  all  Protozoa  oxygen  is  simply  taken  up  by  the 
general  protoplasm  from  the  surrounding  medium,  into 
which  the  waste  carbonic  acid  is  again  passed.  The 
bubbles  which  enter  with  the  food  particles  assist  in 
respiration.  In  parasitic  forms  the  method  of  respiration 
must  be  the  same  as  that  of  the  tissue  cells  of  the  host. 


io8  PROTOZOA. 

Excretion. — Of  the  details  of  this  process  little  is  certainly 
known,  but  the  contractile  vacuoles  are,  without  doubt, 
primitive  excretory  appliances.  In  the  more  specialised 
forms  they  appear  to  drain  the  cell  substance  by  means  of 
fine  radiating  canals,  and  then  to  burst  to  the  exterior.  \  Uric 
acid  and  urates  are  said  to  be  demonstrable  as  waste 
products.  ] 

Growth  and  Reproduction. — In  favourable  conditions, 
when  income  exceeds  expenditure,  the  Amoeba  or  other 
Protozoon  grows  ;  in  reverse  conditions,  or  at  the  limit  of 
growth,  it  reproduces.  The  phenomena  of  reproduction 
we  will  consider  in  greater  detail  later  on. 

Colour. — Pigments  are  not  infrequently  present  in  the  Protozoa  ; 
we  have  already  noticed  the  presence  of  chlorophyll  in  some  forms. 
With  Radiolarians,  the  so-called  "yellow  cells"  are  found  almost 
constantly  associated.  Each  of  these  cells  consists  of  protoplasm, 
surrounded  by  a  cell  wall,  and  containing  a  nucleus.  The  protoplasm, 
is  impregnated  with  chlorophyll,  the  green  colour  of  which  is 
obscured  by  a  yellow  pigment.  Starch  is  also  present.  The  cells 
multiply  by  fission  and  continue  to  live  after  isolation  from  the  proto- 
plasm of  the  Radiolarian.  All  these  facts  point  to  the  conclusion  that 
the  cells  are  symbiotic  Algae,  so-called  Zoochlorellce.  According  to 
some,  the  "chlorophyll  corpuscles"  seen  in  the  primitive  Archerina,  in 
some  flagellate  forms,  as  Euglena,  and  in  many  Ciliata,  as  Stentor^ 
Sty  Ionic  hia,  one  species  of  Paramcecium,  Volvox  and  the  allied  forms, 
are  also  symbiotic  Algce,  which  have  lost  the  power  of  independent 
existence.  The  evidence  for  this  is,  however,  insufficient,  and  this 
explanation  will  not  apply  to  cases  like  that  of  Vorticella  viridis, 
where  the  green  colouring  matter  is  uniformly  distributed  through  the 
protoplasm.  In  many  cases  there  is,  besides  the  chlorophyll,  a  brown 
pigment,  identical  with  the  diatomin  of  Diatoms.  In  many  of  the 
Flagellata  there  are  one  or  more  bright  pigment  spots  at  the  anterior 
end  of  the  cell ;  these  may  be  specially  sensitive  areas.  In  some  of  the 
simpler  Gregarines  the  medullary  protoplasm  is  coloured  with  pigment 
which  is  apparently  a  derivative  of  the  haemoglobin  of  the  host. 

Psychical  Life. — As  to  the  psychical  life  of  the  Protozoa, 
we  find  that  they  often  behave  in  a  way  which  suggests  con- 
scious effort  and  intelligence,  but  as  cut-off  fragments  also 
act  with  apparent  reasonableness,  and  as  the  nucleus 
cannot  be  regarded  as  a  brain,  there  seems  no  reason  to 
credit  them  with  more  than  that  diffuse  consciousness  which 
is  possibly  co-extensive  with  life.  Verworn  has  decided, 
after  much  labour,  that  the  Protozoa  do  not  exhibit  what 
even  the  most  sanguine  could  call  intelligence,  but  this  is 


STRUCTURE.  109 

no  reason  why  he  or  any  other  evolutionist  should  doubt 
that  they  have  in  them  the  indefinable  rudiments  of 
thought. 

Structure. — The  Protozoa  are  sometimes  called  "  struc- 
tureless," but  they  are  only  so  relatively.  For  though  they 
have  not  stomachs,  hearts,  and  kidneys,  as  Ehrenberg 
supposed,  they  are  not  like  drops  of  white  of  egg.  Our 
eyes,  when  aided  by  the  microscope,  can  distinguish  struc- 
ture in  these  simplest  animals.  They  are  simple  as  an  egg 
is  simple  when  compared  with  a  bird. 

The  cell  substance  consists  of  a  living  network  or  foam, 
in  the  meshes  or  vacuoles  of  which  there  is  looser  material. 
Included  with  the  latter  are  granules,  some  of  which  are 
food  fragments  in  process  of  digestion,  or  waste  products  in 
process  of  excretion. 

The  cell  substance  includes  one  or  more  nuclei,  special- 
ised areas  which  are  essential  to  the  life  and  multiplication 
of  the  unit.  In  the  Protozoa  there  are  several  conditions 
under  which  the  nucleus  may  exist. 

(1)  In  some  adult  forms,   and   in  many  spores  or  young  forms,  no 
nucleus  has  yet  been  discovered.     It  is,  however,  unnecessary  to  preserve 
the  term  "Monera"  for  such  simple  forms,  as  it  is  probable  that  nuclear 
material  does  exist  in  some  form  even  in  these  cases. 

(2)  In  some  of  the  Ciliata  the  nucleus  is  diffuse,  that  is,  it  exists  in  the 
form  of  a  powder  scattered  through  the  medullary  protoplasm,  and  is 
only  discernible  after  death  by  means  of  careful  staining.     In  Opalin 
opsis  the  fine  powder  sometimes  coalesces  into  a  single  nucleus. 

(3)  In  the  majority  of  cases,  notably  in  the  Gregarines.  the  nucleus 
is  single,  often  large,  and  placed  centrally  ;  from  a  consideration  of  the 
cells  of  Metazoa  we  may  call  this  the  typical  case. 

(4)  In  many  of  the  Ciliata,  e.g.,  Paramcecium,  the  nucleus  is  double. 
There  is  a  large  oblong  nucleus  and  beside  it  a  smaller  spherical  one. 

(5)  In  Opalina,  from  the  intestine  of  the  frog,  and  a  few  other  forms, 
there  are  very  numerous  nuclei,  arranged  in  a  symmetrical  manner  in 
the  cell  substance.      In  some  cases  these  isolated   nuclei   have   been 
observed  to  unite  to  form  one  large  nucleus  just  before  binary  fission 
takes  place.     Of  these  various  cases  the  diffuse  condition  is  apparently 
very  primitive. 

The  nucleus,  when  stained  and  examined  under  high  powers,  is 
observed  to  be  complex  in  structure.  It  consists  of  a  nuclear  network, 
or  a  coil  of  chromatin  threads.  In  the  division  of  many  Protozoa,  as  in 
the  cells  of  higher  animals  it  plays  an  important  part.  During  division 
it  passes  from  the  resting  to  the  active  condition.  The  nuclear  threads 
or  "  chromatin  filaments  "  loosen  themselves  from  their  coiled  state, 
and  arrange  themselves  in  a  star  at  the  equator  of  the  cell,  whence  they 


I  io  PROTOZOA. 

divide  into  two  groups,  which  retreat  from  one  another,  and  become  the 
daughter  nuclei  of  two  daughter  cells.  In  short,  karyokinesis  has  been 
observed  here  as  elsewhere  (see  p.  45). 

While  we  cannot  at  present  define  the  physiological  import  of  the 
nucleus,  we  must  recognise  its  importance.  Thus,  Bruno  Hofer  has 
shown  that  when  an  Amoeba  is  cut  in  two,  the  part  with  the  nucleus 
lives  and  grows  normally,  while  the  part  without  any  nucleus  sooner  or 
later  dies  ;  and  Balbiani  has  observed  that  in  the  case  of  Infusorians  cut 
into  pieces,  those  parts  which  have  nuclei  survive,  while  if  no  nucleus  is 
present  in  the  fragment,  the  wound  may  remain  unhealed  and  death 
ensues.  There  seems  no  reason  why  one  may  not  combine  the  view  of 
Weismann  that  the  nucleus  bears  the  essential  hereditary  substances 
with  the  view  that  it  is  a  trophic,  or,  at  any  rate,  a  vital  centre  in  the 
cell. 

In  naked  Protozoa,  the  outer  part  of  the  cell  substance 
("  ectoplasm  ")  is  often  clearer  and  less  granular  than  the 
inner  part  ("  endoplasm  "),  but  this  difference  is  a  physical 
one  of  little  importance.  In  corticate  Protozoa  there  is  a 
more  definite  rind  or  thickened  margin  of  cell  substance. 
Outside  this  there  may  be  a  "cuticle"  distinct  from  the 
living  matter,  sometimes  consisting  of  chitin,  or  gelatin,  or 
rarely  of  cellulose.  The  cuticle  may  form  a  cyst,  which  is 
either  a  protection  during  drought,  or  a  sheath  within  which 
the  unit  proceeds  to  divide  into  numerous  spores.  More- 
over, the  cuticle  may  become  the  basis  of  a  shell  formed 
from  foreign  particles,  or  made  by  the  animal  itself  of  lime, 
flint,  or  "  horny  "  material. 

In  the  cell  substance  there  may  be  bubbles  of  water  taken 
in  with  food  particles  (food  vacuoles),  contractile  vacuoles, 
fibres  which  seem  to  be  specially  contractile  (in  Gregarines), 
spicules  of  flint  or  threads  of  horn-like  material  which  may 
build  up  a  connected  framework,  and  the  pigments  already 
mentioned. 

Reproduction  of  Protozoa. — Growth  and  reproduction  are 
on  a  different  plane  from  the  other  functions.  Growth  occurs 
when  income  exceeds  expenditure,  and  when  constructive 
or  anabolic  processes  are  in  the  ascendant.  'Reproduction 
occurs  at  the  limit  of  growth,  or  sometimes  in  disadvantag- 
eous conditions  when  disruptive  or  katabolic  processes  gain 
some  relative  predominance. 

As  it  is  by  cell  division  that  all  embryos  are  formed  from 
the  egg,  and  all  growth  is  effected,  the  beginnings  of  this 
process  are  of  much  interest.  (a)  Some  very  simple 


RE  PROD  UCTION.  1 1 1 

Protozoa  seem  to  reproduce  by  what  looks  like  the  rupture 
of  outlying  parts  of  the  cell  substance.  (£)  The  production 
of  a  small  bud  from  a  parent  cell  is  not  uncommon,  and 
some  Rhizopods  (e.g.,  Arcella,  Pelomyxa)  give  off  many 
buds  at  once,  (c)  Commoner,  however,  is  the  definite  and 
orderly  process  by  which  a  unit  divides  into  two — ordinary 
cell  division,  (d)  Finally,  if  many  divisions  occur  in  rapid 
succession  or  contemporaneously,  and  usually  within  a  cyst 
enclosing  the  parent  cell,  i.e.,  in  narrowly  limited  time  and 
space,  the  result  is  the  formation  of  a  considerable  number 
of  small  units  or  spores.  In  the  great  majority  of  cases, 
each  result  of  division  is  seen  to  include  part  of  the  parent 
nucleus. 

A  many  celled  animal  multiplies  in  most  cases  by  liberat- 
ing reproductive  cells — ova  and  spermatozoa — different 
from  the  somatic  cells  which  make  up  the  "  body."  A 
Protozoon  multiplies  by  dividing  wholly  into  daughter  cells. 
This  difference  between  Metazoa  and  Protozoa  in  their 
modes  of  multiplication  is  a  consequence  of  the  difference 
between  multicellular  and  unicellular  life.  Each  part  of  a 
divided  Protozoon  is  able  to  live  on,  and  will  itself  divide 
after  a  time,  whereas  the  liberated  spermatozoa  and  ova  of 
a  higher  animal  die  unless  they  unite. 

By  sexual  reproduction,  we  mean  (a)  the  liberation  of 
special  reproductive  cells  from  a  "  body,"  and  (b)  the 
fertilisation  of  ova  by  spermatozoa.  It  is  obvious  that 
unicellular  Protozoa  can  show  nothing  corresponding  to 
sexual  reproduction  in  the  first  sense.  Moreover,  Pro- 
tozoa can  live  on,  dividing  and  multiplying,  for  prolonged 
periods  without  the  occurrence  of  anything  like  fertilisation. 

So  it  is  often  stated  as  a  characteristic  of  Protozoa  that 
"  they  have  no  sexual  reproduction."  But  if  this  mean 
that  the  unicellular  Protozoa  have  no  special  reproductive 
cells,  then  it  is  a  truism.  If,  however,  the  statement  mean 
that  the  Protozoa  are  without  anything  corresponding  to 
fertilisation,  then  it  is  not  true.  For  in  many  of  the 
Protozoa,  there  occurs  at  intervals  a  process  of  "  conjuga- 
tion "  in  which  two  individuals  unite  either  permanently  or 
temporarily.  This  is  an  incipiently  sexual  process  ;  it  is 
the  analogue  of  the  fertilisation  of  an  ovum  by  a  spermato- 
zoon. 


112  PROTOZOA. 

It  is  one  of  the  recurrent  phases  in  the  life  history  of  some  of  the 
simplest  Protozoa  (Proteomyxa  and  Mycetozoa)  (see  p.  98),  that  a 
number  of  amoeboid  units  flow  together  into  a  composite  mass,  which 
has  been  called  a  "  plasmodium" 

It  is  known  that  more  than  two  individual  Gregarines  and  other 
forms  occasionally  unite.  To  this  the  term  "  multiple  conjugation  "  has 
been  applied. 

Commonest,  however,  is  the  union  of  two  apparently  similar  individ- 
uals, either  permanently  so  that  the  two  fuse  into  one,  or  temporarily  so 
that  an  exchange  of  material  is  effected.  Permanent  conjugation  has 
been  observed  in  several  Rhizopods,  Infusorians,  and  Gregarines. 
Temporary  conjugation  is  well  known  in  not  a  few  ciliated  Infusorians, 
and  it  is  possible  that  a  curious  end-to-end  union  of  certain  Gregarines 
is  of  the  same  nature,  or  it  may  be  of  the  nature  of  a  "  plasmodium  " 
formation. 

Fourthly,  there  are  some  cases  where  one  of  the  conjugating  individ- 
uals is  larger  and  less  active  than  the  other.  Thus  in  Vorticella,  a 
small  free  swimmin'g  form  unites  and  fuses  completely  with  a  stalked 
individual  of  normal  size.  To  call  this  "dimorphic  conjugation  "  is 
hardly  necessary,  since  it  is  evidently  equivalent  to  the  fertilisation  of  a 
passive  ovum  by  an  active  spermatozoon,  one  of  the  well-known  charac- 
teristics of  reproduction  in  the  Metazoa. 

In  Volvox  this  is  even  more  obvious,  for  the  small  and  active  cells, 
both  in  shape  and  method  of  formation,  recall  the  spermatozoa  of 
higher  forms.  The  conjugation  of  ciliated  Infusorians,  such  as  ParamtE- 
duni,  has  been  studied  with  great  care  by  Gruber,  Maupas,  R.  Hertwig, 
and  others,  and  though  their  results  are  not  quite  harmonious,  the  main 
facts  are  secure.  In  many  ciliated  Infusorians  there  are  two  nuclear 
bodies,  one  large,  the  other  small.  The  smaller  or  micronucleus  lies 
by  the  side  of  the  larger  or  macronucleus.  The  micronucleus  divides 
into  parts,  while  the  macronucleus  degenerates.  Two  individual 
Infusorians  (A  and  B)  lie  side  by  side  in  close  contact,  a  portion  of  the 
micronucleus  of  A  passes  into  B,  and  fuses  with  a  portion  of  the  micro- 
nucleus  of  B,  similarly  a  portion  of  the  micronucleus  of  B  passes  into 
A,  and  fuses  with  a  portion  of  the  micronucleus  of  A.  In  short,  mutual 
fertilisation  occurs,  the  conjugating  individuals  separate,  a  new  micro- 
nucleus  and  a  new  macronucleus  are  established  in  each. 

The  precise  interpretation  of  the  process  is  to  some  extent  a  matter  of 
mere  opinion.  We  may  regard  it  as  a  mutual  rejuvenescence,  each 
unit  supplying  some  substances  or  qualities  which  the  other  lacks  ;  or 
we  may  regard  it  rather  as  a  process  by  which  the  average  character  of 
the  species  is  sustained,  peculiarities  or  pathological  variations  of  one 
individual  being  counteracted  by  other  characters  in  the  neighbour 
(apparently  no  near  relation)  with  which  it  conjugates  ;  or  we  may  see 
in  it  a  source  of  variation  as  the  result  of  new  combinations  among  the 
essential  hereditary  substances.  The  researches  of  M.  Maupas  have 
thrown  much  light  on  the  facts,  and  some  of  his  results  deserve 
summary. 

It  has  been  often  alleged  that  the  subsequent  dividing  is  accelerated 
by  conjugation  ;  but  Maupas  finds  that  this  is  by  no  means  the  case. 
The  reverse  in  fact  is  true.  While  a  pair  of  Infusorians  ( Onychodromus 


BIONOMICS.  113 

grandis]  were  engaged  in  conjugation,  a  single  individual  had,  by 
ordinary  asexual  division,  given  rise  to  a  family  of  from  forty  thousand 
to  fifty  thousand  individuals.  Moreover,  the  intense  internal  changes 
preparatory  to  fertilisation,  and  the  general  inertia  during  subsequent 
reconstruction,  not  only  involve  loss  of  time,  but  expose  the  Infusorians 
to  great  risk.  Conjugation  seems  to  involve  danger  and  death  rather 
than  to  conduce  to  multiplication  and  birth. 

The  riddle  was,  in  part  at  least,  solved  by  a  long  series  of  careful 
observations.  In  November  1885,  M.  Maupas  isolated  an  Infusorian 
(Sty Ionic kia  pustulata)  and  observed  its  generations  till  March  1886. 
By  that  time  there  had  been  two  hundred  and  fifteen  generations  pro- 
duced by  ordinary  division,  and  since  these  lowly  organisms  do  not  con- 
jugate with  near  relatives,  there  had  been  no  conjugation. 

What  was  the  result?  At  the  date  referred  to,  the  family  was 
observed  to  have  exhausted  itself.  The  members  were  being  born  old 
and  debilitated.  The  asexual  division  came  to  a  standstill,  and  the 
powers  of  nutrition  were  lost. 

Meanwhile,  before  the  generations  had  exhausted  themselves,  several 
of  the  individuals  had  been  restored  to  their  natural  conditions,  where 
they  conjugated  with  unrelated  forms  of  the  same  species.  One  of 
these  was  again  isolated,  and  watched  for  five  months.  In  this  case  up 
till  the  one  hundred  and  thirtieth  generation,  it  was  found  that  on 
removal  to  fresh  conditions  the  organisms  were  capable  of  conjugating 
with  unrelated  forms.  Later  this  power  was  lost,  and  at  the  one 
hundred  and  eightieth  generation  the  individuals  of  the  same  family 
were  observed  making  a  vain  attempt  to  conjugate  with  each  other. 

We  thus  see  that  without  normal  conjugation  the  whole  family 
becomes  senile,  degenerates  both  morphologically  and  physiologically. 
Morphologically,  the  individuals  decrease  in  size,  until  they  measure  only 
a  quarter  of  their  original  proportions,  the  micronucleus  atrophies  com- 
pletely or  partially,  the  chromatin  of  the  macronucleus  gradually 
disappears,  other  internal  structures  also  degenerate.  Physiologically, 
the  powers  of  nutrition,  division,  and  conjugation  come  to  a  standstill, 
and  this  senile  decay  of  the  isolated  individuals  or  family  inevitably 
ends  in  death. 

The  general  conclusion  is  evident.  Sexual  union  in  those  Infusorians, 
dangerous,  perhaps,  for  the  individual  life,  and  a  loss  of  time  so  far  as 
immediate  multiplication  is  concerned,  is  absolutely  necessary  for  the 
species.  The  life  runs  in  strictly  limited  cycles  of  asexual  division. 
Conjugation  with  allied  forms  must  occur,  else  the  whole  life  ebbs. 
Without  it,  the  Protozoa,  which  some  have  called  "immortal,"  die  a 
natural  death.  Conjugation  is  the  necessary  condition  of  their  eternal 
youth. 

Bionomics. — Many  Protozoa  raise  organic  debris  once 
more  into  the  circle  of  life,  and  many  form  part  of  the  food 
of  higher  animals.  Thus,  those  pelagic  Foraminifera  and 
Radiolarians,  which  dying  sink  to  the  great  oceanic  depths, 
form  along  with  more  substantial  debris  the  fundamental 
food  supply  in  that  plantless  world.  Fundamental,  since  it 

8 


H4  PROTOZOA. 

is  plain  that  the  deep  sea  animals  cannot  all  be  living  on  one 
another. 

Almost  every  kind  of  nutritive  relation  occurs  among  the 
Protozoa.  Predatory  life  is  well  illustrated  by  most  In- 
fusorians,  and  thorough  going  parasitism  by  the  Gregarines ; 
Opalina  in  the  rectum  of  the  frog  may  serve  as  a  type  of 
those  which  feed  on  decaying  debris,  and  Volvox  of  those 
which  are  holophytic.  Radiolarians,  with  their  partner  Algae, 
exhibit  the  mutual  benefits  of  symbiosis,  the  plants  utilising 
the  carbon  dioxide  of  their  transparent  bearers,  the  animals 
being  aerated  by  the  oxygen  which  the  plants  give  off  in 
sunlight,  and  probably  nourished  by  the  carbohydrates  which 
they  build  up.  Some  of  the  parasitic  forms,  especially 
among  the  Sporozoa,  are  of  serious  importance  to  higher 
animals. 

Though  Protozoa  may  be  seriously  infected  by  Bacteria, 
Acineta  parasites,  some  fungi,  like  Chytridium,  &c.,  fatal 
infection  is  rare,  because  of  the  power  of  intracellular 
digestion  which  most  Protozoa  possess.  "The  parasite," 
Metchnikoff  says,  "makes  its  onslaught  by  secreting  toxic 
or  solvent  substances,  and  defends  itself  by  paralysing  the 
digestive  and  expulsive  activity  of  its  host ;  while  the  latter 
exercises  a  deleterious  influence  on  the  aggressor  by  digesting 
it  and  turning  it  out  of  the  body,  and  defends  itself  by  the 
secretions  with  which  it  surrounds  itself."  With  this  struggle 
should  be  compared  that  between  phagocytes  and  Bacteria 
in  most  multicellular  animals. 

Few  Protozoa  come  into  direct  touch  with  human  life, 
but  man  has  several  Protozoon  parasites,  e.g.,  Amoeba  coli, 
associated  with  inflammation  of  the  intestinal  mucous  mem- 
brane, Coccidium  oviforme  (Sporozoa),  affecting  the  liver,  and 
various  Infusorians.  On  the  other  hand,  the  shells  of 
Protozoa  deposited  as  ooze  in  ancient  days,  have  formed 
important  deposits,  such  as  chalk  and  Barbadoes  Earth. 

General  Zoological  Interest. — The  Protozoa  illustrate,  in 
free  and  single  life,  forms  and  functions  like  those  of  the 
cells  which  compose  the  many  celled  animals.  Typically, 
they  show  great  structural  or  morphological  simplicity,  but 
great  physiological  complexity.  Within  its  single  cell,  the 
Protozoon  discharges  all  the  usual  functions,  while  in  a  higher 
animal  distinct  sets  of  cells  have  been  specialised  for  various 


GENERAL   ZOOLOGICAL   INTEREST.  115 

activities,  and  each  cell  has  usually  one  function  dominant 
over  the  others.  The  Metazoan  cells,  in  acquiring  an  in- 
creased power  of  doing  one  thing,  have  lost  the  Protozoan 
power  of  doing  many  things. 

The  Protozoa  remain  at  the  level  represented  by  the 
reproductive  cells  of  higher  forms,  and  are  comparable  to 
\  reproductive  cells  which  have  not  formed  bodies.  In  the 
sexual  colonies  of  Volvox,  however,  we  see  the  beginning  of 
that  difference  between  reproductive  cells  and  body  cells 
which  has  become  so  characteristic  of  Metazoa.  The 
Protozoa  are  self-recuperative,  and  in  normal  conditions 
they  are  not  so  liable  to  "  natural  death  "  as  are  many  celled 
animals.  Weismann  and  others  maintain  that  they  are 
physically  immortal. 

They  illustrate  (a)  the  beginnings  of  reproduction,  from 
mere  breakage  to  definite  division,  either  into  two  as  in 
fission,  or  in  limited  time  and  space  into  many  units,  as  in 
the  formation  of  spores  within  a  cyst ;  (b)  the  beginnings  of 
fertilisation,  from  "  the  flowing  together  of  exhausted  cells  " 
and  multiple  conjugation  to  the  specialised  sexual  union  of 
some  Infusorians,  where  two  individuals  become  closely 
united ;  (c)  the  beginnings  of  sex,  in  the  difference  of  size 
and  of  constitution  sometimes  observed  between  two  con- 
jugating units ;  (d}  the  beginnings  of  many  celled  animals 
in  the  associated  groups  or  colonies  which  occur  in  several 
of  the  Protozoan  classes.  These  colonies  show  a  gradation 
in  complexity.  Raphidiophrys  and  other  Heliozoa  form 
loose  colonies,  which  arise  by  the  want  of  separation  of  the 
products  of  fission.  Among  the  Radiolarians,  there  are  several 
colonial  forms,  in  these  the  individuals  are  united  by  their 
extra-capsular  protoplasm,  but  are  all  equivalent.  In  Pro- 
teropongia  the  cells  show  considerable  morphological  dis- 
tinctiveness,  some  are  flagellate,  some  amoeboid,  some 
encysted  and  spore  forming.  Again,  in  Volvox,  as  we 
noticed  above,  the  cells  of  the  colonies  show  a  distinction 
into  nutritive  and  reproductive  units. 

Lastly,  in  their  antithesis  of  passivity  and  activity,  con- 
structive and  destructive  preponderance,  anabolism  and 
katabolism,  the  Protozoa  illustrate  the  phases  of  the  cell 
cycle,  and  so  furnish  a  key  to  the  variation  of  higher  animals. 


CHAPTER    VIII. 


PORIFERA  -  SPONGES. 

A.  Calcarea  (Calcispongise). 

B.  Non-Calcarea. 

Hexactinellida. 


Demospongke.         onaxona. 

^  letractmellida. 

SPONGES  seem  to  have  been  the  first  animals  to  attain 
marked  success  in  the  formation  of  a  "body."  For  though 
their  details  are  often  complex,  their  general  structure  is 
simpler  than  the  average  of  any  other  class  of  Metazoa,  and 
some  of  the  simplest  forms  do  not  rise  high  above  the  level 
of  the  gastrula  embryo.  A  "  body  "  has  been  gained,  but  it 
shows  relatively  little  division  of  labour  or  unified  life  ;  it  is 
a  community  of  cells  imperfectly  integrated.  There  are  no 
definite  organs,  and  the  tissues  are,  as  it  were,  in  the 
making.  Sponges  are  passive,  vegetative  animals,  and  do 
not  seem  to  have  led  on  to  anything  higher  ;  but  they  are 
successful  in  the  struggle  for  existence,  and  are  strong  in 
numbers  alike  of  species  and  of  individuals. 

General  Characters. 

Sponges  are  diploblastic  (two  layered)  Metazoa,  the  middle 
stratum  of  cells  —  the  mesoglcea  —  not  attaining  to  the  definiteness 
of  a  proper  mesoderm.  There  is  no  cwlome  or  body  cavity. 
The  longitudinal  axis  of  the  body  corresponds  to  that  of  the 
embryo  :  in  other  words,  the  general  symmetry  of  tjie  gastrula 
is  retained.  In  these  three  characters  the  Sponges  agree  with 
the  Ccelentera  and  differ  from  higher  (triploblastic  and 
cwlomate)  Metazoa. 

The  body  varies  greatly  in  shape,  even  within   the  same 


DESCRIPTION  OF  A   SIMPLE  SPONGE. 


117 


species.  It  is  traversed  by  canals,  through  which  currents  of 
ivater  bear  food  inwards  and  waste  outwards.  Numerous 
minute  pores  on  the  surface  open  into  afferent  canals,  leading 
into  a  cavity  or  cavities  lined  by  endoderm  cells,  many  or  all 
of  which  are  flagellate.  To  the  activity  of  the  flagella  the  all- 
important  water  currents  are  due.  The  endodermic  or  gastric 
cavity  may  be  a  simple  tube,  or  it  may  have  radially  outgrow- 
ing chambers,  or  it  may  be  represented  by  branched  spaces, 
from  which  efferent  canals  lead  to  the  exterior.  Where  there 
is  a  distinct  central  cavity  there  is  usually 
but  one  large  exhalent  aperture  (osculum), 
but  in  other  cases  there  are  many  exhalent 
apertures. 

The  ectoderm  is  the  least  important 
layer;  it  covers  the  body,  and  is  perhaps 
continued  into  the  afferent  canals;  the 
endoderm  lines  most  of  the  internal  cavities, 
and  is  typically  flagellate ;  the  intervening 
mesoglcea  contains  a  skeleton  of  lime,  flint, 
or  spongin  ;  amoeboid  cells  or  phagocytes,  im- 
portant in  digestion  and  excretion ;  repro- 
ductive cells,  and  other  elements. 

Budding  is  very  common,  and  in  a  few 
cases  buds  are  set  adrift.  Both  herma- 
phrodite and  unisexual  forms  occur.  The 
sexually  produced  embryo  is  almost  always 
developed  within  the  mesoglcea,  and  leaves 
the  sponge  as  a  ciliated  larva.  With  the 
exception  of  one  family,  all  are  marine. 

Description  of  a  Simple  Sponge. 

A  very  simple  sponge,  such  as  Ascetta, 
is  a  hollow  vase,  moored  at  one  end  to  rock  or  seaweed, 
with  a  large  exhalent  aperture  at  the  opposite  pole,  and 
with  numerous  minute  inhalent  pores  through  the  walls. 
These  walls  consist  of  (i)  a  flat  ectoderm ;  (2)  a  mesoglcea 
containing  triradiate  calcareous  spicules,  phagocytes,  and 
reproductive  elements ;  and  (3)  an  endoderm  lining  the 
central  cavity,  and  composed  of  collared  flagellate  cells, 
almost  exactly  like  some  of  the  monad  Infusorians.  This 
simple  sponge  is  not  much  above  the  gastrula  level ;  it 


FIG.  31. — Simple 
Sponge  (Ascetta 
primordialis}. 

(After  H^CKEL.) 

Note  the  vase-like 
form,  the  apical  oscu- 
lum, the  inhalent  pores 
in  the  walls. 


PORIFERA— SPONGES. 


agrees  generally  with  a  simple  Ccelenterate,  such  as  Hydra, 
but  differs  from  it  in  the  absence  of  tentacles  and  stinging 
cells,  and  in  the  greater  development  of  the  mesoglcea. 

More  Complicated  Forms. 

But  a  description  of  a  simple  sponge  like  Ascetta  conveys 
little  idea  of  the  structure  of  a  complex  form  such  as  the 
bath  sponge  (Euspongia).  Let  us  consider  the  origin  of 
complications: — 

(a)  Sponges — long  regarded  as  plants— are  plant-like  in 
being  sedentary  and  passive.     They 

seem  also  to  feed  easily  and  well. 
Like  plants,  they  form  buds,  the  out- 
come of  surplus  nourishment.  These 
buds,  like  the  suckers  of  a  rose  bush, 
often  acquire  some  apparent  inde- 
pendence, and  the  sponge  looks  like 
many  vases,  not  like  one.  More- 
over, as  they  grow  these  buds  may 
fuse,  like  the  branches  of  a  tree  tied 
closely  together.  Thus  the  structure 
becomes  more  intricate. 

(b)  In    the    simple    sponge    the 
gastric  cavity  of  the  vase  is  com- 
pletely lined  by  the  collared  endo- 
derm  cells   (Ascon  type).     But  the 
endoderm  may  grow  out  into  radial 
chambers,   and   the  walls    of  these 
may  also  be  folded  into  side  aisles 
(Sycon   type).     The    outgrowing    of 
the   endoderm   into    the    mesoglcea 

may  be  continued  even  further,  and  the  cells  may  become 
pavement-like  except  in  minute  flagellate  chambers,  where 
the  characteristic  collared  type  is  retained  (Leucon  type). 
(See  Fig.  33.) 

[Speculatively  it  may  be  suggested  that  the  characteristic 
folding  or  outgrowth  of  the  endoderm  is  necessitated  by  the 
fact  that  the  endoderm  cells  are  better  nourished  and 
multiply  more  rapidly  than  those  of  the  ectoderm,  which 
thus  fails  to  keep  pace  with  the  inner  layer.] 

(c)  By  infoldings  of  the  skin — ectoderm  and  a  subjacent 


FIG.  32.— Section  of  a 
Sponge.  (After  F.  E. 
SCHULZE.) 

Showing  inhalent  canals, 
flagellate  chambers,  a  gastrula 
forming  in  the  mesoglcea,  &c. 


COMPLICATED  FORMS   OF  SPONGES. 


119 


sheath   of  mesogloea — subdermal 


FIG.  33. — Diagram  showing  types  of 
Canal  System.  (After  KORSCHELT  and 
HEIDER.)  The  flagellate  regions  are 
dark  throughout,  the  mesogloea  is  dot- 
ted, the  arrows  show  the  direction  of 
the  currents.  All  the  figures  represent 
cross  sections  through  the  wall. 

A.  Simple  A  scon  type,  EC.  ectoderm,  En. 

endoderm,  Mg.  mesogloea. 

B.  Sycon     type,     with     flagellate     radial 

chambers  (r.c.). 

C.  Leucon  type,  with  flagellate  side  aisles 

^on  the  main  radial  chambers. 

D.  Still   more   complex   type,  with   small 

flagellate  chambers,/;  ch. 

chambers,  by  flat  epithelium  with 


spaces  may  be  formed  ; 
an  outer  cortex  may  be 
distinctly  differentiated 
from  the  internal  region 
in  which  the  flagellate 
chambers  occur  ;  the 
pores  may  collect  into 
sieve -like  areas  which 
open  into  dome  -  like 
cavities;  these  and  many 
other  complications  are 
common. 

(d}  The  ectoderm  is 
usually  described  as  a 
covering  layer  of  flat 
epithelium,  but  flask 
shaped  cells  have  also 
been  observed  (Bidder). 
It  may  be  folded  in- 
wards, as  we  have 
noticed,  and,  according 
to  some,  it  also  lines 
the  incurrent  or  afferent 
canals  in  whole  or  in 
part.  In  a  few  cases, 
e.g.,  Oscarella  lobularis, 
it  is  ciliated,  and  its  cells 
may  also  exhibit  con- 
tractility, as  around  the 
osculum  of  Ascetta  cla- 
thrus,  though  the  con- 
tractile elements  usu- 
ally belong  to  the  meso- 
glrea. 

The  endoderm  con- 
sists typically  of  collared 
flagellate  cells,  but  in  the 
more  complex  sponges 
these  are  replaced,  ex- 
cept in  the  flagellate 
or  without  flagella. 


1 20  PORIFERA— SPONGES. 

The  mesoglota  contains  very  varied  elements,  and  illus- 
trates the  beginnings  of  different  kinds  of  tissue.  Thus 
there  are  migrant  amoeboid  cells  (phagocytes) ;  irregular 
connective  tissue  cells  embedded  in  a  little  jelly  ;  spindle 
shaped  connective  tissue  cells,  united  into  fibrous  strands  ; 
contractile  cells,  e.g.,  those  forming  a  sphincter  around  the 
oscula  of  some  forms,  such  as  Pachymatisma ;  skeleton 
making  cells ;  pigment  containing  cells ;  supposed  nerve 
cells,  projecting  on  the  surface,  and  believed  to  be  connected 
internally  with  multipolar  (ganglion  ?)  cells ;  and  lastly,  the 
reproductive  cells,  which  are  connected  by  transitional 
forms  with  the  ordinary  phagocytes. 

(e)  The  skeleton  consists  of  calcareous  or  siliceous 
spicules,  or  of  spongin  fibres,  or  of  combinations  of  the 
two  last.  A  calcareous  spicule  is  formed  of  calcite,  with  a 
slight  sheath  and  core  of  organic  matter  ;  a  siliceous  spicule 
is  formed  of  colloid  silica  or  opal ;  the  spongin  is  chemically 
somewhat  like  silk.  Uniradiate,  biradiate,  triradiate,  quadri- 
radiate,  sexradiate,  and  multiradiate  spicules  occur,  and  in 
a  general  way  it  may  be  said  that  they  are  arranged  so  that 
they  give  most  architectural  stability.  Each  is  formed 
within  a  single  cell,  and  may  be  speculatively  regarded  as 
an  organised  excretion.  "  During  its  growth,"  Prof.  Sollas 
says,  "  the  spicule  slowly  passes  from  the  interior  to  the 
exterior  of  the  sponge,  and  is  finally  (in  at  least  some 
sponges — Geodia,  Stelletta),  cast  out  as  an  effete  product." 
The  fibres  of  spongin  are  formed  as  the  secretions  of 
mesoglcea  cells  known  as  spongioblasts. 

Ordinary  Functions. 

Excepting  the  fresh  water  Spongillidae,  all  Sponges  are 
marine,  occurring  from  between  tide  marks  to  great  depths. 
After  embryonic  life  is  past,  they  live  moored  to  rocks, 
shells,  seaweeds,  and  the  like.  Their  motor  activity  is 
almost  completely  restricted  to  the  lashing  movements  of 
the  flagella,  the  migrations  of  the  phagocytes,  and  the  con- 
traction of  muscular  mesoglceal  cells,  especially  around  the 
exhalent  apertures.  In  the  closure  of  the  inhalent  pores, 
sponges  show  sensitiveness  to  injurious  influences,  but  how 
far  this  is  localised  in  specialised  cells  is  uncertain. 


REPRODUCTION  OF  SPONGES.  121 

The  most  important  fact  in  the  life  of  a  Sponge  is  that 
which  Robert  Grant  first  observed, — that  currents  of  water 
pass  gently  in  by  the  inhalent  pores,  and  more  forcibly 
out  by  the  exhalent  aperture  or  apertures.  This  may  be 
demonstrated  by  adding  powdered  carmine  to  the  water. 
The  instreaming  currents  of  water  bear  dissolved  air  and 
supplies  of  food,  such  as  Infusorians,  Diatoms,  and  particles 
of  organic  debris.  The  outflowing  current  carries  away 
waste.  When  a  sponge  is  fed  with  readily  recognisable 
substances,  such  as  carmine  or  milk,  and  afterwards  sec- 
tioned, the  grains  or  globules  may  be  found  (a)  in  the 
collared  endoderm  cells ;  (b)  in  the  adjacent  phagocytes  of 
the  mesoglrea ;  (c)  in  the  phagocytes  surrounding  the  sub- 
dermal  spaces,  if  these  exist.  It  is  uncertain  whether  the 
epithelium  of  the  subdermal  spaces  or  the  collared 
endoderm  is  the  more  important  area  of  absorption, 
but  it  is  certain  that  the  phagocytes  play  an  important 
part  in  engulfing  and  transporting  particles,  in  digesting 
those  which  are  useful,  and  in  getting  rid  of  the  useless. 
In  an  extract  of  several  sponges,  Krukenberg  found  a 
(tryptic)  digestive  ferment,  probably  formed  within  the 
phagocytes. 

Many  sponges  contain  much  pigment,  thus  the  lipochrome 
pigment  (see  Chap.  XXIX.)  zoonerythrin  is  common,  and 
like  some  others,  such  as  floridine,  is  regarded  as  helping  in 
respiration.  The  green  pigment  of  the  fresh  water  sponge 
is  closely  analogous,  if  not  identical,  with  chlorophyll,  and 
probably  renders  some  measure  of  holophytic  nutrition 
possible. 

Reproduction. 

Sponge  growers  often  cut  a  sponge  into  pieces,  and  bed 
these  out  in  suitable  places.  The  parts  regenerate  the 
whole — a  fact  which  illustrates  the  relatively  undifferentiated 
state  of  the  sponge  body.  It  is  possible  that  fission  may 
also  occur  naturally. 

The  frequent  budding  is  merely  a  kind  of  continuous 
growth,  but  when  buds  are  set  adrift,  as  sometimes  happens, 
we  have  discontinuous  growth  or  asexual  reproduction. 

In  the  fresh  water  Spongillidoe  there  is  a  peculiar  mode  of  reproduc- 
tion by  statoblasts  or  gemmules.  A  number  of  mesogloeal  cells  occur 


122  PORIFERA—  SPONGES. 

in  a  clump,  some  forming  an  internal  mass,  others  a  complex  protective 
capsule  with  capstan-like  spicules,  known  as  amphidiscs.  According  to 
W.  Marshall,  the  life  history  is  as  follows  :  In  autumn  the  sponge  suffers 
from  the  cold  and  the  scarcity  of  food,  and  dies  away..  But  throughout  the 
moribund  parent  gemmules  are  formed.  These  survive  the  winter,  and 
in  April  or  May  they  float  away  from  the  dead  parent,  and  develop 
into  new  sponges.  Some  become  short  lived  males,  others  more  stable 
females.  The  ova  produced  by  the  latter  and  fertilised  by  spermatozoa 
from  the  former,  develop  into  a  summer  generation  of  sponges,  which  in 
turn  die  away  in  autumn  and  give  rise  to  gemmules.  The  life  history 
thus  illustrates  what  is  called  alternation  of  generations  (see  p.  55). 
Interpreted  from  a  utilitarian  point  of  view,  the  formation  of  gemmules 
is  a  life  saving  expedient.  As  Prof.  Sollas  says,  "  the  gemmules  serve 
primarily  a  protective  purpose,  ensuring  the  persistence  of  the  race, 
while,  as  a  secondary  function,  they  serve  for  dispersal." 

All  Sponges  produce  sex  cells,  which  seem  to  arise  from 
amoeboid  mesoglcea  cells  retaining  an  embryonic  character. 
In  the  case  of  the  ovum,  the  amoeboid  cell  increases  in  size, 
and  passes  into  a  resting  stage ;  in  the  case  of  the  male 
elements,  the  amoeboid  cell  divides  into  a  spherical  cluster 
of  numerous  minute  spermatozoa.  The  similar  origin  of 
the  ova  and  spermatozoa  is  of  interest.  Most  sponges  are 
unisexual,  but  many  are  hermaphrodite.  In  the  latter  case, 
however,  either  the  production  of  ova  or  the  production  of 
spermatozoa  usually  preponderates,  probably  in  dependence 
on  nutritive  conditions. 

Development. 

It  is  not  surprising  to  find  that  there  is  great  variety  of 
development  in  the  lowest  class  of  Metazoa  ;  it  seems  almost 
as  if  numerous  experiments  had  been  made,  none  attended 
with  progressive  success. 

The  minute  ovum,  without  any  protective  membrane, 
usually  lies  near  one  of  the  canals,  and  is  fertilised  by  a 
spermatozoon  borne  to  it  by  the  water.  It  exhibits  a  certain 
power  of  migration  as  in  some  Hydroids.  Previous  to 
fertilisation,  the  usual  extrusion  of  polar  bodies  has  been 
observed  in  a  few  cases,  and  is  doubtless  general.  Seg- 
mentation is  total  and  usually  equal,  and  results  in  a 
spherical  or  oval  embryo  more  or  less  flagellate.  This 
leaves  the  parent  sponge,  swims  about  for  a  time,  then 
settles  down,  and  undergoes  a  larval  metamorphosis  often 
difficult  to  understand.  It  is  peculiarly  difficult  to  bring  the 


DEVELOPMENT  OF  SPONGES.  123 

history  of  the  germinal  layers  in  Sponges  into  line  with  that 
in  other  Metazoa. 

(a)  In  the  small  calcareous  sponge  Sycandra  raphanus,  as  described  by 
F.  E.  Schulze,  the  segmentation  results  in  a  hollow  ball  of  cells — the 
blastula.  A  few  cells  at  the  lower  pole  remain  large,  and  are  filled  with 
nutritive  granules  ;  the  other  cells  divide  rapidly  and  become  small, 
clear,  columnar,  and  flagellate.  The  large  granular  cells  become 

temporarily  invaginated,  form- 
ing what  is  called  a  "pseudo- 

^0j^^  gastrula"       This     leaves      the 

;fE;  parent   and    the    granular   cells 

$  right    themselves,    forming    the 

posterior     hemisphere     of     the 

^^^^^  embryo,  now  called  an  amphi- 

blastula.     It  swims  for  a   time 

xf^y^x  actively,  but  the  flagellate  cells 

of  the  anterior  hemisphere  are 
invaginated  into  or  overgrown 
by  the  large  granular  cells,  and 
thus  what  is  generally  called 
the  gastrula  stage  results.  This 
soon  settles  down,  on  rock  or 
seaweed,  with  the  blastopore  or 
gastrula  mouth  downwards,  and 
is  moored  by  amoeboid  pro- 
cesses from  the  granular  cells, 
which  likewise  obliterate  the 
blastopore.  The  granular  cells 
lose  their  granules,  for  the  larva 
is  not  yet  feeding ;  the  now  in- 
ternal flagella  disappear  in  the 
absence  of  the  stimulating  water ; 
a  mesoglcea  with  spicules  begins 
to  be  formed  between  the  inner 

FIG.  34. — Development  of 
Sycandra  raphanus.  (After  F. 
E.  SCHULZE.) 


1.  Ovum. 

2.  Section  of  16  cell  stage. 

3.  Blastula  with  8  granular  cells 

(^r.c.)  at  lower  pole. 

4.  Free  swimming  amphiblastula, 
En  with     upper     hemisphere    of 

flagellate  cells  (f.c.\  and 
lower  hemisphere  of  granu- 
'ar  cells. 

5.  Gastrula   stage   settled    down. 
^^  EC. ,  outer  layer  (ectoderm  ?  ) ; 

En. ,  inner  layer(endoderm  ?  ) ; 
bl.,  closing  blastopore;  am.p., 
mooring  amoeboid  processes. 


124 


PORIFERA— SPONGES. 


and  outer  layer,  probably  by  migrants  from  the  latter.  But  this  dis- 
advantageous state  of  affairs  cannot  last.  Pores  open  through  the 
walls,  the  entrance  of  water  enables  the  inner  cells  to  recover  their 
flagella,  and  an  exhalent  aperture  is  ruptured  at  the  upper  pole.  The 
young  sponge  is  now  in  an  Ascon  stage,  from  which,  by  the  out- 
growth (?)  of  the  inner  layer  into 
radial  chambers,  it  passes  into  the  per- 
manent Sycon  form,  grows  into  a  cylin- 
der, and  becomes  differentiated  in 
detail  (Fig.  34). 

(b]  In  Oscarella  (Halisarca]  lobularis, 
a    sponge    without    any    skeleton,    the 
ovum  segments  equally  into  a  blastula, 
which  is  flagellate  all  over.     This  free 
swimming    stage   may   be   invaginated 
from  either  pole  to  form  a  hemispherical 
gastrula,    which  settles    mouth    down- 
wards.    Pores,   an    osculum,    and   the 
mesoglcea   are   formed  as   before,   and 
the   inner   layer    becomes   folded   into 
flagellate   chambers.     It   may   be   sug- 
gested that  this  folding  is  due  to  the 
higher  nutrition,   and   consequent  dis- 
proportionate   growth,    of    the    inner 
layer,  for  a  rapidly  growing  sac  within 
one  of  less  rapid  growth  must  become 
folded  on  itself  (HEIDER). 

(c]  Another  type,  seen   for   instance 
in   a   horny  sponge,   Spongelia^   results 
in   a   flagellate   larva,   whose  cavity  is 
rilled  up  with  what  may  be  called  gela- 
tinous  connective    tissue,   from    which 
mesogloea   and    endoderm    are    subse- 
quently differentiated.     Such  a  larva  is 
called  a  parenchymula. 

As  these  are  not  all  the  types  of  de- 
velopment which  occur  among  sponges, 
the  general  fact  is  impressive  that 
in  this  lowest  class  of  Metazoa  there 
has  been  considerable  plasticity  in 
development. 


FIG.  35.  —  Diagrammatic 
representation  of  develop- 
ment of  Oscarella  lobularis. 
(After  HEIDER.) 

Bl.  Free  swimming  blastula  with 
flagella  ;  G.  gastrula  settled  down. 

Next  figure  shows  folding  of 
Endoderm  (En). 

Lowest  figure  shows  radial 
chambers  (A\C.)-  Mesoglcea  (Mg)  ; 
inhalent  pore  (/*.);  exhalent  oscu- 
lum (0.). 


Classification. 
A.  Port/era  Calcarea,  with  skeleton 

of  calcareous  spicules  : — 
Order  I. — Homocoela. — Endo- 
derm wholly  composed  of  collared  flagellate  cells, 
e.g.,  Ascetta^  Leucosolenia. 

Order  II. — Heterocoela. — Endoderm  consists  of  collared  flagel- 
late cells  in  radial  tubes  or  chambers,  and  of  flat 
epithelium  elsewhere,  e.g.,  Grant  ia,  Sycon. 


BIONOMICS.  125 

B.  For  if  era  non-Cakarea,  skeleton  of  silica  or  of  spongin,  or  of  both. 

(1)  Hexactinellida,    with    sexradiate    siliceous    spicules,    canal 

system  usually  simple,  with  Sycon  chambers.  The 
members  live  chiefly  in  deep  water,  e.g.,  Venus' 
Flower  Basket  (Euplectella)  and  the  Glass  Rope 
Sponge  (Hyalonenid]. 

(2)  Monaxonida,  with  siliceous  spicules  (which  are  not  quadri-  or 

sex-radiate),  or  with  "horny"  skeleton,  or  with 
both. 

Order  I, — Monaxona,  with  spicules  only,  e.g.,  Mermaid's 
Gloves  (Chalina  oculata),  Crumb  of  Bread 
Sponge  (Halichondria  or  Amorphina paniced), 
FreshWater  Sponge  (Spongilla}. 

Order  II. — Ceratosa,  "  horny  "  sponges  with  or  without 
spicules,  e.g. ,  the  Bath  Sponge  (Euspongia}. 

(3)  Tetractinellida,  mostly  with  quadriradiate  spicules,  or  with 

tritenes,  in  which  a  main  shaft  bears  at  one  end 
three  branches  diverging  at  equal  angles,  e.g., 
l^etilla,  Geodia,  Pachymatisma,  Plakina. 

There  are  also  a  few  sponges  (Myxospongioe)  without  any  skeleton, 
perhaps  survivals  of  primitive  types  (Oscarella,  Halisarca)  or  degraded 
for m s  ( Chondrosia ) . 

History. — Sponges,  as  one  would  expect,  date  back  almost  to  the 
beginning  of  the  geological  record.  Thus  the  siliceous  Protospongia 
occurs  in  Cambrian  rocks,  and  in  the  next  series — the  Silurian— the  main 
groups  are  already  represented.  From  that  time  till  now  they  have 
continued  to  abound  and  vary. 

Bionomics. — Sponges  are  living  thickets  in  which  many 
small  animals  play  hide-and-seek.  Many  of  the  associations 
are  practically  constant  and  harmless,  but  some  burrowing 
worms  do  the  sponges  much  damage.  The  spicules  and  a 
frequently  strong  taste  or  odour  doubtless  save  sponges 
from  being  more  molested  than  they  are ;  the  numerous 
phagocytes  wage  successful  war  with  intruding  micro- 
organisms. Some  sponges,  such  as  Clione  on  oyster  shells, 
are  borers,  and  others  smother  forms  of  life  as  passive  as 
themselves.  Several  crabs,  such  as  Dromia,  are  masked  by 
growths  of  sponge  on  their  shells,  and  the  free  transport  is 
doubtless  advantageous  to  the  sponge  till  the  crab  casts 
its  shell.  A  compact  orange  coloured  sponge  (Suberites 
domunculd)  of  peculiar  odour  often  grows  round  a  whelk 
shell  tenanted  by  a  hermit  crab,  and  gradually  eats  into  the 
shell  substance.  Within  several  sponges,  minute  Algae  live, 
like  the  "  yellow  cells  "  of  Radiolarians,  in  mutual  partner- 


1 26  PORIFERA— SPONGES. 

ship  or  symbiosis.      Finally,   sponges  deserve  mention  as 
factors  in  human  civilisation. 

General  Zoological  Interest  and  Position. — Sponges  have 
/this  great  interest,  that  they  form  the  first  successful  class 
Jof  Metazoa.     They  illustrate  the  beginnings  of  a  "  body  "- 
'  the  beginnings  of  tissues.     Along  with  the  Ccelentera,  from 
which  it  is  the  almost  unanimous  opinion  that  they  must 
be  held  distinct,  they  differ  markedly  from  the  triploblastic, 
Coelomate  Metazoa,  which  do  not  retain  the  radial  symmetry 
of  the  gastrula. 

Their  origin  is  wrapped  in  obscurity,  though  there  is 
much  to  be  said  for  the  view  that  they  are  the  non-pro- 
gressive descendants  of  primitive  gastrula-like  ancestors  of 
sluggish  constitution.  It  does  not  seem  likely  that  they 
have  led  on  to  anything  higher,  they  rather  represent  a 
by-road  in  Metazoan  evolution. 


MESOZOA. 


127 


APPENDIX    TO    SPONGES.       MESOZOA. 

The  title  Mesozoa  was  applied  by  Van  Beneden  to  some 
very  simple  organisms  which  appear  to  occupy  a  very 
humble  position  in  the  Metazoan  series.  The  name  sug- 
gests a  grade  between  the  Protozoa  and  the  Metazoa. 
Hseckel  called  some  of  them  Gastrseadae,  regarding  them 


A  B 

FIG.  36. — A.  Young  Dicyema.     (After  WHITMAN.) 

B.    Female  Orthonectid  (Rhopalura  Giardii}. 
(After  JULIN.) 

e.  Ectoderm  ;    en,   inner  endoderm  cell  with   nucleus  (n)  ;    and 
embryo  (em). 

as  slight  modifications  of  the  hypothetical  gastrula-like 
ancestors  of  the  Metazoa,  while  Hatschek,  comparing  them 
to  precociously  reproductive  planulae  speaks,  of  them  as 
Planuloidse.  It  is  also  possible  that  some  of  them  may  be 


128 


MESOZOA. 


It  will 


parasitic  degenerations  of  Turbellarian-like  worms, 
be  enough  here  merely  to  notice  four  types  : — 

(i.)  Dicyemidre  (type  Dicyema]  occur  as 
parasites  in  Cephalopods  ;  the  body  con- 
sists of  a  ciliated  outer  layer,  enclosing  a 
single  multinucleate  inner  cell,  within 
which  egg-like  germs  develop,  apparently 
without  fertilisation,  into  dimorphic  em- 
bryos (see  Fig.  36,  A). 

(2.)  Orthonectidne  (type  Rhopalura] 
occur  as  parasites  in  Turbellarians,  Brittle 
stars,  and  Nemerteans  ;  the  body  is  slightly 
ringed,  and  consists  of  a  ciliated  outer 
layer,  a  subjacent  sheath  of  contractile 
fibres,  and  an  internal  mass  of  cells,  among 
which  ova  and  spermatozoa  appear.  The 
sexes  are  separate  and  dimorphic  (see 
Fig.  36  B.). 

(3.)  Professor  F.  E.  Schulze  has  dis- 
covered a  small  marine  organism — Tricho- 
plax  adh<zrens — in  the  form  of  a  thin, 
three  layered,  externally  ciliated  plate  ;  and 
Monticelli  records  a  similar  form  under  the 
title  Treptoplax  adhcerens. 

(4.)  Professor  J.  Frenzel  has  discovered 
in  brine  solutions  a  minute  Turbellarian- 
like  organism — Salinella  salve — whose 
body  consists  of  one  layer  of  cells.  There 
is  an  anterior  mouth,  a  ciliated  food  canal, 
and  a  posterior  anus.  The  ventral  surface 
is  finely  ciliated,  the  rest  of  the  cells  bears 
short  bristles.  The  animal  reproduces  by 
transverse  fission,  but  conjugation  and 
encystation  also  occur.  The  larva  is  uni- 
cellular. 

These    forms    are    obviously    of    much 

interest  to  those  who  ponder  over  the  possible  nature  of  the  earliest 
multicellular  animals. 


Fig.  37. — Salinella. 
(After  FRENZEL.) 

1.  Longitudinal  section-  a, 
anterior  ;  /,  posterior. 

2.  Transverse  section. 


CHAPTER    IX. 

CCELENTERA. 

Class  ,  HYOROZOA  {  *$?£$£  «"      '•  H™^{ 

C/«*  2.  SCYPHOZOA  |  Acraspeda,  or    Class    II.  ACTINOZOA. 

|  Actmozoa. 
Class  3.  CTENOPHORA.  Class  III.  CTENOPHORA. 

THE  Coelentera  —  including  zoophytes,  jelly  fish,  sea 
anemones,  corals,  and  the  like — form  a  very  large  series  of 
Accelomate  Metazoa,  i.e.,  of  multicellular  animals  without  a 
body  cavity.  Their  simplest  forms  are  not  much  above  the 
level  of  the  simplest  sponges,  but  the  series  has  been  more 
progressive.  Thus  many  illustrate  the  beginnings  of  definite 
organs.  In  their  variety  they  seem  almost  to  exhaust  the 
possibilities  of  radial  symmetry,  and  some  types  may  be 
regarded  as  pioneers  of  the  yet  more  progressive  bilateral 
"worms."  Many  are  very  vegetative,  deserving  the  old 
name  of  zoophytes  (which  should  rather  be  read  backwards 
— Phytozoa),  and  in  their  budded  colonies  afford  most  in- 
teresting illustrations  of  organic  co-operation  and  division 
of  labour. 

General  Characters. 

The  Coelentera  are  simple,  usually  marine,  forms  in  which 
the  primary  long  axis  of  the  gastrula  becomes  the  long  axis  of 
the  adult,  which  is  almost  always  radially  symmetrical  about 
this  axis.  There  is  no  body  cavity  or  ccelome,  distinct  from 
the  primitive  digestive  cavity  (enter on)  and  its  outgrowths. 
In  the  lower  members  of  the  series,  the  primary  opening  of 
this  cavity  becomes  the  mouth  of  the  adult,  but  in  the  more 
specialised  types  there  is  an  (ectodermic)  oral  invagination, 
which  forms  a  gullet  tube.  Between  the  ectoderm  and 
endoderm  of  the  body  ivall,  there  is  a  supporting  layer,  or 

9 


130  CCELENTERA. 

mesoglcea,  of  jelly-like  consistency.  In  the  simplest  cases  this 
is  quite  devoid  of  'cells ',  but  secondarily,  these  may  migrate  into 
it  from  the  endoderm.  Stinging  cells  of  varying  complexity 
are  almost  invariably  present,  but  in  most  members  of  the 
class  Ctenophora  they  are  modified  into  adhesive  cells. 

The  Ccelentera  exhibit  tivo  divergent  types  of  structure, 
which  recur  constantly,  in  modified  forms,  throughout  the 
group,  and  may  even  be  both  present  in  the  course  of  one  life 
history,  when  they  illustrate  the  phenomenon  of  alternation  of 
generations  or  metagenesis.  Of  the  two,  the  more  primitive 
type  is  the  sessile  tubular  hydroid,  which  may  be  compared  to  a 
gastrula,  fixed  by  one  end,  and  furnished  with  a  crown  of 
tentacles  placed  round  the  central  aperture  of  the  other  pole. 
The  other  derived  form,  which  has  become  specialised  in 
various  directions,  is  the  active  medusoid  or  jelly  fish  type.  In 
several  divisions  the  formation  of  a  calcareous  "skeleton"  by 
the  hydroid  type  may  result  in  the  production  of  "corals" 
Multiplication  by  budding  is  common,  and  often  results  in  the 
formation  of  colonies,  some  of  which  shoiv  considerable  divi- 
sion of  labour. 

The  preservation  of  the  primary  axis,  the  absence  of  true 
mesoderm  and  of  a  ccelome,  are  often  said  to  distinguish 
Ccelentera  and  Sponges  from  other  Metazoa,  or  Ccelomata, 
but  the  results  of  recent  researches  on  the  nature  of  the 
mesoderm  seem  to  rob  this  distinction  of  part  of  its  precision. 

General  Survey. 

The  Ccelentera,  or  "  Nettle  animals "  of  the  Germans, 
include  a  large  number  of  familiar  and  beautiful  forms  of 
life.  The  graceful  zoophytes  which  fringe  shells  and  stones, 
and  the  tiny  transparent  bells  which  float  in  the  pools ;  the 
sea  anemones  which  cluster  in  the  nooks  of  the  rocks,  and 
the  active  jelly  fish  which  swim  on  the  waves,  are  but 
different  expressions  of  the  antithesis  so  characteristic  of 
the  series,  and  illustrate,  the  former  in  the  class  Hydrozoa, 
the  latter  in  the  Scyphozoa,  the  two  physiological  tendencies 
of  the  Ccelentera.  The  delicate  irridescent  globes,  which 
represent  the  third  class,  the  Ctenophora,  illustrate  the 
climax  of  activity,  for  among  them  there  is  no  sessile  hydroid 
type. 

In  our  survey  of  the  series,  however,  we  must  pass  over 


GENERAL  SURVEY  OF  CCELENTERA.  131 

these  familiar  types,  and  begin  with  the  little  fresh  water 
Hydra,  which  is  often  to  be  found  attached  to  the  stems 
and  leaves  of  water  plants.  The  structure  here  is  extremely 
simple,  but  the  simplicity  is  probably  due  to  degeneration. 
This  Hydra  was  first  described  in  1703  by  Leeuwenhoek, 
and  was  studied  by  some  of  the  older  naturalists,  such  as 
Rosel  von  Rosenhof  and  the  Abbe  Trembley,  with  much 
eagerness.  In  favourable  conditions  it  may  be  frequently 
observed  giving  off  daughter  buds,  which  remain  for  a  time 
attached  to  the  parent,  and  then  separate  as  independent 
Hydra.  The  bud  itself,  before  leaving  the  parent,  may 
also  bud,  so  that  three  generations  are  present.  If  we 
suppose  this  process  of  gemmation,  combined  with  imper- 
fect separation  of  the  units,  to  continue  indefinitely,  we  can 
understand  the  formation  of  hydroid  colonies,  such  as  the 
zoophytes,  where  the  colony  is  supported  by  an  organic 
axis  of  varying  complexity. 

The  members  of  such  a  colony  would,  however,  with  an 
exception  which  we  will  consider  later,  be  all  similar  and 
equivalent,  and  this  is  by  no  means  true  of  all  hydroid 
colonies.  In  Hydr actinia,  for  example,  which  is  common 
on  shells  at  the  shore,  the  colony  consists  of  polypes  of 
varied  structure  and  function.  It  may  be  that  these  differ- 
ences are  caused  by  differences  in  nutrition,  the  fact  at  any 
rate  is  that  some  of  the  polypes  are  nutritive  "  persons," 
like  Hydra  in  appearance ;  some  are  mouthless  reproductive 
u  persons,"  which  produce  sperms  and  eggs,  and  so  eventu- 
ally start  a  new  colony ;  others  also  mouthless,  are  long, 
slender,  sensitive,  and  abundantly  furnished  with  stinging 
cells ;  while  the  little  protecting  spines  at  the  base  of  the 
colony  may  perhaps  be  abortive  "  persons."  All  these 
polypes  are  united  by  connecting  canals  at  the  base,  and 
all  are  fed  at  the  expense  of  the  nutritive  "persons." 
Hydractinia  thus  exhibits  division  of  labour  among  the 
members  of  the  colony,  and  a  tendency  towards  this  is 
common  in  the  Coelentera. 

If  we  now  return  to  the  simpler  zoophyte  colony,  we  find 
that  this  tendency  can  be  recognised  even  here.  Like 
Hydractinia,  the  colony  at  intervals  exhibits  reproductive 
"  persons,"  different  from  the  ordinary  polypes.  These,  as 
in  Hydractinia,  may  be  sessile  and  mouthless,  or  they  may 


132 


CCELENTERA. 


after  a  time  become  detached  and  float  away  as  delicate, 
pulsating,  swimming  bells.  These  swimming  bells  are  male 
and  female,  they  give  rise  to  male  and  female  elements,  and 
so  to  embryos,  which,  after  a  time,  settle  down  and  form  new 
zoophyte  colonies.  This  is  an  instance  of  the  phenomenon 
of  alternation  of  generations  (see  p.  55). 

Again,  just  as  the  predominance  of  passivity  is  exhibited 
in    Hydractinia    and    some    zoophytes,    where    the    active 


FIG.  38. — Diagram  of  Coelenterate  structure,  encloderm 
darker  throughout. 

1.  To  left,  shows  longitudinal  section  of  Hydra;  to  right,  of  Sea 
anemone,    g,  gut ;  gl. ,  incipient  gullet. 

2.  To  left,  shows  cross  section  of  Hydra  ;  to  right,  of  Sea  anemone. 

3.  To  left,  shows  vertical  section  of  Craspedote  Medusoid  (with 
velum)  ;  to  right,  of  Acraspedote  Medusa,  without  velum,    g;  gut ; 
gl. ,  gullet. 

Note  anatomical   correspondence   of   the   polype   and    medusoid 
forms. 

swimming  bell  is  left  out  of  the  life  history,  so  the  pre- 
dominance of  activity  is  exhibited  in  the  permanent 
medusoids,  e.g.,  Geryonia,  where  the  colonial  hydroid  stage  is 
omitted,  and  the  embryo  becomes  at  once  a  swimming  bell. 
Finally,  the  medusoids  themselves  may  become  colonial, 
and  we  have  active  floating  colonies,  like  those  of  the 


GENERAL   SURVEY  OF  CCELENTERA.  133 

Portugese  man-of-war,  which  show,  on  a  different  plane,  as 
much  division  of  labour  as  Hydractinia. 

All  these  types  belong  to  the  class  Hydrozoa,  but  the 
same  general  conclusions  apply  to  the  next  class,  that  of 
the  sea  anemone  and  jelly  fish.  The  jelly  fish  present  a 
strong  general  resemblance  to  the  medusoids,  but  are 
separated  from  them  by  their  usually  greater  size,  as  well  as 
by  greater  complexity  and  distinct  anatomical  differences. 
It  is  in  accordance  with  this  increased  complexity  that  the 
alternation  of  active  and  passive  forms,  though  as  real,  is 
less  obvious.  Yet  even  here  we  find  one  type  (Pelagid) 
always  locomotor,  another  (Aurelia)  whose  early  life  is 
sedentary,  and  others  (Lucernarians)  which  in  their  adult 
life  are  predominantly  passive,  and  attach  themselves  by  a 
stalk. 

The  sea  anemones  and  their  numerous  allies  may  be 
regarded  as  bearing  a  relation  to  the  jelly  fish,  somewhat 
similar  to  that  which  the  hydroid  polypes  bear  to  the  swim- 
ming bells  (Fig.  38).  They  are,  however,  much  more  com- 
plicated in  structure  than  the  hydroids.  Simple  forms  are 
much  commoner  than  in  the  Hydrozoa,  but  the  colonial 
type  is  nevertheless  very  frequent.  The  colonies  may  be 
supported  by  an  organic  framework  only,  but  very  com- 
monly there  is  a  tendency  to  accumulate  lime  in  the  tissues, 
which  results  in  the  formation  of  corals. 

It  may  be  well  to  note  here  explicitly  that  various  polypoid  types  I 
may  form  corals.  In  fact  the  formation  of  a  framework  of  carbonate  of 
lime  may  be  regarded  as  an  expression  of  the  sedentary  constitution. 
The  most  important  reef  building  corals  are  included  in  the  Scyphozoa, 
but  among  the  Hydrozoa  the  Millepores  form  very  considerable  lime 
sheathed  colonies. 

The  corals  present  many  problems  of  great  interest.  How  do  they 
get  their  carbonate  of  lime  ?  Is  that  salt  particularly  abundant  about 
the  reefs,  or  is  there,  as  Irvine  and  Murray  suggest,  an  interaction 
between  the  waste  products  of  the  polypes  and  the  sulphate  of  lime 
abundant  in  sea  water?  On  what  do  they  feed?  Do  their  bright  pig- 
ments, as  Hickson  suggests,  enable  them  to  utilise  carbonic  acid  after 
the  manner  of  plants  ?  We  may  think  also  of  the  struggle  for  standing- 
room  among  the  coral  polypes,  and  of  the  struggle  for  existence  among 
the  many  brightly  coloured  animals  which  browse  and  hide  on  the 
reef. 

Finally,  as  the  corals  are  predominantly  passive,  so  there 
is  a  climax  of  activity  in  the  Ctenophores,  which  move  by 


134 


CCELENTERA. 


cilia  united  into  combs,  and  often  shine  with  that  "  phos- 
phorescence "  which  is  an  expression  of  intensity  of  life  in 
many  active  animals.  The  Ctenophores  have  probably 
arisen  from  a  modified  Hydrozoon  medusoid. 

As  to  diet,  the  active  Ctenophores  are  carnivorous, 
attaching  themselves  by  adhesive  cells  to  one  another,  or  to 
other  small  animals ;  many  of  the  larger  forms,  e.g.,  sea 
anemones  and  jelly  fish  are  able  to  engulf  booty  of  con- 
siderable size ;  the  majority,  however,  feed  on  small  organ- 
isms, in  seizing  and  killing  which  the  tentacles  and  stinging 
cells  are  actively  used,  but  what  the^corals  eat  no  one  seems 
to  know. 

TYPES  OF  CCELENTERA. 
First  Type — HYDRA,  illustrative  of  the  Class  HYDROZOA. 

General  Life.  —  The  genus  Hydra  is  represented  by 
several  species,  e.g.,  the  green  Hydra  vindis  and  the 
brownish  Hydra  fusca,  both  widely 
distributed  in  fresh  water.  It  is  one 
of  the  simplest  of  Ccelentera,  for  the 
body  is  but  a  two-layered  tube,  with 
a  crown  of  (6-10)  hollow  tentacles 
around  the  mouth,  and  with  no  organs 
except  those  concerned  in  reproduc- 
tion. The  body  is  usually  fixed  by 
its  base  to  some  aquatic  plant,  often  to 
the  underside  of  a  duckweed  leaf.  It 
may  measure  \-\  inch  in  length,  but  it 
is  as  thin  as  a  needle,  and  contracts 
into  a  minute  knob. 

The  animal  sways  its  body  and 
tentacles  in  the  water,  and  it  can  also 
loosen  its  base,  lift  itself  by  its  ten- 
tacles, stand  on  its  head,  or  creep  by 
looping  movements.  Usually,  how- 
ever, it  prefers  a  quiet  life.  It  feeds  on  small  organisms, 
which  are  paralysed  or  killed  by  stinging  cells  on  the 
tentacles,  and  are  swept  into  the  tubular  cavity  of  the  body 
by  the  action  of  flagella  on  the  internal  cells.  Sometimes 
animals  as  large  as  water  fleas  (e.g.,  Daphnid)  are  caught, 
and  in  part  digested.  Infusorians  (Euplotes,  etc.)  are  often 


FIG.  39.  —  Hydra, 
hanging  from  water 
weed. 

(After  GREENE.) 

ov,  Ovary.     /,  Testes. 


HYDRA.  135 

seen  wandering  to  and  fro  on  the  surface  of  the  Hydra,  but 
these  wonted  visitors  do  not  seem  to  provoke  the  stinging 
cells  to  action. 

So  simple  is  Hydra  that  a  cut  off  tentacle,  or  a  fragment, 
containing  samples  of  the  various  kinds  of  cells  in  the  body, 
and  not  too  minute,  may  grow  into  an  entire  animal.  Thus 
the  Hydra  may  be  multiplied  by  being  cut  in  pieces.  If  the 
animal  be  turned  inside  out  (a  delicate  operation),  the  status 
quo  is  soon  restored.  The  Abbe  Trembley  who  first  made 
this  experiment  thought  that  the  out-turned  inner  layer 
or  endoderm  assumed  the  characters  of  the  outer  layer 
or  ectoderm,  and  that  the  inturned  ectoderm  assumed  the 
characters  of  endoderm.  But  this  is  not  the  case.  Either 
the  animal  rapidly  rights  itself  by  turning  outside  in,  or,  if 
this  be  prevented,  the  inturned  ectoderm  disappears  intern- 
ally, and  by  growing  over  the  out-turned  endoderm,  from 
the  lips  downwards,  restores  the  normal  state. 

In  favourable  nutritive  conditions,  the  Hydra  forms  buds, 
and  on  these  a  second  generation  of  buds  may  be  devel- 
oped. A  check  to  nutrition  or  some  other  influence  causes 
the  buds  to  be  set  adrift.  Besides  this  asexual  mode  of 
multiplication,  the  usual  sexual  reproduction  occurs. 

General  Structure.^—  The  tubular  body  consists  of  two 
layers  of  cells,  i.e.,  the  animal  is  diploblastic.  The  cavity  is 
the  gut,  and  it  is  continuous  with  the  hollow  tentacles. 
These,  when  fully  extended,  may  be  longer  than  the  body. 
The  mouth  is  slightly  raised  on  a  disc  or  hypostome.  Of 
the  two  layers  of  cells,  the  outer  or  ectoderm  is  transparent, 
the  inner  or  endoderm  usually  contains  abundant  pigment. 
On  the  tentacles  especially,  even  with  low  power,  one  can 
see  numerous  clumps  of  clear  stinging  cells.  The  male 
organs  appear  as  ectodermic  protuberances  a  short  distance 
below  the  bases  of  the  tentacles ;  the  ovary,  with  a  single 
ovum,  is  a  larger  bulging  further  down.  Both  male  and 
female  organs  may  occur  on  the  same  animal,  either  at  one 
time  or  at  different  times,  but  often  they  occur  on  different 
individuals.  The  buds  have  the  same  structure  as  the  parent 
body,  but  in  origin  they  appear  to  be  wholly  ectodermic. 

Minute  Structure. — The  outer  layer  or  ectoderm  includes  the  follow- 
ing different  kinds  of  cells  : — 

(i)   Large  covering  or  epithelial  cells,   within  or  between  some  of 


ler 

1S     1 

is    • 


136  CCELENTERA. 

which  lie  the  stinging  cells.  The  epithelial  cells  are  somewhat 
conical,  broader  externally  than  internally,  and  in  the  interspaces  lie 
interstitial  cells.  By  certain  methods,  a  thin  shred  can  be  peeled  off 
the  external  surface  of  the  ectoderm  cells.  This  is  a  cuticle ,  i.e.,  a 
pellicle  no  longer  living,  produced  by  the  underlying  cells. 

(la. )  Many  of  these  large  cells  have  contractile  basal  processes,  or 
roots,  running  parallel  to  the  long  axis  of  the  body,  and  lying  on  a 
middle  lamina  which  separates  ectoderm  from  endoderm  (Fig.  40,  E). 
The  cells  themselves  are  contractile,  but  there  is  special  contractility  in 
the  roots.  Like  the  muscle  cells  of  higher  animals,  they  contract  under 
certain  stimuli,  and  are  often  called  "  neuro-muscular."  But  the  dis- 
covery of  special  nerve  cells  (Jickeli)  shows  that  even  in  Hydra  there 
a  differentiation  of  the  two  functions  of  contractility  and  irritability. 

(2.)  Small  stinging  cells  or  cnidoblasts  occur  abundantly  on  the  upper 
parts  of  the  body,  especially  on  the  tentacles.  Each  contains  a  pro- 
trusible  structure  called  a  nematocyst.  This  consists  of  a  sack,  the  neck 
of  which  is  doubled  in  as  a  pouch,  usually  bearing  internal  barbs,  and 
prolonged  into  a  long,  hollow,  spirally  coiled  filament  or  lasso.  This 
lasso  is  bathed  in  a  fluid,  presumably  poisonous,  for  it  is  able  to  paralyse 
small  animals.  On  its  free  surface  the  stinging  cell  usually  bears  a 
delicate  trigger  hair  or  cnidocil.  Under  stimulus,  whether  directly  from 
the  outside  or  from  a  nerve  cell,  the  cnidoblast  contracts,  and  the  pressure 
of  the  fluid  causes  the  forcible  evagination  of  the  barbed  pouch  and  the 
long  lasso.  Besides  the  ordinary  stinging  cells,  there  are  others  of  small 
size  which  do  not  seem  to  explode. 

(3.)  Scattered  about  there  are  minute  nerve  cells,  with  fine  connect 
tions,  especially  with  the  muscular  and  the  stinging  cells  (Fig.  40,  B). 

(4.)  Small  interstitial  or  indifferent  units  fill  up  chinks  in  the  ecto- 
derm, and  seem  to  grow  into  reproductive,  stinging,  and  other  cells. 

(5.)  Granular  glandular  cells  on  the  basal  disc  or  "foot"  probably 
secrete  a  glutinous  substance.  They  are  also  said  to  put  out  pseudopodia 
and  so  mave  the  animal  slowly. 

The  inner  layer  or  endoderm  is  less  varied  in  structure,  as  is  to  be 
expected  from  the  fact  that  it  is  not,  like  the  ectoderm,  exposed  to  the 
varying  action  of  the  environment.  Its  cells  are  pigmented,  often 
vacuolated,  and  most  of  them  are  either  flagellate  or  amoeboid.  The 
pigment  bodies  in  H.  viridis  seem  comparable  to  the  chlorophyll  cor- 
puscles of  plants  ;  in  H.fusca  they  are  brownish  and  without  chlorophyll. 
The  active  lashing  of  the  flagella  causes  currents  which  waft  food  in  and 
waste  out.  If  some  small  animal,  stung  by  the  tentacles,  is  thus  wafted 
in,  it  may  be  directly  engulfed  by  the  amoeboid  processes  of  some  of  the 
cells,  and  it  has  been  noticed  that  the  same  cell  may  be  at  one  time 
flagellate  and  at  another  time  amoeboid  (cf.  the  cell  cycle,  p.  97).  After 
this  direct  absorption  the  food  is  digested  within  the  cells,  and  while 
some  of  the  dark  granules  seen  in  these  cells  may  be  decomposed  pig- 
ment bodies,  others  seem  to  be  particles  of  indigestible  debris.  Thus 
Hydra  illustrates  what  is  called  intracellular  digestion  (T.  J.  Parker), 
such  as  occurs  in  Sponges,  some  other  Coelentera,  and  some  simple 
"  worms."  But,  according  to  Miss  Greenwood,  the  food  is  digested  in 
the  gut  cavity,  and  subsequently  absorbed.  It  seems  likely  that  both  i 
intracellular  and  extracellular  digestion  occur. 


HYDRA. 


137 


Some  of  the  endoderm  cells  have  muscular  roots  like  those  of  the 
ectoderm.     They  lie  on  the  inner  side  of  the  middle  lamina,  in  a  trans- 
verse or  circular  direction.     A  few  cells  near  the  mouth  and  base  are 
described  as  glandular,  and  the  presence  of  a  few  stinging  cells  has  been  / 
recorded,  though  some  suggest  that  the  last  are  discharged  ectodermic/ 
nematocysts  which  have  been  swallowed. 

The  middle  lamina,  representing  the  mesoglcea,  is  a  thin  homogeneous 
plate,  on  each  side  of  which  lie  the  muscular  roots  of  ectodermic  and 
endodermic  cells  (Fig.  40,  D). 

It  is  historically  interesting  to  notice  the  important  step  which  was 


Ervd 


f  f  !•  'f  1-  *(' 


FIG.  40. — Minute  structure  of  Hydra.     (After  T.  J.  PARKER 
and  JICKELI.) 

A.  Ect.  ectoderm  ;  ing.  mesogloeal  plate  ;  st.  c.  stinging  cell  ;  End. 
endoderm  with  flagella  and  amoeboid  processes. 

B.  nc.  nerve  cell,  and  st.  c.  stinging  cell  together. 

C.  Stinging  cell  with  ejected  thread  ;  n.  nucleus. 

D.  Mesogloeal  plate  with  contractile  roots  resting  on  it. 

E.  m.c.  muscular  cell  with  contractile  roots,  c.r. 

made  when,  in  1849,  Huxley  definitely  compared  the  outer  and  inner 
layers  of  the  Coelentera  with  the  epiblast  and  hypoblast  which  embryo- 
logists  were  beginning  to  demonstrate  in  the  development  of  higher 
animals.  Not  long  afterwards  Allman  applied  to  the  two  layers  of 
Hydroids,  the  terms  ectoderm  and  endoderm. 


138 


CCELENTERA. 


The  division  of  labour  among  the  cells  of  Hydra  is  not  very  strict, 
but  already  the  essential  characteristics  of  ectoderm  and  endoderm  are 
evident.  We  may  summarise  these  as  follows,  comparing  them  with  the 
characteristics  of  epiblast  and  hypoblast  in  higher  animals  : — 


OUTER  LAYER. 

MEDIAN  LAYER. 

INNER  LAYER. 

The  ectoderm  forms  — 
Covering  cells. 
Stinging  cells, 
Nerve  cells, 
Muscle  cells,  &c. 

The  endoderm  forms  — 
Digestive   cells   lining   the 
food  canal,  and  also  Muscle 
cells,  &c. 

The   embryonic   epiblast 
of    higher    animals    grows 
into      epidermis,     nervous 
system,  and  essential  parts 
of  sense  organs. 

The  mesoblast  of  higher 
animals  becomes  muscular, 
connective,  skeletal  tissue. 

The  embryonic  hypoblast 
of  higher  animals  always 
lines  the  digestive  part  of 
the  food  canal. 

The  Reproductive  Organs. — (a)  From  nests  of  repeatedly  dividing 
interstitial  cells,  several  (1-20)  simple  male  organs  or  testes  are  formed. 
Each  consists  merely  of  a  clump  of  male  elements  or  spermatozoa, 
bounded  by  the  distended  ectoderm.  Through  this  the  spermatozoa  are 
extruded  at  intervals,  and  one  may  fertilise  the  ovum  of  the  Hydra.  In 
other  words,  self-fertilisation,  which  is  very  rare  among  animals,  may 
occur.  The  spermatozoon  is  a  motile  cell,  with  a  minute  cylindrical 
"head"  consisting  of  nucleus,  a  minuter  middle  piece,  and  a  long 
thread-like  vibratile  tail  (Fig.  41,  i). 

(b)  Usually  there  is  but  one  female  organ  or  ovary,  but  in  H.  fusca 
as  many  as  eight  have  sometimes  been  observed.  The  ovary  arises  like 
the  testes  from  a  nest  of  interstitial  cells,  one  of  which  becomes  the 
ovum.  In  rare  cases  there  are  two  ova.  The  ovum  is  at  first  amoeboid 
and  transparent,  but  like  many  other  ova  it  feeds  on  its  neighbours,  loses 
its  amoeboid  form,  and  becomes  rich  in  nutritive  material.  It  also 
becomes  pigmented  (Fig.  41,  2). 

Development.'*- — The  ovum  of  Hydra  is,  as  we  have  seen,  the  success- 
ful central  cell  in  a  nest  of  interstitial  elements  which  form  the  rudi- 
mentary ovary.  It  is  at  first  amoeboid,  and  becomes  more  and  more  \ 
rich  at  the  expense  of  its  neighbours.  Their  remains  (perhaps  nuclei) 
accumulate  within  the  ovum  as  "yolk  spherules"  or  "pseudo-cells." 
With  increase  of  size  the  ovum  changes  its  form  from  amoeboid  to  cake- 
like,  and  from  that  to  spherical.  Around  the  spherical  ovum  a  gelatinous 
sheath  is  formed.  When  the  limit  of  growth  is  reached  the  nucleus  or 
germinal  vesicle  divides  twice  in  the  usual  way,  and  two  polar  bodies  are 
extruded  at  the  distal  pole.  Thereafter,  the  ectoderm  of  the  parent 
Hydra  yields  to  the  increasing  strain  put  upon  it;  and  ruptures,  allowing 
the  ovum  to  protrude.  By  a  broad  base  it  still  remains,  however, 
attached  to  the  parent,  and  in  this  state  it  is  fertilised,  the  spermatozoon 
entering  by  the  distal  pole  (Fig.  41,  4). 

1  As  the  accounts  given  by  various  investigators  do  not  agree,  it  may 
be  noted  that  we  havre  here  followed  that  of  Brauer  (1891). 


HYDRA. 


139 


The  segmentation  which  follows  is  total  and  equal,  and  results  in  the 
formation  of  a  blastosphere  (Fig.  41,  5).  By  inwandering,  or  by  division 
of  the  cells  of  the  blastosphere,  an  internal  endoderm  is  formed,  and  this 
formation  takes  place  on  all  sides.  In  a  word,  it  is  multipolar.  The 
segmentation  cavity  of  the  blastosphere  is  thus  filled  up,  and  the  two 
layers  become  differentiated  from  one  another. 

The  outer  or  ectodermic  layer  forms  (a)  an  external  "chitinoid"  shell 
of  several  layers  ;    (b)  an  internal  membrane,  homogeneous,  thin,  and 
elastic  ;  and  (c)  the  future  ectoderm  of  the  adult.     In  Hydra  fusca  the 
egg  is  separated  from   the  parent  before  the  shell  is  formed,  and  is 
fastened    by    its   gelatinous   sheath    to  aquatic   plants  ;    in    H.  viridis 
and  H.  grisea  the  egg  falls  off  after  the  outer  shell  has  been  formed,  j 
In  all  species  the  separation  from  the  parent  appears  to  be  followed  by  | 
a  period  of  quiescence  lasting  from  one  to  two  months. 

Within  the  shell,  differentiation  at  length  recommences,  but  it  proceeds 


FIG.  41.— Development  of  Hydra.     (After  BRAUER.) 

i.  sp.,  spermatozoa.    2.  Amoeboid  ovum  ;  g.v.,  germinal  vesicle  or 
nucleus  ;  y.s.,  yolk  spherules. 

3.  Ovum  with  lobecl  envelope  (sh)  around  it. 

4.  Ovum  protruding  ;  n.,  the  nucleus  ;  ect.,  the  ruptured  ectoderm  ; 
end.,  the  endoderm. 

5.  Section  of  blastosphere — Ect.,  ectoderm  ;    End.,    endoderm — 
being  formed. 

6.  Section  of  larva.     Ect.,  ectoderm  ;  End.,  endoderm  ;  g.c.,  gut- 
cavity  ;  s/i.,  ruptured  envelopes. 

slowly.  Interstitial  cells  arise  in  the  ectoderm  ;  a  middle  lamella  is 
formed  ;  a  gastric  cavity  begins  to  appear  in  the  midst  of  the  endoderm. 
Thereafter  the  shell  bursts,  and  development  proceeds  more  rapidly. 
The  embryo  elongates,  acquires  a  mouth  by  rupture  at  the  distal  (some- 
times called  vegetative)  pole.  The  inner  sheath  is  also  lost,  and  the 
young  Hydra  fixes  itself  and  begins  to  live  as  its  parent  or  parents  did. 
The  multipolar  formation  of  the  endoderm  is  perhaps  a  primitive 


140  CGELENTERA. 

mode  ;  it  occurs  only  in  Coelentera,  and  in  those  which  have  no  free 
swimming  blastosphere  stage.  Perhaps,  as  Brauer  suggests,  it  was  origi- 
nally characteristic  of  free  swimming  spherical  blastospheres,  which 
rotated  in  any  direction.  But  when  the  blastosphere  is  oval,  and  swims 
with  one  pole  always  forwards,  the  food  particles  are  swept  towards  the 
posterior  pole.  It  is  therefore  advantageous  that  the  formation  of  the 
endoderm  should  occur  there,  either  by  immigration  of  cells,  or  by  gas- 
trular  invagination  (Korschelt  and  Heider). 

Forms  like  Hydra. 

Even  simpler  than  Hydra  is  Protohydra,  a  form  without 
tentacles,  occurring  both  in  the  sea  and  in  fresh  water.  A 
similar  fresh  water  form  has  been  described  under  the  name 
of  Microhydra,  and  a  strange  simple  polype — Polypodium 
— whose  history  is  incompletely  known,  has  been  found  as 
a  parasite  on  the  eggs  of  sturgeons.  But  further  details  in 
regard  to  all  these  forms  are  much  wanted. 

Second  Type  of  CCELENTERA — A  MEDUSOID.     Class  HYDRO-' 
ZOA.     Sub-Class  Hydromedusse  or  Craspedota. 

Hydra  is  too  simple  to  be  thoroughly  typical  of  the 
Hydrozoa.  The  class  includes  the  hydroid  colonies  or  zoo- 
phytes, which  may  be  compared  to  Hydra  with  many  buds, 
and  also  free  medusoid  forms,  which  may  be  (a)  liberated 
members  of  a  hydroid  colony,  or  (^)  independent  organisms. 
Besides  these  there  are  complex  colonies  of  medusoid  forms 
(Siphonophorae). 

The  hydroid  type,  except  in  minor  details,  usually 
resembles  Hydra.  In  some  cases  the  tentacles  are  solid, 
instead  of  hollow  as  in  Hydra,  and  they  may  be  arranged  in 
two  circles, — an  outer  and  an  inner  (Tubularia).  In  some 
of  the  hydroid  colonies,  notably  the  Millepores  and  Hydrac- 
tinia,  the  polypes  are  very  dissimilar  to  one  another,  and 
have  become  specialised  for  the  performance  of  different 
functions. 

The  medusoid  type  is  like  an  inflated  hydroid  adapted 
for  swimming.  It  is  bell- shaped,  and  down  the  middle  of 
the  bell  hangs  a  prolongation  —  the  manubrium — which 
terminates  in  the  mouth.  Around  the  margin  of  the  bell 
there  is  a  little  shelf,  the  velum  or  craspedon,  which  projects 
inwards,  and  is  furnished  with  muscle  cells.  The  margin 
of  the  bell  also  bears  tentacles,  usually  hollow,  and  abun- 
dantly furnished  with  stinging  cells  (Fig.  38,  3). 


MEDUSOID.  .       141 

On  the  convex  surface  of  the  bell  the  ectoderm  forms 
simply  an  epithelial  layer  ;  on  the  concave  surface  it  is 
differentiated  into  muscle  cells  on  the  velum,  the  manu- 
brium,  and  the  tentacles,  nerve  cells  at  the  base  of  the 
velum,  and  stinging  cells  on  the  tentacles.  The  endoderm 
is  ciliated ;  it  lines  the  food  space,  and  extends  also  into  the 
tentacles.  The  mesoglcea  forms  a  thickened  jelly,  present 
more  especially  on  the  convex  (ex-umbrellar)  surface. 

The  mouth  opens  into  the  canal  of  the  manubrium,  which 
leads  to  the  central  cavity  of  the  dome.  With  this  a  varying 
number  of  unbranched  radial  canals  communicate ;  these 
also  open  into  a  marginal  circular  vessel,  which  again  com- 
municates with  the  cavities  of  the  tentacles.  Digestion  is 
intracellular,  and  probably  goes  on  throughout  the  whole  of 
this  "  gastro-vascular  "  system. 

The  movements  of  the  bell  are  caused  by  the  contractions 
of  the  muscle  cells  mentioned  above. 

The  nervous  system  consists  of  a  double  ring  of  nerve 
fibres  around  the  margin  of  the  bell.  With  these  are 
associated  ganglionic  cells,  which  apparently  control  the 
muscular  contractions. 

Sense  organs  may  be  present,  in  the  form  of  "  eyes,"  at 
the  base  of  the  tentacles  (Ocellatae),  or  there  may  be 
"  auditory "  vesicles  developed  as  pits  in  the  velum  (Vesti- 
culatoe). 

The  reproductive  organs  develop  either  in  the  manu- 
brium or  in  the  course  of  the  radial  canals.  The  products 
always  (?)  ripen  in  the  ectoderm,  and  often  seem  to  arise 
there  ;  but  Weismann  and  others  have  shown  that  the  repro- 
ductive cells  of  a  medusoid  derived  from  a  hydroid,  or  of 
the  reduced  and  fixed  reproductive  persons  of  many  hydroids, 
have  considerable  powers  of  migration,  and  may  originate 
(sometimes  in  the  endoderm)  in  the  hydroid  colony  at  some 
distance  from  the  place  where  they  are  matured  within  the 
medusoid  bud.  The  sexes  are  usually  separate.  The 
commonest  kind  of  free  swimming  larva  is  the  planula,  which 
is  oval,  ciliated,  and  diploblastic,  devoid  of  an  opening,  and 
usually  of  a  central  cavity.  In  those  medusoids  which  arise 
as  the  liberated  sexual  persons  of  a  fixed  asexual  hydroid 
colony,  the  planula  settles  down  and  develops  into  a  new 
hydroid. 


142  CCELENTERA. 

In  many  Hydrozoa,  as  has  been  already  noticed,  the 
sexual  persons  are  not  set  free,  but  remain  attached  as  buds 
to  the  parent  hydroid.  These  fixed  "  gonophores "  show 
many  stages  of  degeneration  ;  some,  notably  in  the  floating 
colonies  of  Siphonophorae,  differ  little  structurally  from  true 
medusoids,  while  others,  as  in  Hydractinia^  are  simply  small 
closed  sacs  enclosing  the  genital  products.  (Fig.  49.) 

Third  Type  of  CCELENTERA.  The  common  Jelly  fish— 
Aurelia  aurita.  Class  SCYPHOZOA.  Sub-Class  Scypho- 
medusae  or  Acraspeda. 

This  medusa  is  almost  cosmopolitan,  and  in  the  summer 
months  occurs  abundantly  around  the  British  coasts.  We 
often  see  hundreds  gently  swimming  in  shoals,  and  many  are 
washed  shorewards  and  stranded  on  flat  beaches.  The 
glassy  disc  usually  measures  about  four  inches  in  diameter, 
but  may  be  twice  as  large.  The  jelly  fish  feeds  on  small 
animals,  such  as  crustaceans,  which  are  entangled  and  stung 
to  death  by  the  long  lips. 

External  Appearance. 

The  animal  consists  of  a  gelatinous  disc,  slightly  convex  on 
its  upper  (ex-umbrellar)  surface,  and  bearing  on  the  centre  of 
the  other  (sub-umbrellar)  surface  a  four  cornered  mouth,  with 
four  long  much-frilled  lips.  The  circumference  of  the  disc 
is  fringed  by  numerous  short  hollow  tentacles,  by  little 
lappets,  and  by  a  continuation  of  the  sub-umbrella  forming 
a  delicate  muscular  flap  or  velarium.  Conspicuously  bright 
are  the  four  reproductive  organs  which  lie  towards  the 
under  surface.  Nor  is  it  difficult  to  see  the  numerous  canals 
which  radiate  from  the  central  stomach  across  the  disc,  the 
eight  marginal  sense  organs,  and  the  muscle  strands  on  the 
lower  surface. 

The  Three  Layers. 

The  ectoderm  which  covers  the  external  surface  bears 
stinging  cells,  especially  on  the  tentacles,  and  to  this  layer 
belong  sensory  and  nervous  cells  aggregated  at  eight  centres, 
also  a  plexus  of  ganglion  cells  beneath  the  skin  on  the  under 
surface,  and,  finally,  the  muscle  cells.  According  to  some, 
the  ectoderm  lines  part  of  the  mouth  tube  or  manubrium. 
The  endoderm  lines  the  digestive  cavity,  is  continued  out 


STRUCTURE   OF  AURELIA.  143 

into  its  radiating  canals,  and  is  ciliated  throughout.  The 
mesoglcea  is  a  gelatinous  coagulation  containing  wandering 
amoeboid  cells  from  the  endoderm.  The  whole  animal  is  very 
watery,  indeed  the  solid  parts  amount  to  not  more  than  ten 
per  cent,  of  the  total  weight. 

Nervous  System. 

The  nervous  system  consists  (a)  of  a  special  area  of 
nervous  epithelium,  associated  with  each  of  the  eight  sense 
organs,  and  (b)  of  numerous  much  elongated  bipolar 
ganglion  cells  lying  beneath  the  epithelium  on  the  under 
surface  of  the  disc.  This  condition  should  be  contrasted 
with  that  in  Craspedote  medusoids,  but  too  much  must  not 
be  made  of  the  contrast,  for  a  nerve  ring  is  described  in 
Cubomedusae,  one  of  the  orders  of  Acraspedote  jelly  fish. 
In  Aurelia,  the  sense  organs  are  less  differentiated  than  in 
many  other  jelly  fish.  Each  of  the  eight  organs,  protected 
in  a  marginal  niche,  consists  of  a  pigmented  spot,  a  club- 
shaped  projection  with  numerous  calcareous  "  otoliths  "  in 
its  cells,  and  a  couple  of  apparently  sensitive  pits  or 
grooves.  We  are  not  warranted  in  calling  these  organs 
"  optic,"  "  auditory,"  and  "  olfactory,"  in  Aurelia  at  any 
rate.  The  sense  organs  arise  as  modifications  of  tentacles, 
and  are  often  called  "  tentaculocysts  "  or  "  rhopalia."  Their 
cavities  are  in  free  communication  with  branches  of  the 
radial  canals. 

Muscular  System. 

Between  the  plexus  of  nerve  cells  and  the  sub-umbrellar 
mesoglcea,  there  are  cross-striped  muscle  fibres,  each  of 
which  has  a  large  portion  of  non-contractile  cell  substance 
attached  to  it.  They  lie  in  ring-like  bundles,  and  by  their 
contractions  the  medusa  moves.  Unstriped  muscle  fibres 
are  found  about  the  tentacles  and  lips. 

Alimentary  System. 

The  four  corners  of  the  mouth  are  extended  as  four  much 
frilled  "  arms,"  each  with  a  ciliated  groove  and  stinging 
cells,  and  with  an  axis  of  mesoglcea.  They  exhibit  con- 
siderable mobility.  Their  crumpled  and  mobile  bases 
surround  and  almost  conceal  the  mouth.  A  short  tube,  the 
"  manubrium  "  or  gullet,  connects  the  mouth  with  the 


144  CCELENTERA. 

central  digestive  cavity  which  occupies  the  centre  of  the 
disc.  From  this  central  chamber,  sixteen  gastro-vascular 
canals  of  approximately  equal  calibre  radiate  to  the  circum- 
ference, where  they  open  into  a  circular  canal,  with  which 
the  hollow  tentacles  are  connected.  Eight  of  the  radial 
canals  are  straight,  but  the  other  eight  are  branched,  and 
thus  in  an  adult  Aurelia  the  total  number  of  canals  is  large. 
These  canals  are  really  due  to  a  partial  obliteration  of  the 
gastric  cavity,  to  a  fusion  of  its  ex-umbrellar  and  sub- 
umbrellar  walls  along  definite  lines.  They  are  all  lined  by 
ciliated  endoderm. 

Where  the  manubrium  or  tube  from  the  mouth  passes  into 
the  central  digestive  cavity,  there  are  four  strong  pillars  of 
thickened  sub-umbrellar  material.  Outside  each  of  these 


FIG.  42. — Surface  view  of  Aurelia.     (From  ROMANES.) 

Showing  four  genital  pockets  in  centre,  much  branched  radial 
canals,  eight  peripheral  niches  for  sense,  organs,  and  peripheral 
tentacles. 

pillars,  and  still  near  the  base  of  the  manubrium,  there  are 
four  patches  where  the  sub-umbrellar  surface  remains  thin. 
These  are  the  gastro-genital  membranes,  lined  internally  by 
germinal  epithelium  (Fig.  43,  R). 

To  the  inside  of  these  genital  organs,  within  the  digestive 
cavity,  are  four  groups  of  mobile  gastric  filaments  (g/^  Fig. 
43),  which  are  very  characteristic  of  jelly  fish.  In  appearance 
these  are  very  similar  to  the  small  tentacles  of  the  margin, 
and,  like  them,  are  hollow.  They  are  covered  with  endo- 
derm— with  ciliated,  glandular,  muscular,  and  stinging  cells. 


REPRODUCTIVE  SYSTEM  OF  AURELIA.  145 

The  body  is  mapped  out  into  regions  by  the  following  convention. 
The  first  tentacles  to  appear  in  the  larva  are  four  in  number,  and 
correspond  to  the  four  angles  of  the  mouth  ;  the  radii  on  which  they 
appear  are  called  "  perradial."  Halfway  between  these,  four  "  inter- 
radials"  are  then  developed.  Then  eight  "adradials"  may  follow, 
between  perradii  and  interradii. 

Reproductive  System. 

The  sexes  are  separate.  The  reproductive  organs — ovaries 
or  testes — consist  of  plaited  ridges  of  germinal  epithelium, 
situated  on  the  four  patches  already  mentioned,  within  sacs 
which  are  derived  from  and  communicate  with  the  floor  of 
the  gastric  cavity.  They  are  of  a  reddish  violet  colour,  and  at 
first  of  a  horse-shoe  shape,  with  the  closed  part  of  the  curve 
directed  outwards.  Afterwards  the  ridges  become  circular 
and  extend  all  round  the  walls  of  the  sacs  in  which  they  lie. 


FIG.  43. — Vertical  section  of  Aurelia.     (After  GLAUS.) 

m.,  Mouth;  st.,  stomach;  r.c.,  radial  canal;  /?.,  reproductive 
organs;  gf.  gastric  filaments;  g.p.,  genital  pocket:  t.,  marginal 
tentacle  ;  s.,  sense  organ  ;  the  shaded  part  is  mesoglcea. 

But  the  sub-umbrellar  surface  is  modified  beneath  each 
genital  sac  in  such  a  way  that  the  sac  comes  to  lie  in  a  sub- 
genital  cavity  communicating  with  the  exterior  (g.p.,  Fig.  43). 
The  contractions  of  the  umbrella  produce  a  rhythmic  move- 
ment of  the  water  which  enters  the  sub-genital  sacs,  and 
this  constant  renewal  of  the  water  suggests  some  respiratory 
significance  for  the  sacs.  It  must  be  understood  that 
the  genital  sacs  containing  the  plaited  ridges  of  germinal 
epithelium  communicate  with  the  gastric  cavity  only,  while 
the  sub-genital  cavities  containing  water  and  enveloping  the 
genital  sacs  communicate  with  the  exterior  only. 

The  ova  and  spermatozoa  pass  from  the  frills  of  germinal 
epithelium  into  the  sacs,  and  thence  into  the  gastric  cavity. 
They  find  exit  by  the  mouth,  but  young  embryos  may  be 

10 


146 


CCELENTERA. 


found  swimming   in    the   gastro-vascular   canals,   and   also 
within  the  shelter  of  the  long  lips. 

Life  History  of  Aurelia. — According  to  the  most  recent  investigation, 
the  fertilised  ovum  divides  completely,  but  not  quite  equally,  to  form  a 
blastosphere  with  a  very  narrow  slit-like  cavity.  From  the  larger  celled 
hemisphere,  single  cells  migrate  into  the  cavity,  and  fill  this  up 
entirely  with  a  solid  mass  of  endoderm.  The  archenteron  arises  as  a 
central  cleft  in  this  cell  mass,  and  opens  to  the  exterior  temporarily  by 
the  primitive  mouth.  During  these  processes  the  embryo  elongates,  the 


FIG.  44. — Diagram  of  life  history  of  Aurelia. 
H^ECKEL.) 


(After 


i,  Free  swimming  embryo;  2-6,  various  stages  of  Hydra-tuba; 
7-8,  Strobila  stage  j  9,  liberation  of  Ephyrae ;  10-11,  growth  of 
Ephyrae  into  Medusae. 

outer  cells  become  ciliated,  the  mouth  closes,  and  the  embryo  swims 
freely  as  an  oval  plamda. 

After  a  short  period  of  free  life,  this  planula  settles  down  on  a 
stone  or  seaweed,  attaching  itself  by  the  pole  where  the  mouth  formerly 
opened.  At  a  very  early  stage  the  mesogloea  appears  between  the  two 
layers.  At  the  free  pole  an  ectodermic  invagination  next  occurs,  an 
opening  breaks  through  at  its  lower  end,  and  thus  a  gullet  lined  with 
ectoderm  *  is  formed,  which  hangs  freely  in  the  general  cavity.  During 

1  The  statement  as  to  the  ectodermic  gullet  is  due  to  Gotte  (1887) ;  its 
existence  is  denied  by  Glaus,  who  is  followed  by  Chun. 


LIFE  HISTORY  OF  AURELIA.  147 

this  process  there  are  formed  first  two  and  then  four  diverticula  of  the 
general  cavity,  which  are  arranged  round  the  gullet  above,  and  open 
freely  into  the  digestive  cavity  below.  In  the  gullet  region  these  are 
separated  by  broad  septa,  which  are  continued  into  the  lower  region  of 
the  body  as  four  interradial  ridges  or  taeniolae.  Although  the  develop- 
ment is  different,  these  may  perhaps  be  compared  to  the  mesenteries  of 
Anthozoa.  The  tentacles  bud  out  from  the  region  of  the  mouth,  the 
first  four  corresponding  in  position  to  the  four  pouches.  Interradially 
above  the  four  septa,  four  narrow  funnel  shaped  invaginations  arise,  these 
are  produced  by  the  ingrowth  of  ectoderm,  which  then  forms  the  muscle 
fibres  which  run  down  the  tceniolae  (contrast  the  endodermic  muscles  of 
Anthozoa). 

The  larva  now  forms  a  "  Hydra-tuba  "  or  "  Scyphistoma,"  it  is  about 
an  eighth  of  an  inch  in  height.  By  lateral  budding,  or  by  the  formation 
of  creeping  stolons,  it  may  give  rise  to  larvae  like  itself.  The  gradual 
widening  of  the  central  cavity  renders  the  gullet  tube  less  obvious,  and 
results  in  an  increasing  resemblance  to  the  medusa  type. 

In  late  autumn,  however,  a  more  fundamental  change  occurs.  We 
will  begin  with  the  simplest  case,  (a)  Occasionally,  as  has  been  observed 
by  Haeckel,  the  Scyphistoma  becomes  detached  and  converted  into  a 
free  swimming  Ephyra,  which  in  turn  becomes  a  jelly  fish,  (b]  In  Aurelia, 
in  unfavourable  conditions,  a  furrow  appears  round  the  upper  region  of 
the  Scyphistoma,  the  upper  portion  is  converted  into  an  Ephyra,  and 
floats  away,  while  the  lower  portion  reforms  its  oral  region  by  regeneration, 
and  produces  another  Ephyra.  (c)  In  ordinary  conditions  the  Scyphis- 
toma elongates,  and  displays  a  succession  of  annular  constrictions.  This 
stage,  often  compared  to  a  pile  of  saucers,  is  technically  called  a  Strobila. 
Each  disc  is  separated  off  in  its  turn  as  a  free  swimming  Ephyra,  which 
becomes  a  jelly  fish.  The  still  undivided  basal  portion  may  rest  for  a 
time,  and  then  undergo  further  constriction.  This  is  probably  an  ab- 
breviation of  the  primitive  mode  of  development. 

In  the  conversion  of  the  Scyphistoma  into  the  Ephyrae,  the  diverticula 
coalesce  into  a  general  cavity,  the  entrances  to  the  septal  invaginations 
probably  persist  as  the  sub-genital  pits,  the  gastric  filaments  sprout  out 
from  the  remains  of  the  septa,  and  so  mark  the  place  where  the  ectoder- 
mal  gullet  passed  into  the  endodermal  cavity. 

The  first  Ephyra  differs  from  those  which  come  after  it  in  bearing  the 
original  tentacles  of  the  Hydra-tuba.  From  its  margin  eight  bifid  lobes 
grow  out,  each  embracing  the  base  of  a  perradial  or  interradial  tentacle. 
The  bases  of  these  eight  tentacles  become  the  sense  organs  or  rhopalia. 
The  other  eight  adradial  tentacles  atrophy.  On  the  Ephyrae  which  fol- 
low there  are  at  first  no  tentacles,  only  the  eight  bifid  marginal  lobes 
which  bear  the  sense  organs  in  their  niches. 

This  development  illustrates  alternation  of  generations.  From  the 
fertilised  ovum  a  fixed  asexual  Scyphistoma  results.  This  grows  into  a 
Strobila,  from  which  transverse  buds  or  Ephyrae  are  liberated.  Each  of 
these  grow  into  a  sexual  jelly  fish,  producing  ova  or  spermatozoa.  The 
first  two  cases  mentioned  (a  and  b]  show  how  readily  this  alternation 
might  pass  into  a  "  direct "  development. 

Relatives  of  Aurelia. — The  Medusae,  or  true  jelly  fish,  include  forms 
which  agree  with  the  Anthozoa,  in  relative  complexity  of  structure  as 


148 


CCELENTERA. 


compared  with  Hydrozoa,  in  the  possession  of  an  ectodermal  gullet  (see 
footnote  on  page  146),  but  differ  in  possessing  ectodermal  septal  muscles 
and  in  some  histological  features. 
Among  those  closely  allied  to 
Aurelia,  some,  e.g.,  Pelagia,  have 
a  direct  development  without  the 
intervention  of  Scyphistoma  or 
Strobila  stages,  but  this  may 
occur  exceptionally  in  Aurelia. 
Cyanea  is  often  very  large,  "it 
may  measure  "]\  feet  across  the 
bell,  with  tentacles  120  feet 
long."  Chrysaora  is  hermaphro- 
dite, and  has  diffuse  sperm  sacs 
even  upon  the  arms.  In  the 

Rhizostomse— e.g.,  Cassiopeia*.^  pIGp  45._Lucernaria.     (After 

Pilema,  the  mouth  is  obliterated,  KOROTNEFF.) 

and  replaced  by  numerous  small 

pores  on  the  four  double  arms.  Lucernaria  and  its  allies  are  interesting 
sessile  forms  which  have  been  compared  to  sexual  Scyphistomas — that 
is,  are  regarded  as  persistently  larval  forms. 

Contrast  between  Hydrozoon  and  Scyphozoon  medusoids. 


HYDROZOON.     (CRASPEDOTA.) 


The  majority  are  small  "  swimming 
bells." 

A  flap  or  velum  (craspedon)  projects  in- 
wards from  the  margin  of  the  bell. 

No  taeniolae,  nor  gastric  filaments. 


A  double  nerve  ring  around  the  margin. 


Naked  sense  organs  either  optic  or  audi- 
tory. They  are  usually  derived  from 
the  skin,  but  the  auditory  sacs  may 
be  modified  tentacles. 

Reproductive  organs  on  the  radial  canals 
or  by  the  side  of  the  manubrium. 
The  reproductive  cells  are  usually 
derived  from  the  ectoderm. 

With  the  exception  of  the  Trachymedusae, 
all  arise  as  the  liberated  reproductive 
persons  of  Hydroid  colonies. 

True  Hydrozoa. 


SCYPHOZOON.    (ACRASPEDA.) 


Many  are  large  "jelly  fish." 

No  velum.  (The  velarium  of  Aurelia 
is  a  mere  fringe,  very  inconspicuous 
in  the  adult,  and  not  inturned.) 

In  the  Scyphistoma  there  are  four  taeniolae, 
from  part  of  which  the  gastric  fila- 
ments of  the  adult  grow. 

Eight  separate  nervous  centres  beside 
the  sense  organs,  and  a  sub-umbrellar 
nervous  plexus. 

Sense  organs  are  modified  tentacles,  and 
probably  have  almost  always  a  triple 
function.  They  are  usually  protected 
by  a  hood. 

Reproductive  organs  in  special  pockets 
on  the  floor  of  the  gastric  cavity. 
The  reproductive  cells  arise  in  the 
endoderm. 

Have  no  connection  with  hydroids,  but 
may  have  a  small  sedentary  polype 
stage  (or  Scyphistoma)  in  the  course 
of  their  life  history. 

Probably  more  nearly  related  to  Anthozoa 
than  to  Hydrozoa. 

We  may  note  here  that  Chun,  while  agreeing  provisionally  to  the 
separation  of  the  Acraspeda  from  the  Hydrozoa,  strongly  opposes  their 
association  with  the  Anthozoa,  basing  his  opposition  especially  on  the 
existence  of  Scyphistomas  of  great  simplicity  (e.g.,  Spongicold]. 


STRUCTURE   OF  A   SEA   ANEMONE.  149 

Fourth  Type  of  CCELENTERA.     A  SEA  ANEMONE,  such  as 
Tealia  crassicornis. 

Class,  SCYPHOZOA.     Sub-Class,  ANTHOZOA  or  ACTINOZOA. 

Most  sea  anemones  live  fixed  to  the  rocks  about  low- 
water  mark.  Some,  e.g.,  Tealia  crassicornis,  are  often  half 
buried  in  sand ;  a  few  are  unattached.  The  sedentary  forms 
are  able  to  shift  their  positions  by  short  stages.  They  feed 
on  small  animals, — molluscs,  crustaceans,  worms,  which  are 
caught  and  stung  by  the  tentacles,  but  many  must  depend 
largely  on  minute  organisms,  while  others  may  be  seen  trying 
to  engulf  molluscs  decidedly  too  large  for  them.  A  few 
anemones,  without  pigment  or  with  little,  have  symbiotic 
Algae  in  their  endoderm  cells ;  the  bright  pigments  of  many 
others  seem  to  help  in  respiration.  Besides  the  normal 
sexual  reproduction  (in  which  the  young  are  sometimes 
developed  within  the  parent),  some  sea  anemones  exhibit  a 
power  of  asexual  multiplication  by  detaching  portions  from 
near  the  base,  and  fission  occurs  in  a  few  forms. 

External  Appearance. 

The  cylindrical  body  is  fixed  by  a  broad  base ;  it  bears 
circles  of  hollow  tentacles  around  the  oral  disc  ;  the  mouth 
is  usually  a  longitudinal  slit.  The  tentacles  are  contracted 
when  the  animal  is  irritated,  and  the  whole  body  can  be 
much  reduced  in  size.  Just  below  the  margin  of  the  oral 
disc  there  is  a  powerful  sphincter  muscle,  this  contracts,  and 
pulls  together  the  body  wall  over  the  mouth  and  retracted 
tentacles.  Water  may  pass  out  gently  or  otherwise  by  a  pore 
at  the  tip  of  each  tentacle,  and  long  white  threads,  richly 
covered  with  stinging  cells,  are  often  ejected  through  the 
walls  of  the  body.  In  certain  states,  especially  if  dying,  the 
sea  anemone  protrudes  its  gullet,  and  turns  itself  partially 
inside  out. 

General  Structure  of  the  Body. 

The  Anthozoon  polype  differs  markedly  from  the  Hydroid 
— not  only  because  an  invagination  from  the  oral  disc  in- 
wards has  formed  a  gullet  tube,  which  hangs  down  freely 


150 


CCELENTERA. 


into  the  general  cavity ;  but  also  because  a  number  of  parti- 
tions or  mesenteries  extend  from  the  body  wall  towards  this 
gullet.  Some  of  the  partitions  are  "  complete,"  i.e.,  they 
reach  the  gullet ;  others  are  "  incomplete,"  i.e.,  do  not  extend 
so  far  inwards.  The  complete  mesenteries  are  attached  to 
the  oral  disc  above,  to  the  side  of  the  gullet,  and  to  the  base, 
and  all  the  mesenteries  are  ingrowths  of  the  body  wall.  The 
cavity  of  the  anemone  is  thus  divided  into  a  number  (some 


FIG.  46. — Structure  of  Sea  anemone.    (After  ANDRES.) 

t,  Tentacles  ;  0,  mouth  ;  ces,  oesophagus  ;  c,  c',  apertures  through 
a  mesentery  ;  a,  a',  acontia  ;  g;  genital  organs  on  mesentery;  m.f., 
mesenteric  filaments  ;  ;/z./.,  longitudinal  muscles  ;  s,  primary  septum 
or  mesentery  ;  s',  secondary  septum;  s",  tertiary  septum  ;  v,  base  of 
gut  cavity. 

multiple  of  six)  of  radial  chambers.  These  are  in  com- 
munication at  the  base,  so  that  food  particles  from  the  gullet 
may  pass  into  any  of  the  chambers  between  the  partitions. 
Moreover,  each  partition  is  perforated,  not  far  from  the 
mouth,  by  a  pore,  besides  which  there  is  often  another  nearer 


STRUCTURE   OF  A   SEA   ANEMONE.  151 

the  body  wall.  The  tentacles  are  continuous  with  the  cavi- 
ties between  the  mesenteries,  and  thus  all  the  parts  of  the 
body  are  in  communication.  The  mouth  is  usually  a  longi- 
tudinal slit,  and  its  two  corners  are  often  richly  ciliated.  The 
gullet  is  marked  with  longitudinal  grooves,  two  of  which,  the 
"  siphonoglyphes,"  correspond  to  the  angles  of  the  mouth, 
and  are  especially  broad  and  deep.  Along  these  two  grooves, 
and  by  these  two  corners,  food  particles  usually  pass  in  ;  but 
in  some,  one  side  is  an  incurrent,  the  other  an  excurrent 
channel.  Occasionally,  only  one  corner  of  the  mouth  and 
side  of  the  gullet  is  thus  modified.  The  gullet  often  extends 
far  down  into  the  cavity  of  the  anemone.  It  admits  of  a 
certain  amount  of  extrusion.  The  mesenteries  bear  (a) 


FIG.  47. — Section  through  Sea  anemone  (across  arrow  in 
Figure  46).     (After  ANDRES.) 

A.  B.,  directive  septa;  m.f.,  mesenteric  filaments;  g,  genital 
organs  ;  m.L,  longitudinal  muscles  ;  s,  primary  septum  ;  s',  second- 
ary septa  ;  s",  tertiary  septum.  The  arrow  enters  between  two 
primary  septa  (an  intra-septal  cavity)  and  passes  out  between  two 
tertiary  septa. 

mesenteric  filaments  ;  (/>)  retractor  muscles  ;  (c)  ridges  of 
reproductive  cells,  almost  always  either  ova  or  spermatozoa, 
rarely  both ;  and  (d)  in  some  cases  offensive  threads 
(acontia),  rich  in  stinging  cells,  and  extrusible  through  the 
body  wall.  The  mesenteric  filaments  seem  to  be  closely 
applied  to  the  food  and  perhaps  secrete  digestive  juice. 
Intracellular  digestion  also  occurs.  Sea  anemones  have  no 
sense  organs  ;  the  sapphire  beads,  which  are  so  well  seen  at 


152  CCELENTERA. 

the  bases  of  the  outermost  tentacles  of  the  common  Actinia 
mesembryanthemum,  are  batteries  of  stinging  cells.  The 
nervous  system  is  uncentralised,  and  consists  of  superficial 
sensory  cells  connected  with  a  plexus  of  sub-epithelial 
ganglion  cells. 

7^he  Layers  of  the  Body. — The  ectoderm  which  clothes  the  exterior  is 
continued  down  the  inside  of  the  gullet.  The  endoderm  lines  the  whole 
of  the  internal  cavity,  including  mesenteries  and  tentacles.  The  meso- 
glcea  is  a  supporting  plate  between  these  two  layers,  and  forms  a  basis 
for  their  cells. 

The  ectoderm  consists  of  ciliated,  sensory,  stinging,  and  glandular 
cells,  and  also  of  sub-epithelial  muscle  and  ganglion  cells  based  on  the 
mesoglcea,  but  mainly  restricted  to  the  circumoral  region. 

The  endoderm  consists  mainly  of  flagellate  cells,  with  muscle  fibres  at 
their  roots.  These  form  the  main  muscle  bands  of  the  wall,  the  mesen- 
teries, and  the  gullet.  Nor  are  glandular  and  even  sensory  cells  wanting 
from  the  endoderm. 

The  Mesenteries. — In  sea  anemones  and  nearly  related  Anthozoa 
twelve  primary  mesenteries  are  first  formed.  These  are  grouped  in  pairs, 
and  the  cavity  between  the  members  of  a  pair  is  called  intra-septal,  in 
contrast  to  the  inter-septal  cavities  between  adjacent  pairs.  In  these 
inter-septal  chambers  other  mesenteries  afterwards  appear  in  pairs.  Two 
pairs  of  mesenteries,  however,  differ  from  all  the  rest,  those,  namely, 
which  are  attached  to  each  corner  of  the  mouth  and  to  the  correspond- 
ing grooves  of  the  gullet.  These  two  pairs  of  mesenteries  are  called 
"  directive,"  and  they  divide  the  animal  into  bilaterally  symmetrical 
halves.  Anatomically,  a  pair  of  directive  mesenteries  differs  from  the 
other  paired  mesenteries,  because  the  retractor  muscles  which  extend  in 
a  vertical  ridge  along  them,  are  turned  away  from  one  another,  and  run 
on  the  inter-septal  surfaces,  whereas  in  the  other  mesenteries  the 
retractor  muscles  run  on  the  intra-septal  surface,  those  of  a  pair  facing 
one  another.  The  arrangement  of  these  muscles  is  of  great  importance 
in  classifying  Anthozoa.  It  is  possible  that  the  mesenteries  are 
homologous  with  the  taeniolge  of  jelly  fish,  and  the  mesenteric  with  the 
gastric  filaments. 

From  the  above  description,  it  will  be  noticed  that  the  funda- 
mental radial  symmetry  of  the  Ccelentera  has  here  become  profoundly 
modified. 

Development. — Comparatively  little  is  known  in  regard  to  the  early 
stages  of  development  in  sea  .anemones.  From  the  fertilised  ovum,  a 
blastosphere  may  result  which  by  invagination  becomes  a  gastrula.  Or 
the  two  layers  may  be  established  by  a  process  known  as  delamination, 
in  which  a  single  layer  of  cells  is  divided  into  an  inner  endodermic  and 
an  outer  ectodermic  layer. 

Related  Forms. — The  sea  anemones  are  classified  in  the  sub-class 
Anthozoa  or  Actinozoa,  and  along  with  many  corals  are  distinguished  as 
Zoantharia  or  Hexacoralla  from  the  Alcyonaria  or  Octocoralla,  like 
Alcyonium  and  related  corals.  This  contrast  is  not  perfectly  satisfactory, 
but  it  rests  on  such  distinctions  as  the  following  : — 


CORALS. 
ANTHOZOA  OR  ACTINOZOA. 


153 


ZOANTHARIA,  HEXACORALLA,  e.g., 

SEA  ANEMONE. 


ALCYONARIA,  OCTOCORALLA,  e.g. 
DEAD  MEN'S  FINGERS 


Many  are  simple,  many  colonial. 

Tentacles  usually  simple,  usually  some 
multiple  of  six,  often  dissimilar. 

Mesenteries  usually  some  multiple  of  six, 
complete  and  incomplete. 

Retractor  muscles  never  as  in  Alcyonarla. 

Two  gullet  grooves  or  siphonoglyphes,  or 

only  one. 
Dimorphism  only  in  some  Antipatharia, 

and  in  one  Madrepore  coral. 
Calcareous  skeleton  if  present  is  derived 

from  the  basal  ectoderm. 

Types. 

Actiniaria.  Sea  anemones. 

Madreporaria.      Reef  building  corals. 
Antipatharia.       Black  corals. 


All  colonial,  except  a  small  family  includ- 
ing Monoxcnia.  and  Haimea. 
Tentacles  eight,  feathered,  uniform. 

Mesenteries  eight,  complete. 

Retractor  muscles  always  on  one  (ventral) 
side  of  each  mesentery. 

One  (ventral)  gullet  groove  or  siphono- 
glyphe,  or  none. 

Occasional  dimorphism  among  members 
of  a  colony. 

There  are  usually  calcareous  spicules  (of 
ectodermic  origin)  in  the  mesoglcea. 
Examples. 

Alcyonium  (Dead  men's  fingers),  with 
diffuse  spicules  of  lime. 

Tubipora  (Organ  pipe  coral),  with 
spicules  fused  into  tubes  and  trans- 
verse platforms. 

Corallium  rubruin  (Red  coral),  with  an 
axis  of  fused  spicules. 

fszs,  with  an  axis  of  alternately  limy  and 
"  horny  "  joints. 

Pennatula  (Sea  pen),  a  free  phosphor- 
escent colony,  with  a  "  horny"  axis 
possibly  endodermic. 

Heliopora,  blue  coral. 


>  S 

Z  ^  A 

FIG.  48. — Z,  Diagrammatic  section  of  Zoantharian  ;  A>  of 
Alcyonarian.     (After  CHUN.) 

The  line  6" — ^  in  Z  is  through  the  siphonoglyphes  (a).  The 
retractor  muscles  are  represented  by  dark  thickenings  on  the  mesen- 
teries—all on  one  (the  ventral)  side  in  Alcyonaria. 

Coral  Making. — We  have  already  noticed  that  there  are  "corals  "among 
the  Hydrozoa,  viz.,  the  Millepores.  Leaving  these  out  of  account,  we 
have  to  recognise  that  both  divisions  of  Anthozoa  include  many  corals. 


154  CCELENTERA. 

With  the  doubtful  exception  of  the  Sea  pens  and  their  allies,  in  which 
the  axial  skeleton  is  believed  by  some  to  be  endodermic,  the  "  coral  "  is 
due  to  ectoderm  cells,  which  either  remain  in  the  ectoderm  or  wander 
into  the  mesogloea. 

Taking  as  a  basis  the  hard  parts  only,  corals  may  be  classified  in 
various  ways : — 

According  to  Composition — 

(i.)  Discontinuous  calcareous  spicules — Alcyonium,  &c. ;   these 

may  also  occur  along  with  some  forms  of  (2). 
(2.)  Continuous  skeleton. 

(a)  Organic  and  "horny,"  e.g.,  axis  of  many  Gorgonids, 

axis  of  Pennatulids. 

(I)}  "  Plorny"  and  calcareous,  e.g.,  axis  of  his. 
(c)  Wholly  calcareous,  in  the  great  majority. 
According  to  extent  of  the  hard  parts — 
(l.)  Diffuse  spicules,  e.g.,  Alcyonium. 

(2.)  Paused  in  an  external  tube,  e.g.,  7\ibipora  (Organ  pipe  coral). 
(3.)  Fused  in  an  axis,  e.g.,  Corallium  rubrum  (Red  coral). 
(4.)  Invading  the  outer  wall  (theca),  the  base,  and  forming  cal- 
careous septa  between  the  mesenteries,  and  often,  also,  a 
centra]  pillar  (columella),  e.g.,  massive  reef  building  corals. 
The  terms  Sclerodermic  and  Sclerobasic  were  formerly  much  used  in 
the  description  of  corals.     The  former  denoted  corals  in  which  the  hard 
parts  are  laid  down  by  the  individual  polypes  themselves,  and  support 
their  soft  tissues,  as  in  Tubipora,  Fungia,  and  numerous  others ;  the  latter 
was  used  in  describing  cases,  like  the  Red  Coral,  the  Sea  Pens,  &c., 
where  there  is  a  calcareous  skeleton  in  the  connecting  substance  of  the 
colony. 

According  to  position  of  the  hard  parts — 

(i.)  " Exoskeletal,"  more  ov  less  directly  continuous  with  the 
ectoderm,  e.g.,  in  Madrepore  corals  (reef  builders),  like 
Astrcea,  Fungia,  Madrepora  ;  in  Gorgonids,  Gorgonia  and 
Isis. 

(2. )  "  Mesoskeletal,"  i.e.,  in  the  mesoglcea, 
e.g.,  spicules  of  Alcyonium, 

fused  spicules  of  Tubipora, 
axis  of  Corallium. 

SYSTEMATIC  CLASSIFICATION  OF  THE  COELENTERA. 

The  Ccelentera  are  often  classified  as  follows : — 

( fV^npdntfl  (  Hydromedusse. 

A.  Hydrozoa,         I  \Siphonophor, 

(^Acraspeda. 
( Alcyonaria. 

B.  Actinozoa, 

(^Zoantharia. 

C.  Ctenophora. 


CLASSIFICATION  OF  CCELENTERA. 


155 


The  complex  structure  of  the  Acraspeda,  or  true  jelly  fishes,  together 
with  the  special  points  already  noticed,  seems,  however,  to  justify  their 
association  with  the  sea  anemones  rather  than  with  the  simpler  Craspedote 
forms.  The  classes  are  then  arranged  thus  : — 

fHydrophora. 

f  Order  I.  Hydromedusoe,  -[  Hydrocorallinae. 

A.  Class  Hydrozoa,     \  ( Trachymedusre. 

[Order  2.  Siphonophone. 

(  Lucernariae. 

Sub-class  i.  Scyphome-   ]  Discomedusoe. 
dusse,  or  Acraspeda.      1  Conomedusse. 


B.  Class  Scyphozoa.  - 


C.  Class  Ctenophora. 


iPeromedusse. 


{Order  (i). 
Zoantharia. 
Order  (2). 
Alcyonaria. 
[Rugosa.] 


A.  Class  HYDROZOA. 

There  are  two  types,  polypoid  and  medusoid,  which  may  be  combined 
in  one  life  history.  The  mouth  leads  directly  into  the  gastric  cavity. 
The  mesogloea  is  simple,  and  without  migrant  cells.  The  reproductive 
cells  seem  to  be  usually  ectodermic. 

i.  Order  Hydromedusse. — Simple  or  colonial  forms  in  which  the 
sexually  reproductive  persons  are  either  liberated  as  free  swimming 
medusoids,  or  are  sessile  gonophores. 

(a)  Ilydrophora. — Two  types  are  included  here.  The  first  includes 
the  Tubularians,  Hydractinia,  and  other  forms  in  which  the  polypes  are 
not  enclosed  in  the  protective  sheath  which  often  surrounds  the  colony 
(gymnoblastic),  and  in  which  the  free  medusoid  forms,  when  present, 
have  their  genital  organs  placed  in  the  wall  of  the  manubrium  (Antho- 
medusDe),  and  are  furnished  with  ocelli  placed  at  the  base  of  the 
tentacles.  Hydra  and  its  allies  may  be  included  here. 

Examples  : — 

Syncoryne  sarsii,  the  free  medusoid  of  which  is  called  Sarsia 
tubulosa. 

Bottgainvillea  ramosa  liberates  the  medusoid  Margelis  ramosa. 

Cordylophora  lacustris  and  Tubularia  larynx  have  sessile  gono- 
phores. 

The  second  type  includes  Campanularians,  Sertularians,  Plumularians, 
and  others,  in  which  the  protective  sheath  surrounding  the  colony  is 
continued  into  little  cups  enclosing  the  polypes  (calyptoblastic).  The 
free  medusoids  have  their  gonads  placed  in  the  course  of  the  radial 
canals  (Leptomedusce),  and  are  either  "ocellate"  or  "  vesiculate." 


I56 


CCELENTERA. 


Examples  : — 

Plumularia  and  Serttilaria  have  sessile  gonophores. 
Campamdaria  geniculata  liberates  the  medusoid  Obelia  geniculata. 
(b}  Hydrocorallinse. — Colonial  forms  which  suggest  the  Hydractinioe 
in  their  polymorphism  and  division  of  labour,  but  are  distinguished  by 
their  power  of  taking  up  lime,  and  so  forming  "corals."     The  colonies 
are  complex  and  divergent,  the  medusoid  persons  are  probably  sessile 
gonophores,  but  a  simple  male  medusoid  has  been  described.    Millepora, 
Stylaster. 

(c)  Trachy  medusae. — These  exist  only  in  the  medusoid  form,  and  are 
divided   into   two   groups,   Tracho- 
medusce  and  Narcomedusoe,  accord- 
ing to  the  position  of  the  gonads. 
Gcryonia,  Carmarina,  Ctinina, 

Aeginopsis. 

2.  Order  Siphonophoroe. — Free 
swimming  colonies  of  modified 
medusoid  persons  (medusomes), 
with  much  division  of  labour. 
Physalia  (Portugese  Man-of-War), 
Diphyes,  Velella,  Porpita. 

B.  Class  SCYPHOZOA. 

There  are  two  types — polypoid 
and  medusoid — very  rarely  occur- 
ring in  one  life  history.  The  gastric 
cavity  has  partitions  with  gastric  or 
mesenteric  filaments,  and  there  is  an 
ectodermic  gullet.  The  mesoglcea 
generally  contains  migrant  cells. 
The  reproductive  cells  are  endo- 
dermic. 

I.  Sub-class    ScyphomedusDe,    or 

Acraspeda — 

Jelly   fish   with   gastric   fila- 
ments, sub-genital  pits, 
and  no  velum — 
(i.)   Lucernarke.  —  Sessile 
forms.     Lucernaria. 
(2. }  Discomedusae.  — Active 
forms,     often      with 
complicated  life  his- 
tory.     Aurelia,    Pe- 
lagia,  Cyanea,  Rhizo- 
stoma. 
(3.)  Conomedusse. — Forms  with  broad  pseudo-velum,  and  other 

peculiar  features.     Charybdea. 

(4.)  Peromedusoe.  —  Forms    with    four    tentaculocysts    only. 
Pericolpa. 


FIG.  49. — Diagram  of  a 
gymnoblastic  HydromedusDe. 
(After  ALLMAN.) 

«,  Stem ;  £,  root ;  c,  gut  cavity ; 
d>  endoderm  (dark) ;  e,  ectoderm ; 
f,  horny  perisarc ;  g~,  hydra  like 
"person"  (hydranth);  g\  the  same, 
contracted  ;  /;,  hypostome  bearing 
mouth  ;  k,  sac  like  reproductive  bud 
(sporosac) ;  ?;z,  a  modified  hydranth 
(blastostyle)  bearing  sporosacs ;  /, 
medusoid  "person." 


CLASSIFICATION  OF  CCELENTERA.  157 

II.  Sub-class  Anthozoa,  or  Actinozoa — 

Polypoid  forms  with  well  developed  gullet  and  septa,  and  cir- 
cu moral  tentacles. 

(i.)  Zoantharia  or  Hexacoralla. 

(a)  Actiniaria.     Sea  anemones. 

Actinia,  Anemonia,  Tealia,  Cerianthus. 

(b)  Madreporaria.     Stone  .or  reef  corals. 

AstrcBa,  Madrepora,  Fungia,  Mcmndrina. 

(c)  Antipatharia.     "  Horny  "  black  corals,  with  an  axial 

skeleton,    and    occasional    dimorphism    between 
nutritive  and  reproductive  "persons,"  Antipathes. 
(2.)  Alcyonaria,  or  Octocoralla. 

Alcyonium  (Dead  men's  fingers),  Tubipora  (Organ  pipe 
coral),  Coral  Hum  (Red  coral),  Gorgonia,  Pennatula 
(Sea  pen),  Monoxenia  (non-colonial). 

The  Rugosa,  or  Tetracoralla,  include  extinct,  or  almost  entirely  extinct, 
forms,  with  numerous  septa  in  some  multiple  of  four. 

C.  Class  CTENOPHORA. 

Delicate  free  swimming  organisms,  generally  globular  in  form,  moving 
by  means  of  eight  meridional  rows  of  ciliated  plates,  or  comb-like  com- 
binations of  cilia.  The  stinging  cells  are  usually  modified  into  "adhesive 
cells."  The  mouth  is  at  one  pole,  and  leads  into  an  ectodermic  gullet. 
The  gastric  cavity  is  usually  much  branched.  The  mesenchyme  is  very 
well  developed,  and  includes  muscular  and  connective  cells.  At  the 
aboral  pole  there  is  a  sensory  organ,  including  an  "  otolith,"  which  seems 
of  use  in  steering.  Here,  also,  there  are  two  excretory  apertures.  Ex- 
cept in  Beroe  and  its  near  relatives,  there  are  two  retractile  tentacles. 
All  are  hermaphrodite.  The  development  is  direct.  They  are  pelagic, 
very  active  in  habit,  carnivorous  in  diet,  and  often  phosphorescent. 
According  to  Lang,  they  have  affinities  with  Planarian  "worms,"  but 
this  is  very  uncertain. 
Examples  : — 

(a)  With  tentacles,  Cydippe  and  the  ribbon  shaped  Venus'  Girdle 
(Cesium  Veneris}. 

(b}  Without  tentacles,  Beroe. 

History. — Of  corals,  as  we  would  expect,  the  rocks  preserve  a  faithful 
record,  and  we  know,  for  instance,  that  in  the  older  (Palaeozoic)  strata, 
they  were  represented  by  a  distinct  series  (Rugosa  or  Tetracoralla),  of 
which  we  have  at  most  two  or  three  survivors.  We  often  talk  of  the 
imperfection  of  the  geological  record,  and  rightly,  for  much  of  the 
library  has  been  burned,'  many  of  the  volumes  are  torn,  whole  chapters 
are  wanting,  and  many  pages  are  blurred.  But  this  imperfect  record 
sometimes  surprises  us,  as  in  the  quite  distinct  remains  of  ancient 
jelly  fish,  which  animals,  as  we  know  them  now,  are  apparently  little 
more  than  animated  sea  water.  We  should  also  grasp  the  conception, 
with  which  Lyell  first  impressed  the  world,  of  the  uniformity  of  natural 
processes  throughout  the  long  history  of  the  earth.  Thus  in  connection 
with  Ccelentera  we  learn  that  there  were  great  coral  reefs  in  the  incalcul- 


158  CCELENTERA. 

ably  distant  past,  just  as  there  are  coral  reefs  still.  So  in  the  Cambrian 
rocks,  which  are  next  to  the  oldest,  there  are  on  sandy  slabs  markings 
exactly  like  those  which  are  now  left  for  a  few  hours,  when  a  large 
jelly  fish  stranded  on  the  flat  beach  slowly  melts  away.  On  the  other 
hand,  some  forms  of  life  which  lived  long  ago,  seem  to  have  been  very 
different  from  any  that  now  remain,  witness,  for  example,  the  very 
abundant  Graptolite  fossils,  which,  though  probably  Coelentera,  do  not 
fit  well  into  any  of  our  modern  classes. 

Pedigree. — As  to  the  pedigree  of  the  Coelentera,  the 
facts  of  individual  life  history,  and  the  scientific  imagination 
of  naturalists,  help  us  to  construct  a  genealogical  tree — a 
hypothetical  statement  of  the  case.  Thus  it  seems  very 
likely  that  the  ancestral  many  celled  animals — ancestral  to 
Sponges,  Ccelentera,  and  all  the  rest — were  small  two  layered 
tubular  or  oval  forms.  The  many  celled  animals  must  have 
begun  as  clusters  of  cells ;  the  question  is,  what  sort  of 
clusters — spheres  of  one  layer  of  cells,  or  mouthless  ovals, 
or  little  discs  of  cells,  or  two  layered  thimble-like  sacs? 
Possibly  there  were  many  forms,  but  Haeckel  and  other 
naturalists  were  led  to  fix  their  attention  especially  on  the 
two  layered  sac  or  gastrula,  because  this  form  keeps  con- 
tinually cropping  up  as  an  embryonic  stage  in  the  life 
history  of  animals,  whether  sponge  or  coral,  earthworm  or 
starfish,  mollusc  or  even  vertebrate,  and  also  because  this  is 
virtually  the  form  which  is  exhibited  by  the  simplest  sponges 
(Ascones),  the  simplest  Coelentera  (Hydra),  and  even  by 
the  simplest  "  worms  "  (Turbellarians). 

If  we  begin  in  our  survey  with  such  a  gastrula-like 
ancestor,  the  probabilities  are  certainly  in  favour  of  the 
supposition  that  it  was  a  free  swimming  organism.  A 
gradual  perfecting  of  the  locomotor  characteristics  might 
yield  the  two  medusoid  types  of  which  we  have  already 
spoken.  But  we  know  that  the  common  jelly  fish  Aurelia 
has  a  prolonged  larval  stage  which  is  sedentary,  vegetative, 
and  prone  to  bud.  If  we  suppose  with  W.  K.  Brooks  that 
many  forms,  less  constitutionally  active  than  others,  relapsed 
into  this  sedentary  state,  with  postponed  sexuality,  and  with 
a  preponderant  tendency  to  bud,  we  can  understand  how 
polypes  arose,  and  these  of  two  types,  one  nearer  the  jelly 
fish  and  Lucernarians  and  leading  on  to  sea  anemones  and 
corals,  the  other  nearer  the  swimming  bell  type  and  leading 
on  to  a  terminus  in  Hydra.  It  is  certainly  suggestive  that 


PEDIGREE   OF  CCELENTERA. 


159 


we  have  jelly  fish  wholly  free  (Pelagia),  jelly  fish  with  a 
sedentary  larval  life  (Aurelta),  jelly  fish  predominantly  pas- 
sive (Lucernaria\  and  related  polypes  (Sea  anemones,  &c.), 
which  only  occasionally  rise  into  free  activity ;  while  in  the 
other  series  we  have  medusoid  types  always  free  (Trachy- 
medusae),  others  which  are  liberated  from  (Campanularian 
and  Tubularian)  sedentary  hydroids,  other  (Sertularian  and 
Plumularian)  zoophytes  whose  buds  though  often  medusoid- 
like  are  not  set  free,  and  finally.  Hydra,  which,  though  it 
may  creep  on  its  side,  or  walk  on  its  head,  is  predominantly 
a  sedentary  animal,  without  any  youthful  free  swimming 
stage.  It  must  be  noticed  that  the  most  frequent  larval 
form  is  the  planula,  so  that  if  we  regard  the  gastrula  as  the 
ancestral  type,  the  life  history  is  not  here  a  recapitulation  of 
the  race  history. 

GENERAL  SCHEME  OF  CCELENTERA. 


PREDOMINANTLY  PASSIVE. 

PREDOMINANTLY  ACTIVE. 

C.  CTENOPHORA,  e.g-.,  Beroe, 
Venus'  Girdle.               , 
(Active  climax.) 

B. 

SCYPHO-* 

ZOA. 

A 

f     II.  Anthozoa    or  Actinozoa. 
(Zoantharia)     Sea  anemones 
and  related  corals. 
(Alcyonaria)     Dead      Men's 
Fingers      and       related 
corals. 

The  embryos  are  free  swimmers,  and  a 
few  adults  also  are  locomotor. 

j     /  Scyphomedusae  or 
\  Acraspeda. 
c.  Adult  Lucernarians 
usually  attached. 
b.  Sedentary  larval  stage. 
a.  No  fixed  stage. 
^. 

c.  Free  embryos. 

b.  Aurelia  type  of  jelly  fish. 
a.  Pelagia  type  of  jelly  fish. 

ANCESTRAL 

GASTR/EA. 

Y 

A. 

HYDRO-  , 

ZOA. 

i.  No  fixed  stage. 
2.  No  fixed  stage. 

3.   Many  Hydroid  colonies. 
(Campanularians  and 
Tubularians.) 
4.  Many   Hydroid    colonies, 
whose  reproductive  per- 
sons are  not  liberated. 
5.  Coralline  Millepores. 

i.  Trachy  medusae  (always  locomotor). 
2.  Siphonophorae  (locomotor  colonies   of 
modified  medusoids). 
3.   Liberated  reproductive  "persons"  of 
these  colonies. 

4.  No  free  stage,  except  as  embryos. 
5.  No  known  free  stage. 

\6.  Hydra  without  any  specially  locomotor  stage. 


160  CCELENTERA. 

Bionomics. — The  Coelentera  are  almost  all  marine.  In 
fresh  water  we  find  the  common  Hydra,  the  minute  Micro- 
hydra  without  tentacles,  the  strange  Polypodium,  which  in 
early  life  is  parasitic  on  sturgeons'  eggs,  the  compound 
Cordylophora,  occurring  in  canals  and  in  brackish  water,  and 
the  fresh  water  Medusoid  (Limnocodium)  found  in  a  tank  in 
the  Regent's  Park  Botanic  Gardens,  and  another  similar 
form  recently  discovered  in  Africa.  Most  of  the  active 
swimmers  are  pelagic,  but  there  are  also  a  few  active  forms 
in  deep  water.  Many  polypes  anchor  upon  the  shells  of 
other  animals  which  they  sometimes  mask,  and  there  are 
most  interesting  constant  partnerships  between  hermit  crabs 
and  sea  anemones,  e.g.,  Bernhardus  prideauxii  and  Adamsia 
palliata. 

The  hermit  crab  is  masked  by  the  sea  anemone,  and  may 
be  protected  by  its  stinging  powers ;  the  sea  anemone  is 
carried  about  by  the  hermit  crab  and  may  get  crumbs  from 
its  abundantly  supplied  table.  This  illustrates  a  mutually 
beneficial  partnership  or  commensalism,  which,  however,  in 
some  other  animals,  may  degenerate  into  parasitism. 


CHAPTER    X. 

UNSEGMENTED    "WORMS." 
Chief  Classes. 

1.  TURBELLARIA  \  Plathelminthes 

2.  TREMATODA  or 

3.  CESTODA  J     Flat-worms. 

4.  NEMATODA. 

5.  NEMERTEA. 

THE  title  " worms"  is  hardly  justifiable  except  as  a  con- 
venient name  for  a  shape.  For  there  is  no  class  of  worms, 
the  animals  to  which  the  name  is  applied  forming  a  hetero- 
geneous mob,  a  collection  of  classes  whose  relationships 
are  imperfectly  discerned. 

But  the  zoological  interest  of  the  diverse  types,  some- 
times called  "worms,"  is  great.  For  amid  the  diversity  we 
discern  affinities  with  Coelentera,  Echinoderms,  Arthro- 
pods, Molluscs,  and  Vertebrates. 

Moreover,  it  is  likely,  as  has  been  already  noted,  that 
certain  "  worms "  were  the  first  definitely  to  abandon  the 
more  primitive  radial  symmetry,  to  begin  moving  with  one 
part  of  the  body  always  in  front,  to  acquire  head  and  sides. 
And  if  one  end  of  the  body  constantly  experienced  the  first 
impressions  of  external  objects,  it  seems  plausible  that 
sensitive  and  nervous  cells  would  be  most  developed  in 
that  much  stimulated,  over-educated,  region.  But  a  brain 
arises  from  the  insinking  of  ectodermic  cells,  and  its  be- 
ginning in  the  cerebral  ganglion  of  the  simplest  "  worms  " 
is  thus  in  part  explained. 

Again  it  may  be  noted  that  with  worm  types  begins  the 
series  of  triploblastic  coelomate  animals,  i.e.,  of  those  which 

11 


1 62  UNSEGMENTED   "  WORMS." 

have  a  well-defined  mesodenn,  and  a  mesoderm  lined  in- 
ternal cavity  distinct  from  the  gut.  But  the  appearance  of 
a  well-developed  coelome  is  very  gradual. 

It  is  not  at  present  possible  to  have  much  confidence  in 
preferring  one  arrangement  of  the  many  classes  of  "worms" 
to  another,  but  it  seems  useful  to  separate  the  segmented 
Annelids  from  the  unsegmented  types. 

Class  TURBELLARIA.     Planarians,  &c. 

Turbellarians  are  unsegmented  " worms"  living  in  fresh, 
brackish,  or  salt  water.  They  represent  the  beginning  of 
definite  bilateral  symmetry. 

The  ectoderm  is  ciliated,  and  contains  peculiar  rod-like 
bodies  (rhabdites),  and  occasionally  stinging  cells.  A  pair  of 
ganglia  in  the  head  region  give  off  lateral  nerve  cords,  and 
there  are  usually  simple  sense  organs.  The  food  canal  has  a 
muscular  pharynx,  is  often  branched,  and  is  always  blind. 
In  diet  the  Turbellarians  are  carnivorous.  There  are  no 
special  respiratory  or  circulatory  organs ;  the  body  cavity  is 
represented  at  most  by  small  spaces ;  the  excretory  system 
usually  CQnsists  of  two  longitudinal  canals  whose  branches 
end  internally  in  ciliated  (flame)  cells.  Excepting  two 
genera,  the  Turbellarians  are  hermaphrodite,  and  the  repro- 
ductive organs  usually  show  some  division  of  labour,  e.g.,  in 
the  development  of  a  yolk  gland,  which  seems  to  have  arisen 
as  an  over-nourished  (hypertrophied)  part  of  the  ovary. 

Classification. 

A.  Rhabdocoelida. — Small  fresh  water  and  marine  forms.  The  body 
tends  to  be  cylindrical.  The  food  canal  is  very  slightly 
branched  or  quite  straight  or  absent. 

(1)  Acoela.       Degenerate    forms    without    intestine,    e.§.9 
Convoluta,  which  contains  green  cells,  regarded  by  some 
as  symbiotic  AlgDe. 

(2)  Rhabdoccela.      With  straight   intestine,   e.g.,     Vortex; 
Microstoma,  a  unisexual  fresh  water  genus,  with  stinging 
cells,   forming  temporarily  united  asexual  chains,   some- 
times of  sixteen  individuals,   suggesting  the  origin  of  a 
segmented  type  ;  Graffilla  and  Attoplodium,  parasitic  (cf. 
next  class). 

(3)  Alloiocoela.     With  lobed  or  irregular  gut.     All  marine 
except  one  from  Swiss  lakes  \Plagiostoma  Lemani}. 


TURBELLARIA. 


163 


B.   Dendroccelida.     Larger,  flatter  forms  with  branched  intestine. 

( i )  Tricladida.  Elongated  flat  "  Planarians  "  ;  the  mouth 
and  tubular  pharynx  lie  behind  the  middle  of  the  body  ; 
intestine  with  three  main  branches,  themselves  branched  ; 
two  ovaries,  numerous  yolk  glands  and  testes,  a  common 
genital  aperture.,  e.g.,  Planaria  and  Dendroccelum  (in 
fresh  water),  the  former  sometimes  divides  transversely; 
Gunda  segmentata  (marine)  showing  hints  of  internal 
segmentation  ;  Geodesmus  and  Bipalium  (in  damp  earth). 


c.F. 


FIG.  50. — Diagrammatic  figure 
of  a  simple  Turbellarian. 

m,  Mouth  ;  ph,  pharynx ;  f,  digestive 
part  of  gut ;  l.e,  longitudinal  excre- 
tory vessels  ;  e.p,  excretory  pore  ; 
Ect,  ciliated  Ectoderm  ;  Ms,  meso- 
derm  ;  End,  endoderm. 


FIG.  51. — Diagrammatic  expres- 
sion of  part  of  the  structure  of 
a  simple  Turbellarian. 

Ect,  Ciliated  ectoderm ;  c.g;  cerebra 
ganglion ;  /.«,  lateral  nerve ;  T, 
testes ;  ov,  ovary. 


(2)  Polycladida.  Large  leaf  -  like  marine  "Planarians," 
with  numerous  intestinal  branches  diverging  from  a  central 
stomach  ;  with  numerous  ovaries  and  testes,  without  yolk 
glands,  mostly  with  two  genital  apertures. 

e.g.,  Cycloporus  (showing  beginning  of  anus),  Leptoplana, 
Thysanozoon. 


1 64  UNSEGMENTED    "  WORMS." 

Relationships. — Two  remarkable  forms  Cceloplana  (Kowalewsky)  and 
Ctenoplana  (Korotneff)  seem  in  some  ways  intermediate  between 
Turbellarians  and  Ctenophora.  Thus  they  have  an  aboral  sense  organ, 
and  retractile  branched  tentacles  ;  the  branching  of  the  food  canal  is 
slightly  suggestive  of  that  in  Ctenophora  ;  and  Ctenoplana  has  eight 
dorsal  bands  of  ciliated  combs.  The  resemblance  has  been  made  much 
of  by  Lang  and  others,  but,  apart  from  direct  affinity,  there  are  likely  to 
be  resemblances  of  "  convergence  "  (see  p.  33)  between  forms  not  far 
removed  from  a  common  stock — that  of  the  primitive  Metazoa. 

The  occasional  presence  of  a  retractile  proboscis  and  of  a  ciliated 
groove  on  each  side  of  the  brain  is  suggestive  of  two  characteristics  of 
Nemerteans. 

The  Turbellaria  are  also  related  to  the  next  class — the  Trematodes. 


Class  TREMATODA.     Flukes,  &c. 

The  Trematodes  are  leaf-like  or  roundish  external  or  in- 
ternal parasites.  With  their  mode  of  life  we  may  associate 
the  absence  of  cilia  on  the  surface  of  the  adults,  the  well- 
formed  and  apparently  cellular  "cuticle"  the  presence  of 
attaching  suckers  (occasionally  with  hooks),  and  the  rarity  of 
sense  organs.  It  is  likely  that  they  have  arisen  from  free 
Turbellarian-like  ancestors,  and  they  resemble  the  Turbel- 
larians in  being  unsegmented,  in  having  anterior  nerve 
centres  from  which  nerves  pass  backward  and  forward,  in 
the  rudimentary  nature  of  the  body  cavity,  in  the  ramifying 
system  of  fine  excretory  canals,  in  the  hermaphrodite  and 
usually  complex  reproductive  system.  The  alimentary  canal 
is  usually  forked,  often  much  branched,  and  always  ends 
blindly.  In  many  cases  the  animals  are  self-impregnating, 
but  cross  fertilisation  also  occurs.  The  development  of  the 
external  parasites  is  usually  direct,  of  the  internal  parasites 
usually  indirect,  involving  alternation  of  generations.  They 
occur  in  or  on  all  sorts  of  Vertebrates,  but  those  which  have 
an  indirect  development,  and  require  two  hosts  to  complete  their 
life  cycle,  often  pass  part  of  their  life  in  some  Invertebrate. 

Type,  The  Liver  Fluke  (Fasciola  (Distomd)  hepatica). 

The  adult  fluke  lives  in  large  numbers  in  the  liver  and 
bile  duct  of  the  sheep.  It  sometimes  occurs  in  cattle, 
horses,  and  other  domestic  animals,  and  rarely  in  man. 


TREMATODA. 


165 


In  the  sheep  it  causes  the  serious  disease  called  liver  rot. 
The  animal  is  flat,  oval,  and  leaf-like,  measures  about  an 


8— 


L — e.v. 


FIG.  52. — Structure  of  Liver  Fluke.  (After  SOMMER.) 
From  ventral  surface.  The  branched  gut  (gn. )  and  the 
lateral  nerve  (/.;/.)  are  shown  to  the  left,  the  branches  of 
the  excretory  vessel  (e.v. )  to  the  right. 

;;?,  Mouth  ;  ph,  pharynx  ;  g,  lateral  head  ganglion  ;  v.s,  ventral 
sucker  ;  c.s,  position  of  cirrus  sac  ;  an  arrow  indicates  the  excretory 
aperture. 

inch  in  length  by  half  an  inch  across  the  broadest  part, 
varies  from  reddish  brown  to  grayish  yellow  in  colour.  As 
the  word  Distoma  suggests,  there  are  two  suckers, — an 


1 66 


UNSEGMENTED    "  WORMS." 


anterior,  perforated  by  the  mouth ;  a  second,  imperforate 
a  little  further  back  on  the  mid  ventral  line. 


ut — 


iii.  53. — Reproductive  Organs  of  Liver  Fluke. 
(After  SOMMER). 


f.   Female  aperture. 
s.v.  Seminal  vesicle. 
y.gl.  Diffuse  yolk  glands. 
sh.g.  Shell  gland. 
v.d.  Vas  deferens. 
T.  Testes  (anterior). 


ov.  Ovary  (dark). 
ut.  Uterus. 
c.s.  Cirrus  sac. 

p.  Penis. 
m.  Mouth. 

g.  Anterior  lobes  of  gut. 


There  is  a  muscular  pharynx  and  a  blind  alimentary  canal 


LIFE   HISTORY  OF  LIVER   FLUKE.  167 

which  sends  branches  throughout  the  body.  The  nervous 
system  consists  of  a  ganglionated  collar  round  the  pharynx, 
from  which  nerves  go  forward  and  backward ;  of  these,  the 
two  which  run  laterally  are  most  important.  Although  the 
larva  has  eye  spots  to  start  with,  there  are  no  sense  organs 
in  the  adult.  The  body  cavity  is  represented  only  by  a  few 
small  spaces.  Into  these  there  open  the  ciliated  ends  of 
much  branched  excretory  tubes,  which  unite  posteriorly, 
and  communicate  with  the  exterior  by  a  terminal  pore.  The 
reproductive  system  is  hermaphrodite  and  complex.  From 
much  branched  testes,  spermatozoa  pass  by  a  pair  of  ducts 
(vasa  deferentia)  into  a  seminal  vesicle  lying  in  front  of  the 
ventral  sucker.  Thence  they  are  expelled  by  an  ejaculatory 
duct,  which  passes  through  a  muscular  protrusible  .penis. 
The  retracted  penis  and  the  seminal  vesicle  lie  in  a  space  or 
"  cirrus  sac "  between  the  ventral  sucker  and  the  external 
male  genital  aperture.  The  ovary  is  also  branched,  but 
less  so  than  the  testes.  From  its  tubes  ova  are  collected 
into  an  ovarian  duct.  Nutritive  cells  are  gathered  from 
very  diffuse  yolk  glands,  collected  in  a  reservoir,  and  pass 
by  a  duct  into  the  end  of  the  aforesaid  ovarian  duct.  At 
the  junction  of  the  yolk  duct  and  the  ovarian  duct  there  is 
a  shell  gland,  which  secretes  the  "  horny  "  shells  of  the  eggs, 
and  from  near  the  junction,  a  fine  canal  (the  Laurer-Stieda 
canal)  seems  to  pass  direct  to  the  exterior,  opening  on  the 
dorsal  surface.  The  meaning  of  this  is  still  somewhat 
uncertain.  In  some  cases  it  is  said  to  be  a  copulatory 
duct ;  in  others  it  is  regarded  as  a  safety  valve  for  over- 
flowing products.  From  the  junction  of  the  ovarian  duct 
and  the  duct  from  the  yolk  reservoir,  the  eggs  (now  furnished 
with  yolk  cells,  accompanied  by  spermatozoa,  and  encased 
in  shells)  pass  into  a  wide  convoluted  median  tube,  the 
oviduct  or  uterus,  which  opens  to  the  exterior  at  the 
base  of  the  penis.  Self  fertilisation  is  probably  normal, 
but  in  some  related  forms  cross  fertilisation  has  been 
observed. 

Life  History. — The  fertilised  and  segmented  eggs  pass  in 
large  numbers  from  the  bile  duct  of  the  sheep  to  the  in- 
testine, and  thence  to  the  exterior.  A  single  fluke  may 
produce  towards  half  a  million  embryos,  which  illustrates 
the  prolific  reproduction  often  associated  with  the  luxurious 


i68 


UNSEGMENTED   "  WORMSr 
2  3 


FIG.  54. — Life  history  of  Liver  Fluke.     (After  THOMAS.) 

i.  Developing  embryo  in  egg  case;  2.  free  swimming  ciliated 
embryo ;  3.  sporocysl ;  3«.  Lymncea  truncatula ;  4.  division  of 
sporocyst ;  5.  sporocyst  with  rediae  forming  within  it ;  6.  redia  with 
more  rediae  forming  within  it ;  7.  tailed  cercaria  ;  8.  young  fluke. 


LIFE   HISTORY  OF  LIVER   FLUKE.  169 

conditions  of  parasitism,  and  almost  essential  to  the  con- 
tinuance of  species  whose  life  cycles  are  full  of  risks.  Out- 
side of  the  host,  but  still  within  the  egg  case,  the  embryo 
develops  for  two  or  three  weeks,  and  eventually  escapes  at 
one  end  of  the  shell.  Those  which  are  not  deposited  in  or 
beside  pools  of  water  must  die.  The  free  embryo  is  conical 
in  form,  covered  with  cilia,  provided  with  two  eye  spots, 
and  actively  locomotor.  By  means  of  its  cilia  it  swims 
actively  in  the  water  for  some  hours,  but  its  sole  chance  of 
life  depends  on  its  meeting  a  small  amphibious  water-snail 
(Lymncea  truncatula),  into  which  it  bores  its  way.  In 
an  epidemic  among  horses  and  cattle  in  the  Hawaiian 
Islands,  the  host  was  L.  cahuensis  (Lutz).  Within  the 
snail,  e.g.,  in  the  pulmonary  chamber,  the  embyro  becomes 
passive,  loses  its  cilia,  increases  in  size,  and  becomes  a 
sporocyst.  Sometimes  this  sporocyst  divides  transversely 
(Fig.  54,  4). 

Within  the  sporocyst  certain  cells  behave  like  partheno- 
genetic  ova.  Each  segments  into  a  ball  of  cells  or  morula, 
which  is  invaginated  into  a  gastrula,  and  grows  into  another 
form  of  larva — the  redia.  These  rediae  burst  out  of  the 
sporocyst,  and  migrate  into  the  liver  or  some  other  organ, 
killing  the  snail  if  they  are  very  numerous.  Indeed  the 
death  of  the  snail  is  probably  necessary  for  the  escape  of 
the  final  larvae.  Each  redia  is  a  cylindrical  organism  with 
a  short  alimentary  canal  (Fig.  54,  6). 

Like  the  sporocysts,  the  redise  give  rise  internally  to  more 
embryos,  of  which  some  are  simply  rediae  over  again,  while 
the  last  set  are  quite  different, — long  tailed  cercarice,  with 
two  suckers  and  a  forked  food  canal.  These  emerge  from 
the  rediae,  wriggle  out  of  the  snail,  pass  into  the  water,  and 
moor  themselves  to  stems  of  damp  grass.  There  they  lose 
their  tails  and  become  encysted.  If  the  encysted  cercaria 
on  the  grass  stem  be  eaten  by  a  sheep,  it  grows,  in  about 
six  weeks,  into  the  adult  sexual  fluke. 

To  recapitulate,  the  developing  embryo  becomes  a  free 
swimming  form,  which  bores  into  a  snail,  and  changes  into 
a  sporocyst. 

From  certain  cells  of  the  sporocyst  rediae  are  developed, 
and  these  may  similarly  give  rise  to  other  rediae. 

Eventually,  within  the  rediae  the  tailed  cercariae  are  formed, 


170  UNSEGMENTED    "  WORMS? 

and  these  in  favouring  circumstances  grow  into  the  adult 
flukes. 

The  above  history  has  been  independently  worked  out  by  Leuckart 
and  Thomas. 

It  will  be  noted  that  the  sporocyst  is  the  modified  embryo,  but  that  it 
has  the  power  of  giving  rise  asexually  to  redke.  These  develop,  how- 
ever, from  special  cells  of  the  sporocyst  which  we  may  compare  to 
precociously  developed  parthenogenetic  ova.  Though  the  reproduction 
is  asexual,  it  is  not  comparable  to  budding  or  division.  The  same 
power  is  possessed  by  the  redise,  and  there  are  thus  several  (at  least  two) 
asexual  generations  between  the  embryo  and  the  adult.  Finally,  it 
must  be  clearly  understood  that  the  cercaria  is  the  young  fluke. 

The  disease  of  liver  rot  in  sheep  is  common  and  disastrous.  It  has 
been  known  to  destroy  a  million  sheep  in  one  year  in  Britain  alone,  and 
in  the  winter,  1879-80,  the  mortality  attributed  to  fluke  disease  was 
estimated  at  three  millions.  It  is  especially  common  after  wet  seasons, 
and  in  damp  districts.  The  preventives  suggested  are  drainage  of 
pastures  and  dressings  of  lime  and  salt ;  destruction  of  the  eggs,  the 
snails,  infected  manure,  and  diseased  sheep.  It  is  usual  to  give  the 
infected  sheep  some  salt  and  a  little  dry  food. 

Classification. 

Trematodes  with  direct  development — Monogenetic. 

e.g.,  Polystoimim  integerrimum.  This  form  with  many  suckers 
is  often  found  in  the  bladder  of  the  frog.  It  attaches  itself 
in  its  youth  to  the  gills  of  tadpoles,  passes  thence  through 
the  food  canal  to  the  bladder,  where  it  develops  slowly 
for  years. 

Gyrodactylus,  found  on  the  gills  and  fins  of  fresh-water  fishes. 
It  is  viviparous,  but  the  embryo,  before  it  is  extruded, 
itself  contains  an  embryo,  and  this  in  turn  another,  so 
that  three  generations  of  embryos  are  represented  sim- 
ultaneously. 

Diplozoon  paradoxum,   consists  of  two  individuals  united. 

The  single  embryo  (Diporpa)  is  at  first  free  swimming, 

but  becomes  a  parasite  on  the  gills  of  a  minnow,  and  there 

two  individuals  unite  very  closely  and  permanently. 

Tristomtim,  with  three  suckers,  is  not  uncommon  on  the 

skin  of  some  marine  fishes. 

Trematodes  with  indirect  development — Digenetic. 
e.g.,  Fasciola  or  Distonia. 

Bilharzia,  or  Gyncecophorus  h&matobius,  a  dangerous  parasite 
of  man,  widely  distributed  in  Africa.     It  infests  the  urinary 
and  visceral  blood  vessels.     The  sexes  are  separate,  and 
the  male  carries  the  female  inserted  in  a  groove. 
Monostomum,  a  form  with  one  sucker. 

The  relationships  of  the  class  are  on  the  one  hand  with  the  free  living 
Turbellarians,  on  the  other  hand  with  the  parasitic  Cestodes. 


CESTODA.  171 

Class  CESTODA.     Tapeworms. 

The  Cestodes  are  internal  parasites,  whose  life  history  in- 
cludes a  bladdenvorm  -(proscolex)  and  a  tapeworm  (strobila) 
stage,  the  former  in  a  Vertebrate  or  Invertebrate  host,  the 
latter  (with  one  exception)  in  the  gut  of  .a  Vertebrate.  In  a 
few  cases  the  body  is  unsegmented,  e.g.,  Archigetes  and  Caryo- 
phyllaeus,  with  one  set  of  gonads  ;  in  a  few  cases,  e.g.,  Ligula, 
there  is  a  serial  repetition  of  gonads  without  distinct  segmenta- 
tion of  the  body  ;  in  most  cases,  e.g. ,  Taenia  and  Bothriocephalus, 
the  body  of  the  tapeivorm  forms  a  chain  of  numerous  joints  or 
proglottides,  each  with  a  set  of  gonads.  Thus  the  class  in- 
cludes transitions  from  unsegmented  to  segmented  forms,  but 
the  latter  are  imperfectly  integrated.  The  general  form  of 
the  body  is  tape-like  and  bilaterally  symmetrical,  with  hooks, 
grooves,  or  suckers  ensuring  attachment  to  the  gut  of  the  host. 
The  nervous  system  consists  of  longitudinal  nerve  strands  and 
anterior  ganglionated  commissures  ;  there  are  no  special  sense 
organs.  There  is  no  alimentary  system  ;  the  parasite  floating 
in  the  digested  food  of  its  host  absorbs  soluble  material  by  its 
general  surface.  There  is  no  vascular  nor  respiratory  system, 
and  the  body  cavity  is  represented  merely  by  irregular  spaces. 
Into  some  of  these  spaces  there  open  ciliated  funnels,  the  ends 
of  the  fine  branches  of  longitudinal  excretory  tubes,  which  are 
connected  transversely  at  each  joint  and  open  terminally  by  one 
or  more  pores.  All  tapeworms  are  hermaphrodite,  and  most, 
if  not  all,  are  probably  self-fertilising.  The  male  reproductive 
organs  include  diffuse  testes,  a  vas  deferens,  and  a  protrusible 
terminal  cirrus.  The  female  organs  include  a  pair  of  ovaries, 
yolk  glands,  a  shell  gland,  a  vagina  by  ivhich  spermatozoa 
enter,  a  receptacle  for  storing  spermatozoa,  and  a  uterus  in 
which  the  ova  develop.  The  embryo  develops  within  another 
host  into  a  proscolex  or  bladdenvorm  stage,  which  forms  a 
"head"  or  scolex.  When  the  host  of  the  bladderworm  is 
eaten  by  the  final  host,  the  scolex  develops  into  an  adult  sexual 
tapeworm.  With  the  conditions  of  endoparasitic  life,  we  may 
associate  the  occurrence  of  fixing  organs,  the  absence  of  sense 
organs,  the  low  though  somewhat  complex  character  of  the 
nervous  system,  the  entire  absence  of  a  food  canal,  and  the 
prolific  reproduction. 

Life  History  of  Tcenia  solium. — This  is  one  of  the  most 


172 


UNSEGMENTED   "  WORMS." 


frequent  of  the  tapeworms  infesting  man.  In  its  adult  state 
it  is  often  many  feet  in  length,  and  is  attached  by  its  "  head  " 
to  the  wall  of  the  intestine.  The  head  bears  four  suckers 
and  a  crown  of  hooks,  and  buds  off  a  long  chain  of  joints, 
which  develop  complex  reproductive  organs  as  they  get 


v.d 


FIG.  55. — Diagram  of  reproductive  organs  in  Cestode  joint. 
(Constructed  from  LEUCKART.) 

ov,,  Ovary,  with  short  oviduct ;  «/.,  uterus  ;  t.,  diffuse  testes  ; 
sh.g.,  shell  gland  ;  y.g.,  yolk  gland  ;  v.d.,  vas  deferens  ;  71.  vagina  ; 
r.s.,  receptaculum  seminis  ;  I.e.,  longitudinal  excretory  ducts  ;  f.e., 
transverse  bridges  connecting  these. 

The  dotted  lines  above  and  below  represent  the  anterior  and 
posterior  borders  of  the  proglottis. 

shunted  further  and  further  from  the  head.     The  last  of  the 
joints    or  proglottides,    is    liberated    (singly    or    along   with 


LIFE  HISTORY  OF    T&NIA   SOLIUM.  173 

others)  and  passes  down  the  intestine  of  its  host  to  the 
exterior.  It  has  some  power  of  muscular  contraction,  and 
is  distended  with  little  embryos  within  firm  egg  shells. 
When  the  proglottis  ruptures,  these  egg  cases  are  set  free. 

In  certain  circumstances,  the  embryos,  within  their  firmly 
resistent  egg  shells,  may  be  swallowed  by  the  omnivorous 
pig.  Within  the  alimentary  canal  of  this  animal  the  egg 
shells  are  dissolved,  and  the  embryos  bearing  six  anterior 
hooks  are  liberated.  They  bore  their  way  from  the  intestine 
into  the  muscles  or  other  structures,  and  there  encyst. 
They  increase  in  size  and  become  passive,  vegetative, 
asexual  "  bladderworms."  A  bud  from  the  wall  of  the 
bladder  or  proscolex  grows  into  the  cavity  of  the  same,  and 
forms  the  future  "head"  or  scolex.  It  is  afterwards 
everted,  and  then  the  bladderworm  consists  of  a  small 
head  attached  by  a  short  neck  to  a  relatively  large 
bladder.  But  this  remains  quiescent,  and  without  power 
of  further  development,  unless  the  pig  be  eaten  by  some 
other  Vertebrate. 

When  man  unwittingly  eats  "  measly  "  pork,  that  is  pork 
infested  with  bladderworms,  an  opportunity  for  further  de- 
velopment is  afforded.  The  bladder  is  lost,  and  is  of  no 
importance,  but  the  "  head  "  or  scolex  fixes  itself  to  the  wall 
of  the  intestine.  There  it  is  copiously  and  richly  nourished, 
and  buds  off  asexually  a  chain  of  joints. 

As  these  joints  are  pushed  by  younger  interpolated  buds 
further  and  further  from  the  head,  they  become  sexually 
mature,  developing  complex  hermaphrodite  reproductive 
organs.  The  ova  produced  in  these  are  fertilised,  appar- 
ently by  spermatozoa  from  the  same  joints ;  the  proglottis 
becomes  distended  with  ripe  eggs  and  developing  embryos. 
These  ripe  joints  are  liberated,  and  the  vicious  circle  may 
recommence.  Happily,  however,  the  chances  are  thirty- 
five  millions  to  one  against  the  embryo  becoming  an  adult. 

The  above  history  is  true  mutatis  mutandis  for  many  other  tape- 
worms. It  will  be  observed  that  the  embryo  grows  into  a  proscolex  or 
bladder,  which  buds  off  a  scolex  or  head,  which,  in  another  host,  buds  off 
the  chain  of  proglottides,  but  as  it  is  virtually  the  same  animal  through- 
out, the  life  history  does  not  include  an  "alternation  of  generations." 
It  is  doubtful,  however,  what  term  should  be  applied  to  those  cases  in 
which  the  bladderworm  (Cccnurus  and  Echinococcus]  forms  not  one  head 


174 


UNSEGMENTED   "  WORMS." 


only  but  many,  each  of  which  is  capable  of  becoming  an  adult  tapeworm. 
'The  only  known  exception  to  the  fact  that  sexual  tapeworms  are  parasites 


FIG.  56. — Life  history  of  Tccnia  solium.     (After  LEUCKART.) 

i.  Six-hooked  embryo  in  egg  case  ;  2.  proscolex  or  bladderworm 
stage  with  invaginate  head  ;  3.  bladderworm  with  evaginated  head  ; 
4.  enlarged  head  of  adult,  showing  suckers  and  hooks ;  5.  general 
view  of  the  tapeworm  from  small  head,  and  thin  neck  to  the  ripe 
joints  ;  6.  a  ripe  joint  or  proglottis  with  uterus. 

of  Vertebrates,  is  Archigetez  sieboldii^  a  simple  cestode  which  is  sexual 
within  the  small  fresh  water  worm  Tubifex  rivulorum. 


NEMERTEA. 


175 


Life  Histories. 


ADULT,  SEXUAL,  OR  TAPEWORM 
STAGE. 


NON-SEXUAL,  PROSCOLEX,  OR  BLADDER- 
WORM  STAGE. 


1.  Tcenia  so  Hum,    in   man,  with    four 
suckers  and  many  hooks. 

2.  Tcenia  saginata  or  mediocanellata, 
in  man,  with  four  suckers,  but  no  hooks. 

3.  Bothriocephalus  latus,  in  man,  with 
two  lateral  suckers,  but  with  no  hooks, 
with  less  distinct  separation  of  the  prog- 
lottides  than  in  Tcenia. 

4.  Tcenia  echinococcus,  in  dog. 


5.  Tcenia  ccenurus,  in  dog. 

6.  Tcenia  serrata,  in  dog. 

7.  Tcenia  cucumerina,  in  cat. 

8.  Tcenia  elliptica,  in  dog. 


1.  Cysticercus  celluloses,  in  muscles  of 
the  pig. 

2.  Bladderworm  in  cattle. 

3.  The  ciliated,  free  swimming  embryo 
becomes  a  parasite  in  the  pike  or  burbot, 
but  without  a  distinct  bladder-like  stage. 

4.  Echinococcus  veterinorum,  in  domestic 
animals,  and  sometimes  in  man,  producing 
brood  capsules,  which  give  rise  to  many 
"  heads." 

5.  C ceriums  cerebralis,  causing  sturdie 
in  sheep,  producing  numerous  "  heads." 

6.  Cysticercus  pisiformis,  in  rabbit. 

7.  Cysticercus  fasciolaris,  in  mouse. 

8.  Cysticercus  in  dog  louse  or  perhaps 
in  flea. 


The  Cestodes  are  closely  connected  with  Trematodes  by  such  forms  as 
Amphilina,  Caryophyllcvus,  Archigetes.  Zoologically,  they  are  interest- 
ing, on  account  of  their  life  histories,  the  degeneration  associated  with 
their  parasitism,  the  prevalence  of  self-impregnation,  and  the  complexity 
of  the  reproductive  organs.  Practically,  they  are  of  importance  as  para- 
sites of  man  and  domestic  animals.  The  medical  student  should  consult 
Leuckart's  great  work,  The  Parasites  of  Man,  part  of  which  has  been 
translated  by  W.  E.  Hoyle  (Edin.  1886). 


Class  NEMERTEA.     Nemertines. 

The  Nemertines  are  worm-like  animals,  unsegmented,  and 
generally  elongate  in  form.  Almost  all  are  marine  ;  most,  if 
not  all,  are  carnivorous.  Among  their  characteristics,  the 
following  are  most  noteworthy  : —  The  skin  is  ciliated ;  there 
is  a  remarkable  retractile  proboscis  ;  the  head  bears  a  pair  of 
ciliated  pits ;  the  nervous  system  consists  of  a  brain,  a  com- 
missure around  the  proboscis^  and  two  lateral  nerve  cords ; 
there  is  a  ccelomic  vascular  system,  a  pair  of  anterior  nephridia, 
and  a  simple  reproductive  system.  The  sexes  are  usually 
separate.  In  some  the  development  includes  a  peculiar  pelagic 
larval  stage  ;  in  others  there  is  no  metamorphosis. 

External  Appearance. 

Some  are  ribbon-like,  others  thread-like,  and  the  cross 
section  is  generally  a  flattened  cylinder.     They  vary  greatly 


176  UNSEGMENTED   "  WORMS." 

in  size,  from  a  Lineus,  12  feet  long,  to  the  small  pelagic 
Pelagonemertes,  which  is  under  an  inch.  There  are  no 
appendages.  The  colours  are  often  bright. 

Skin. 

The  ectoderm  is  covered  with  numerous  short  cilia,  and 
many  of  its  cells  are  also  glandular,  secreting  mucus  which 
often  forms  a  tube  around  the  animal,  or  is  exuded  in  move- 
ment. Some  of  the  glandular  cells  extend  into  the  subjacent 
cutis,  which  consists  in  part  of  connective  tissue. 

Muscular  System. 

The  Nemertines  are  remarkably  contractile,  and  in  some 
cases  the  spasms  result  in  the  breakage  of  the  body.  The 
muscles  are  circular  and  longitudinal,  but  their  arrangement 
is  variable  even  in  individuals. 

Body  Cavity. 

In  the  adult  there  is  no  distinct  coelome,  the  space 
between  the  gut  and  the  body  wall  being  filled  up  with  con- 
nective tissue.  In  the  larvae,  however,  a  body  cavity  may 
be  seen,  either  as  an  archicoele,  i.e.,  persistent  segmentation 
cavity  (Lineus  obscurus\  or  as  a  schizocoale,  i.e.,  a  space 
formed  by  the  cleavage  of  the  mesoderm  into  two  layers 
(Pilidium-laiv^).  In  the  adult,  however,  only  the  blood 
spaces  and  the  cavity  of  the  proboscis  sheath  are  coelomic. 

Nervous  System. 

In  the  head  there  is  a  brain,  generally  four  lobed,  with  a 
commissural  ring  surrounding  the  proboscis  and  its  sheath ; 
from  the  lower  brain  lobes  two  longitudinal  nerve  stems  run 
along  the  sides,  and  are  sometimes  united  posteriorly  above 
the  anus  (Fig.  57,  In). 

Hubrecht  suggests  that  the  nerve  stems  and  the  brain  may  "be  looked 
upon  as  local  accumulations  of  nervous  tissue  in  what  was  in  more  primi- 
tive ancestors  a  less  highly  differentiated  nervous  plexus,  situated  in  the 
body  wall,"  as  in  many  Ccelentera.  In  some  cases  (Schizonemertea) 
this  nerve  plexus  persists,  and  then  the  longitudinal  stems  do  not  give 
off  regular  peripheral  branches  as  is  the  case  in  another  sub-class  (Hoplo- 
nemertea)  where  there  is  no  definite  plexus. 

It  is  interesting  to  find  that  in  Drepanophorus  the  lateral  nerve  steins 
are  approximated  ventrally,  and  in  Langia^  dorsally  ;  for  these  two 


NEMERTEA. 


177 


approximations  tend  towards  the  two  positions  most  characteristic  of  the 
nervous  systems  of  Annelids  and  Arthropods  on  the  one  hand,  and  of 
Vertebrates  on  the  other. 

Lateral  Organs. 

On  each  side  of  the  head  there  is  a  ciliated  pit  communi- 
cating with  the  exterior  through  an  open  slit  or  groove,  and 
communicating  internally  either  with  the  brain  itself,  or  with 
adjacent  and  associated  nervous  tissue.  In  those  cases  in 
which  the  development  has  been  studied  these  so-called 


P.s 


P.c 


d.  v.  m. 


FIG.  57. — Transverse  section  of  the  Nemertean  Drepanophorns 
latus.     (After  BURGER.) 


branches  ;  P.,  parenchyma  ;  £-.,  gut ;  l.v.,  lateral  blood  vessel,  beside 
which  excretory  vessel  ;  E.p.,  excretory  pore  ;  d.v.' ,  dorsal  blood 
vessel  ;  Ep.,  epidermis. 

lateral  organs  arise  from  epiblastic  insinkings  and  cesophageal 
outgrowths.  In  the  most  primitive  genus,  Carinella,  they 
are  absent,  except  in  one  species.  It  has  been  suggested 
that  they  conduce  to  the  respiration  of  the  brain,  which  is 
rich  in  haemoglobin,  and  they  have  even  been  compared  with 
gill  slits.  In  some  forms  the  groove  through  which  they 
12 


1 78  UNSEGMENTED    "WORMS." 

open  to  the  exterior  is  rhythmically  contractile.     It  has  also 
been  suggested  that  they  are  sensory. 

Sense  Organs. 

Nemertines  are  very  sensitive,  and  in  many  this  is  to  be 
associated  with  the  superficial  nerve  plexus  already  men- 
tioned. Tactile  papillae  and  hairs  are  also  present  in  some. 
Eyes  and  eye  spots  are  of  general  occurrence,  and  in  some 
cases  otocyst  sacs  have  been  observed. 

Alimentary  System. 

A  ventral  mouth  leads  into  a  plaited  oesophagus,  which  is 
followed  by  an  intestine  with  regularly  arranged  lateral  caeca. 


d.n 


FIG.  58. — Transverse  section  of  a  Nemertean — Carinella. 
(After  BURGER.) 

d.n.,  Dorsal  nerve  ;  fl.c.,  proboscis  cavity  ;  g.,  gut ;  c.vt.,  circular 
muscles  ;  /.;;/.,  longitudinal  muscles  ;  d,v.m.,  dorso-ventral  muscles  ; 
/.?/.,  lateral  vessel. 

Between  the  caeca  run  transverse  muscle  partitions.  The  anus 
is  terminal.  In  the  adults  of  the  primitive  genus,  Carinella, 
the  caeca  are  absent,  but  they  are  present  in  the  larva. 

The  Proboscis. 

In  a  cavity  along  the  dorsal  median  line  there  lies  a 
remarkable  organ,  known  as  the  proboscis.  It  is  protruded 
and  retracted  through  an  opening  above,  or,  in  a  few  cases, 
within  the  mouth,  but  it  has  no  connection  with  the  ali- 


NEMERTEA.  179 

mentary  system.  The  proboscis  is  a  hollow  muscular 
structure,  very  richly  innervated,  and  is  sometimes  protruded 
with  such  force  that  it  separates  from  the  body,  and  then 
"  often  retains  its  vitality  for  a  long  time,  apparently  crawl- 
ing about  as  if  it  were  itself  a  worm  "  (Hubrecht).  It  has 
been  compared  in  its  retracted  state  to  a  glove  finger  drawn 
in  by  a  thread  attached  to  its  tip,  the  thread  being  the 
retractor  muscle.  But  in  front  of  the  attachment  of  the 
retractile  muscle  there  is  a  non-eversible  glandular  region 
which  secretes  an  irritant  fluid.  In  many  cases  there  is  a 
stylet  at  the  tip  of  the  eversible  portion,  and  if  this  be  absent, 
there  are  stinging  cells  or  adhesive  papillae.  There  is  a  hint 
of  a  similar  structure  in  some  Turbellarians,  and  the  organ 
is  usually  interpreted  as  one  which  wras  originally  tactile,  but 
which  has  become  secondarily  aggressive.  It  is  protruded 
by  the  muscular  contraction  of  the  walls  of  the  proboscis 
sheath,  which  forms  a  closed  cavity  containing  fluid  and 
surrounding  the  proboscis.  (Fig.  57,  P.s.) 

Vascular  System. 

In  the  majority  there  are  three  longitudinal  blood  vessels 
or  blood  spaces,  a  median  and  two  laterals,  which  unite 
anteriorly  and  posteriorly,  and  also  communicate  by  numerous 
transverse  vessels.  The  vessels  or  spaces  are  remnants  of  a 
coelome.  The  blood  is  a  colourless  fluid,  sometimes  at 
least  with  corpuscles  in  which  haemoglobin  may  be  present. 

Excretory  System. 

In  most,  if  not  all,  there  are  two  coiled  nephridia, 
one  on  each  side  of  the  oesophagus — opening  anteriorly. 
(Fig.  57,  E.p.) 

Reproductive  System. 

The  sexes  are  usually  separate,  and  the  reproductive 
organs  are  always  simple.  They  consist  of  simple  sacs, 
arranged  in  a  series  on  each  side  between  the  intestinal 
coeca,  and  communicating  with  the  exterior  by  fine  pores. 
The  ova  are  often  laid  in  gelatinous  tubes,  and  are  probably 
fertilised  shortly  before  or  at  the  time  of  extrusion.  In  three 
or  four  forms  known  to  be  viviparous  the  fertilisation  must, 
of  course,  be  internal. 


i8o  UNSEGMENTED   "  WORMS." 

Development. 

(i.)  In  Cerebratulus,  etc.,  the  larva  is  adapted  for  pelagic 
life,  and  is  known  as  the  Pilidium.  "  In  external  shape  it 
resembles  a  helmet  with  spike  and  ear  lobes,  the  spike  being 
a  strong  and  long  flagellum  or  a  tuft  of  long  cilia,  the  ear 
lobes,  lateral  ciliated  appendages"  (Hubrecht).  (2.)  In 
Linens  there  is  a  sedentary  larva,  which  has  been  interpreted 
as  a  reduced  Pilidium,  and  is  known  as  the  "larva  of  Desor." 
(3.)  In  Hoplonemertea,  the  development  is  direct  without 
metamorphosis. 

Habits. 

Most  Nemertines  are  marine,  creeping  about  in  the  mud, 
under  stones,  among  seaweed,  and  the  like ;  many  are  able 
to  swim;  Pelagonemertes  is  pelagic;  a  few  live  in  fresh  water; 
Malacobdella  lives  in  the  mantle  cavity  of  marine  bivalves, 
and  two  others  occur  on  crabs.  Most  seem  to  be  carni- 
vorous, eating  other  "worms."  Many  break  readily  into 
pieces  when  stimulated,  and  the  Schizonemertea  are  able  to 
regenerate  what  they  lose  in  this  way. 

Classification  (after  Hubrecht)  : — 

1.  Palceonemertea :  No  deep  head  fissure  ;  no  stylet ;  mouth  behind 

brain. 

e.g.,  Carinella,  Cephalothrix,  Carinoma,  Polia. 

2.  Schizoneuierlea :  A  deep  head  fissure  with  a  ciliated  duct  to  the 

brain ;    lateral   nerves   between  the   longitudinal   and   inner 
circular  muscles  ;  mouth  behind  brain. 
e.g.)  Lineus,  Cerebratulus^  Langia. 

3.  Hoplonemertea :  No  deep  head  fissures;  lateral  nerves  inside  the 

muscles  ;  stylet  present ;  mouth  generally  in  front  of  brain. 
e.g.)  Amphiporus,  Nemertes,  Drepanophorus,  Malacobdella^ 
The  last  has  no  head  fissures  nor  spines  on  the  pro- 
boscis, but  bears  a  posterior  sucker. 

Relationships. — Some  of  the  characteristics  of  the  Nemerteans  are 
hinted  at  among  the  Turbellarians.  Professor  Hubrecht  has  maintained 
that  Nemerteans  exhibit  affinities  with  Vertebrates.  (See  Chapter  XX.) 


Class  NEMATODA.     Threadworms,  Hairworms,  £c. 

The  Nematodes  are  unsegmented,  more  or  less  thread-like 
"worms"  some  of  which  are  free  living  and  others  parasitic. 
The  body  is  covered  by  a  cuticle,  often  thick.  Ciliated  epithe- 


NEMATODES.  181 

Hum  is  altogether  absent.  From  a  nerve  ring  around  the 
pharynx  six  nerves  run  forwards  and  six  backwards.  An 
alimentary  canal r,  consisting  of  fore,  mid,  and  hind  gut,  is 
usually  developed.  There  is  no  vascular  nor  respiratory 
system,  but  there  is  usually  a  body  cavity,  and  there  are  two 
excretory  tubes  opening  by  an  anterior  ventral  pore.  The 
sexes  are  usually  separate  and  the  reproductive  organs  simple. 
The  life  history  is  often  intricate. 

Form. 

The  body  is  usually  cylindrical  in  cross  section  and 
tapering  at  each  end.  The  male  is  usually  smaller  than 
the  female,  and  his  tail,  concerned  in  copulation,  bears 
sensory  papillae,  and  usually  some  spines  and  a  "  bursa." 

Body   Wall. 

(a.)  Most  externally  there  is  a  chitinoid,  often  wrinkled, 
cuticle,  thick  in  the  larger  forms,  and  perhaps  of  ser- 
vice in  enabling  the  animals  to  resist  drought  and  digestive 
juices.  With  its  presence  may  be  associated  the  almost 
entire  absence  of  cutaneous  glands,  and  the  entire  absence 
of  cilia,  (b.)  Beneath  this  is  the  sub-cuticula  or  hypodermis, 
usually  thickened  in  four  longitudinal  lines — median  dorsal, 
ventral,  and  lateral.  (c.)  Beneath  the  hypodermis  is  a 
layer  of  longitudinal  muscles,  which  sometimes  lie  in  groups 
defined  by  the  above  mentioned  lines.  Many  of  the 
Nematodes  are  very  agile. 

Nervous  System. 

Around  the  pharynx  there  is  a  nerve  ring  from  which 
six  nerves  run  forwards  and  six  backwards.  One  of  the 
latter  runs  along  the  median  dorsal  line  —  a  unique 
position  in  an  Invertebrate.  Here  and  there  on  the 
ring  and  on  the  nerves  there  are  ganglionic  cells,  but  any 
aggregation  of  these  into  ganglia  is  rare.  Some  of  the  free 
living  forms  have  eye  spots ;  and  probably  all  Nematodes 
have  sensory  papillae  on  various  parts  of  the  body. 

Alimentary  System. 

The  mouth  is  terminal  or  almost  terminal ;  the  anus 
is  ventral  and  posterior,  and  occasionally  terminal.  As 


182 


UNSEGMENTED   "  WORMS. 


the  food  consists  chiefly  of  juices  either  from  a  living  host 

or  from  putrefying  organic  matter,  it  is   not  surprising  to 

find  that  the  alimentary  canal  has   usually  but   a   narrow 

cavity.      In  some  forms,  e.g.,  Sphcerularia 

from   the   bee,   it   degenerates   altogether. 

Normally  it  consists  of  three  parts,  a  fore 

gut  or  oesophagus,  lined  by  the  inturned 

cuticle,  a  mid  gut  or  mesenteron  of  endo- 

dermic  origin,  and  a  usually  short  hind  gut 

or   rectum,   lined   by  the  cuticle.     When 

the  external  cuticle  is  shed,  so  is  that  of  the 

fore  gut  and  hind  gut  (cf.  Crayfish). 


Body  Cavity. 

A  ccelome  is  developed  and  contains 
a  clear  fluid,  which  probably  discharges 
some  of  the  functions  of  the  absent  blood. 
There  are  no  amoeboid  phagocytes. 


Excretory  System. 

Imbedded  in  each  lateral  line  there  is  a 
long  tube  containing  clear  fluid,  probably 
drained  from  the  surrounding  tissues.  The 
two  longitudinal  tubes  unite  anteriorly, 
and  open  in  a  ventral  excretory  pore  near 
the  head. 

Reproductive  System. 

The  sexes  are  separate,  except  in  Angio- 
stomum  which  is  hermaphrodite  and  self- 
fertilising.       In    the    male,    the    testis    is 
usually  unpaired,  —  a  coiled  tube  gradually    tureofaNematode 
differentiating   into  vas   deferens,   seminal    (Oxyuris).   (After 
vesicles,  and  ejaculatory  duct.     The  genital    GALEB.) 
aperture  is  close  to  the  anus,  and  beside  it     .'"-*  Mouth;  c.,cu- 

.£  .,,  j      f,  .       -i  ticular  ring  ;  #?.,  oeso- 

there  are  sensory  papillae,  and  often  spicules,    phagus  ;  #.,  bulb  con- 

and  peculiar  membranous  folds  of  varied 

form  \vhich  constitute  what  is   called  the 

copulatory  "bursa."    The  spermatozoa  have 

not   the  typical  form,   and  are  sluggish.       In   the  female, 

the  ovary  is  a  single  or  paired  tube  which  passes   grad- 


FIG.  59. — Illus- 
trating the  struc- 


.  tej,th  ;  tz;'st|": 
?.<£,  vas  deferens  ;#! 


NEMATODES.  183 

ually  into  an  oviduct,  a  uterus,  and  a  short  vagina. 
The  genital  aperture  is  ventral,  usually  about  the  middle 
of  the  body,  but  it  is  occasionally  far  forward  or  far 
back. 

Development. 

The  ova  meet  the  spermatozoa  at  the  junction  of  uterus 
and  oviduct.  Segmentation  is  total  and  may  take  place 
before  or  after  laying.  Indeed  the  embyro  may  be  hatched 
within  the  uterus.  Before  the  embryo  exhibits  adult  char- 
acteristics, several,  e.g.,  three,  moultings  of  the  cuticle  usually 
occur. 

LIFE  HISTORIES. 

1.  The  embryo  grows  directly  into  the  adult,  and  both  live  in  fresh 

or  salt  water,  damp  earth,  and  rotting  plants — Enoplidce,  e.g., 
Enoplus. 

2.  The  larvae  are  free  in  the  earth,  the  sexual  adults  are  parasitic  in 

plants,  or  in  Vertebrate  animals,  e.g.,  Tylenchus  scandens,  a  com- 
mon parasite  on  cereals  ;  Strongylus  and  Dochmius  in  man. 

3.  The  sexual  adults  are  free,  the  larvse  are  parasitic  in  insects,  e.g., 

Mermis.  The  fertilised  females  of  Spharularia  bombi  pass  from 
the  earth  into  the  body  cavity  of  humble-bee  and  wasp,  whence 
their  larvse  bore  into  the  intestine  and  eventually  emerge. 

4.  The  larvee  are  parasitic  in  one  animal,  the  sexual  adults  in  another 

which  feeds  on  the  first.  Thus  Ollulanus  passes  from  mouse  to 
cat,  Cucullamis  from  Cyclops  to  perch. 

There  are  other  life  histories,  and  many  degrees  of  parasitism.  The 
most  remarkable  form  is  Angiostoinum  (or  Ascaris,  or  Leptodera] 
nigrovenosum.  In  damp  earth  males  and  females  occur,  the  progeny  of 
which  pass  into  the  lungs  of  frogs  and  toads.  There  they  mature  into 
hermaphrodite  animals  (the  only  example  among  Nematodes),  which 
produce  first  spermatozoa  and  then  ova.  They  are  self-impregnating, 
and  the  young  pass  out  into  the  earth  as  males  or  females.  Here  there 
is  alternation  of  generations,  and  a  somewhat  similar  story  might  be 
told  of  Rhabdonema  strongyloides  from  the  intestine  of  man  and 
Leptodera  appendiculata  from  the  snail. 

There  are  several  quaint  reproductive  abnormalities,  thus — the  female 
Spharularia  bombi,  which  gets  into  the  body  cavity  of  the  humble- 
bee,  has  a  prolapsed  uterus,  larger  than  itself;  the  male  of  Trichodes 
crassicauda  passes  into  the  uterus  of  the  female. 


1 84 


UNSEGMENTED   "  WORMS." 


Parasitic  in  Man. 


NAME. 

POSITION. 

HISTORY. 

RESULT  ON 
HOST. 

Ascaris  lumbri- 
coides  (common). 

Small  intestine. 

Probably  enter 
the  body  as  larvae, 
along  with  vege- 
table food  or  impure 
water,  Julus  gut- 
tulatus  perhaps  an 
intermediate  host. 

Rarely  danger- 
ous, but  may  per- 
forate intestine,  and 
cause  abscesses. 

OxyuTts  vermi- 
cularis  (common). 

From  stomach  to 
rectum,  mostly  in 
caecum. 

From  food  or 
water. 

Rarely  more  than 
discomfort. 

Trichocephalus  dis- 
par  (common). 

Caecum  and  colon. 

" 

» 

Dochmius  (A  nchy- 
lostoma)  duodcnalis 
(Europe,          Egypt 
Brazil). 
R  hab  done  ma 
strongyloides. 

Small  intestine. 

Associated  with 
Dochmius. 

The  larvae  seem 
to  live  freely  in  the 
earth. 

Dangerous  an- 
aemia. 

Filar  la  sanguinis 
kominis  (Australia, 
China,  India,  Egypt, 
and  Brazil). 

Mature  female  in 
lymphatic  glands, 
embryos  in  blood. 

Larvae  in  a  Mos- 
quito. 

Elephantiasis, 
and  haematuria. 

Dracunculus  {F*il- 
aria)  medinensis 
(Guin'eaworm)  in 
Arabia,  Egypt, 
Abyssinia,  etc. 

The  female  is  1-6 
feet  long,  encysts 
beneath  skin.  The 
male  is  not  known, 
though  his  tail  is 
said  to  have  been 
seen. 

Larvae  in  a  Cy- 
clops. 

Skin  abscesses. 

Trichina  spiralis. 

Becomes  sexually 
mature  in  the  intes- 
tine ;  embryos,  pro- 
duced rapidly  and 
viviparously,  bore 
their  way  to 
muscles,  and  be- 
come encysted. 

From  "  trichi- 
nosed  "  pig's  muscle 
to  man. 

Inflammatory  pro- 
cesses, often  fatal, 
are  brought  about 
by  the  migration 
of  the  young  worms 
from  intestine  to 
muscles. 

Trichina. — The  formidable  Trichina  deserves  fuller  notice.  It  is  best 
known  as  a  parasite  in  man,  pig,  and  rat,  but  occurs  also  in  hedgehog, 
fox,  marten,  dog,  cat,  rabbit,  ox,  and  horse.  The  sexual  forms  live  in 
the  intestine,  the  female  about  3  millimetres  in  length,  the  male  about 
half  as  long.  After  impregnation,  the  female  brings  forth  numerous 
embryos  viviparously,  60  to  So  at  a  time,  and  altogether  about  1500. 
These  bore  through  the  wall  of  the  intestine  into  the  body  cavity  or 
blood  vessels,  and  work  their  way,  especially  through  connective  tissue, 
to  the  muscle  fibres.  There  they  grow,  coil  themselves  spirally,  and 
become  encysted  within  a  sheath,  at  first  membranous  and  afterwards 
calcareous.  In  these  cysts,  which  may  be  sometimes  counted  in  millions, 
the  young  Trichinae  remain  passive,  unless  the  flesh  of  their  host  be  eaten 


ACANTHOCEPHALA.  185 

by  another,  pig  eating  rat,  man  eating  pig.  In  the  alimentary  canal  of 
the  new  host  the  capsule  is  dissolved,  the  embryos  are  set  free,  and  be- 
come rapidly  reproductive. 

Among  the  numerous  other  parasitic  Nematodes  the  following  may  be 
noted : — The  giant  palisade  worm  {Eustrongyhis gigas]  occurs  in  the  renal 
region  of  domestic  animals,  &c. ;  the  female  may  be  3  feet  long.  The 
armed  palisade  worm  (Strongylus  armatus]  occurs  in  the  intestine  and 
intestinal  arteries  of  horse,  causing  aneurisms,  colic,  &c.  The  young 
forms  are  swallowed  from  stagnant  water,  bore  from  gut  into  arteries, 
become  adult,  return  to  gut,  copulate  and  multiply.  Various  other 
species  of  Strongylus  occur  in  sheep,  cattle,  &c.  The  large  Ascaris 
megalocephala  and  the  much  smaller  Oxyuris  curvula  are  not  uncommon 
in  horses.  Syngamus  trachealis  occurs  in  the  trachea  of  birds,  causing 
"gapes."  Various  species  of  Tylenchus,  especially  T.  devastatrix  and 
T.  scandens  (or  T.  tritici],  destroy  cereal  and  other  crops.  Various 
species  of  Heterodera  (especially  H.  schachtii  and  H.  radicicola]  infest  the 
roots  of  many  cultivated  plants — e.g.,  turnip,  radish,  cabbage. 

Classification. 

At  present  the  Nematodes  are  usually  classified  in  families — Ascaridse, 
Anguillulidce,  &c.  With  these  we  need  not  concern  ourselves  here,  but 
it  is  important  to  notice  that  the  Gordiidse,  (e.g.,  Gordius  aquaticus — the 
horse  hair  worm)  are  very  different  from  all  the  others.  In  the  adult  the 
mouth  is  shut  and  the  food  canal  is  partly  degenerate.  The  adults  live 
freely  in  fresh  water  ;  there  are  two  larval  forms,  the  first  in  aquatic 
molluscs,  young  insects,  &c. ,  the  second  in  adult  insects,  fish,  frog,  &c. 

Class  ACANTHOCEPHALA. 

For  a  single  genus  Echinorhynchus ,  whose  larvae  live  in  Arthropods, 
and  the  adults  in  Vertebrates,  a  special  class,  ACANTHOCEPHALA,  has 
been  established.  We  may  provisionally  place  this  genus,  which  has 
about  a  hundred  species,  beside  Nematodes,  but  the  relationship  does 
not  seem  to  be  very  close.  Mouth  and  gut  are  absent.  The  anterior 
end  bears  a  protrusible  hooked  proboscis. 

Echinorhynchus proteus  of  Pike,  larva  in  the  Amphipod  Ganiniarus pulex. 
, ,  angustatus  of  Perch,  larva  in  the  Isopod  Asellus  aqua- 

ticus. 
, ,  gigas  of  Pig,  larva  in  young  Cockchafers. 


CHAPTER    XI. 

SEGMENTED     WORTylS     OR     ANNELIDA. 
Chief  classes — CH^ETOPODA,  DISCOPHORA. 

THE  Annelida  do  not  form  a  well  defined  phylum,  but  in- 
clude segmented  worms,  in  which  the  segmentation  of  the 
body  is  usually  visible  externally.  There  is  usually  a  well 
developed  body  cavity,  which  communicates  with  the  exterior 
by  paired  nephridia  or  segmental  organs.  The  nervous 
system  consists  typically  of  dorsal  cerebral  ganglia,  a  com- 
missural  ring  round  the  gullet,  and  a  ventral  ganglionated 
chain.  Not  infrequently  the  nephridia  function  also  as  genital 
ducts.  The  development  is  either  indirect,  when  it  includes  a 
larval  Trochosphere  stage,  or  direct. 

In  habit,  form,  and  structure  the  Annelids  exhibit  much 
diversity  of  type.  The  Chaetopods,  represented  on  the  one 
hand  by  the  familiar  earthworm,  and  on  the  other  by  the 
marine  worms,  best  exhibit  the  structure  upon  which  the 
Annelid  type  is  founded.  It  seems,  however,  that  with 
these  we  may  also  include  the  aberrant  Echiuridae — e.g., 
Echiurus  and  Bonellia.  A  few  forms  of  primitive  type 
(the  Archi-Annelida),  and  the  Myzostomata,  which  are 
degenerate  parasites  found  on  Crinoids,  may  also  be 
appended  to  the  class  Chaetopoda.  The  divergent 
leeches  (Discophora)  are  probably  Annelids  which  have  be- 
come modified  in  consequence  of  a  peculiar  habit.  Finally, 
some  zoologists  provisionally  include  Sagitta  (Chaetognatha) 
in  this  series  as  an  Annelid  with  three  segments,  and  also 
the  Rotifers  (Rotatoria),  since  their  adult  form  somewhat 
resembles  the  Trochosphere  larvae  of  many  Annelids. 

According  to  Lang,  the  Chaetopods  are  derived  from  a 


CH^TOPODA.  187 

leech-like  type,  this  from  a  Polyclade  Turbellarian,  and  this 
from  a  Ctenophore.  According  to  Sedgwick,  the  Annelids 
are  derived  from  an  Actinozoon-like  ancestor.  But  we 
cannot  here  discuss  these  possibilities,  nor  the  difficult 
questions  concerned  with  the  meaning  of  segmentation  or 
metamerism. 

Class  CH^ETOPODA.     Worms  with  Bristles. 

Segmented  animals  with  seta  developed  in  little  skin  sacs, 
either  on  a  uniform  body  wall  or  on  special  locomotor  pro- 
trusions known  as  parapodia.  The  segments,  indicated 
externally  by  rings,  are  often  marked  internally  by  parti- 
tions running  across  the  body  cavity,  which  is  usually  well 
developed.  The  nervous  system  generally  consists  of  a  double 
ventral  chain  of  ganglia,  connected  with  a  pair  of  dorsal  or 
cerebral  centres,  by  means  of  a  ring  round  the  beginning  of  the 
gut.  Two  excretory  tiibes  or  nephridia  are  typically  present  in 
each  segment,  and  they  or  their  modifications  may  also  function 
as  reproductive  ducts.  The  reproductive  elements  are  formed 
on  the  lining  membrane  of  the  body  cavity,  and  the  development 
is  either  direct  or  with  a  metamorphosis. 

The  two  prominent  divisions  of  this  class  may  be  con- 
trasted as  follows  : — 


OLIGOCHVETA,  e.g.,  Earthworm.  POLYCH/F.TA,  e.g.,  Nereis. 


With  no  parapodia,  and  with  few  setae. 
Other  external  appendages  are  also  want- 
ing, except  that  Branchiura  has  gills. 
Hermaphrodite. 
Development  direct. 
Living  in  fresh  water  or  in  the  soil. 


With  parapodia  and  with  numerous  setae. 
With  antennae,  gills,  and  cirri. 

Sexes  usually  separate. 

A  metamorphosis  in  development. 

Marine. 


TYPE  OF  OLIGOCH^ETA.     The  Earthworm  (Lumbricus}. 

Earthworms  eat  their  way  through  the  ground,  and  form 
definite  burrows,  which  they  often  make  more  comfortable 
by  a  lining  of  leaves.  The  earth  swallowed  by  the  bur- 
rowers  is  reduced  to  powder  in  the  gut,  and,  robbed  of 
some  of  its  decaying  vegetable  matter,  is  discharged  on  the 
surface  as  the  familiar  "  worm  castings."  By  the^burrowing 
the  earth  is  loosened,  and  ways  are  opened  for  plaht  roots 
and  rain  drops  ;  the  internal  bruising  reduces  mineral  matter 
to  more  useful  form  ;  while,  in  burying  the  surface  with  earat 


i88  SEGMENTED    WORMS   OR  ANNELIDA. 

brought  up  from  beneath,  the  earthworms  have  been 
ploughers  before  the  plough.  Darwin  calculated  that 
there  were  on  an  average  over  53,000  earthworms  in  an 
acre  of  arable  ground,  that  ten  tons  of  soil  per  acre  pass 
annually  through  their  bodies,  and  that  they  cover  the 
surface  with  earth  at  the  rate  of  three  inches  in  fifteen  years. 
He  was  therefore  led  to  the  conclusion  that  earthworms 
have  been  the  great  soil  makers,  or  more  precisely,  that  the 
formation  of  vegetable  mould  was  mainly  to  be  placed  to 
their  credit.  According  to  Gilbert  White  (1777),  "the 
earth  without  worms  would  soon  become  cold,  hard  bound, 
void  of  fermentation,  and  consequently  sterile;"  while 
Darwin  (1881)  said  that  "  it  may  be  doubted  whether  there 
are  many  other  animals  which  have  played  so  important  a 
part  in  the  history  of  the  world  as  have  these  lowly  organised 
creatures." 

Though  without  eyes,  earthworms  are  sensitive  to  light 
and  persistently  avoid  it,  remaining  underground  during  the 
day  unless  rain  floods  their  burrows,  and  reserving  their 
public  life  for  the  night.  Then,  prompted  by  "  love  "  and 
hunger,  they  roam  about  on  the  surface,  leaving  on  the 
moist  roadway  the  trails  which  we  see  in  the  morning. 
More  cautiously,  however,  they  often  remain  with  their  tails 
fixed  in  their  holes,  while  with  the  rest  of  their  body  they 
move  slowly  round  and  round.  The  nocturnal  peregrinations, 
the  labour  of  eating  and  burrowing,  the  transport  of  leaves 
to  their  holes,  the  collection  of  little  stones  to  protect  the 
entrance  to  the  burrows,  include  most  of  the  activities  of 
earthworms,  except  as  regards  pairing  and  egg  laying,  of 
which  something  will  afterwards  be  said.  When  an  earth- 
worm is  halved  with  the  spade  it  does  not  necessarily  die, 
for  the  head  portion  may  grow  a  new  tail,  while  a  decapitated 
worm  has  even  been  known  to  grow  a  new  head  and  brain. 
The  earthworm  is  much  persecuted  by  numerous  enemies, 
e.g.,  centipedes,  moles,  and  birds.  The  male  reproductive 
organs  are  always  infested  by  unicellular  parasites  — 
Gregarines  of  the  genus  Monocystis,  and  little  threadworms 
seem  almost  always  to  occur  in  the  excretory  tubes. 

Form  and  External  Characters. 

The  earthworm  is  often  about  six  inches  long,  with  a  pointed  head 
end,  and  a  cylindrical  body  rather  flattened  posteriorly.    The  successive 


STRUCTURE    OF   THE  EARTHWORM. 


189 


rings  seen  on  the  surface  mark  true  segments.  The  mouth  is  over- 
arched by  the  most  anterior  (pre-oral)  segment,  while  the  food  canal 
terminates  at  the  blunt  posterior  end.  The  skin  is  covered  by  a  thin 
transparent  cuticle,  traversed  by  two  sets  of  fine  lines  which  break  up 
the  light  and  produce  a  slight  irridescence.  On  a  region  extending  from 
the  3 1st  to  the  38th  ring,  the  skin  of  mature  worms  is  swollen  and 
glandular,  forming  the  clitellnm  or  saddle,  which  helps  the  worms  as 
they  unite  in  pairs,  and  perhaps  forms  the  slimy  stuff  which  hardens 
into  cocoons.  The  middle  line  of  the  back  is  marked  by  a  special  red- 
ness of  the  skin.  On  the  sides  and  ventral  surface,  we  feel  and  see  four 
rows  of  tiny  bristles  or  setae,  which  project  from  little  sacs,  are  worked 
by  muscles,  and  assist  in  locomotion.  These  bristles  are  fixed  like  pins 
into  the  ground,  at  times  so  firmly  that  even  a  bird  finds  it  difficult  to 
pull  the  worm  from  its  hole.  As  each  of  the  four  longitudinal  rows  is 


FIG.  60. — Anterior  region  of  Earthworm.     (After  HERING.) 

Note  the  eight  setae  (s)  on  each  segment. 

R-Sf-,  Spots  between  9-10,  IO-TI,  indicate  openings  of  recep^acula 
seminis  ;  Ovd.,  openings  of  oviducts  on  segment  14  v.ct.,  openings 
of  yasa  deferentia  on  segment  15. 

double,  there  are  obviously  eight  bristles  to  each  ring.  On  the  skin  of 
the  ventral  surface,  there  are  not  a  few  special  apertures,  which  should 
be  looked  for  on  a  full  grown  worm,  but  careful  examination  of  several 
specimens  is  usually  necessary.  Almost  always  plain  on  the  I5th  ring 
are  the  two  swollen  lips  of  the  male  ducts,  less  distinct  on  the  1 4th  are 
the  apertures  of  the  oviducts  through  which  the  eggs  pass,  while  on 
each  side,  between  segments  9' and  10,  10  and  n,  are  the  openings 
of  two  receptacula  seminis  or  spermathecce  into  which  male  elements 
from  another  earthworm  pass,  and  from  which  they  again  pass  out  to 
fertilise  the  eggs  of  the  earthworm  when  these  are  laid.  Each  segment 
contains  a  pair  of  excretory  tubes,  which  have  minute  ventral-lateral 
apertures,  while  on  the  middle  line  of  the  back,  between  every  two 
adjacent  rings,  there  are  minute  pores,  through  which  fluid  from  the 
body  cavity  may  exude. 


190  SEGMENTED    WORMS   OR   ANNELIDA. 

Skin  and  Bristles. 

Outermost  lies  the  thin  cuticle,  on  which  intersecting  lines 
produce  interference  of  light  and  irridescence.  Like  any 
other  cuticle,  it  is  produced  by  the  cells  which  lie  beneath, 
and  it  is  perforated  by  the  apertures  previously  mentioned. 
The  epidermis  clothing  the  worm  is  a  single  layer  of  cells, 
of  which  most  are  simply  supporting  or  covering  elements, 
while  many  are  slightly  modified,  as  glandular  or  mucous 
cells,  and  as  nervous  cells.  As  the  latter  are  connected  with 
afferent  fibres  which  enter  the  nerve  cord,  the  skin  is 
diffusely  sensitive.  In  a  few  species  the  skin  is  slightly 
phosphorescent.  The  bristles,  which  are  longest  on  the 
genital  segments,  are  much  curved,  and  lie  in  small  sacs 
of  the  skin,  in  which  they  can  be  replaced  after  breakage. 

Muscular  System. 

The  earthworm  moves  by  the  contraction  of  muscle 
cells,  which  are  arranged  in  hoops  underneath  the  skin, 
and  in  longitudinal  bands  more  internally.  The  special 
muscles  about  the  mouth  and  pharynx  have  considerable 
powers  of  grasping,  while  less  obvious  muscular  elements 
occur  in  the  wall  of  the  gut,  in  the  partitions  which  run 
internally  between  the  segments,  and  on  the  outermost 
portions  of  the  excretory  tubes. 

The  Body  Cavity. 

Unlike  the  leech,  the  earthworm  has  a  very  distinct  body 
cavity,  through  the  middle  of  which  the  gut  extends,  and 
across  which  run  the  partitions  or  septa  incompletely 
separating  successive  segments.  In  this  cavity  there  is  some 
fluid  with  cellular  elements,  of  which  the  most  numerous  are 
yellow  cells  detached  from  the  walls  of  the  gut.  Possible 
communications  with  the  exterior  are  by  the  dorsal  pores, 
and  also  by  the  excretory  tubes  which  open  internally  into 
the  cavities  of  the  segments. 

The  Nervous  System. 

Along  the  middle  ventral  line  lies  a  chain  of  nerve 
centres  or  ganglia,  really  double  from  first  to  last,  but 
compactly  united  into  what  to  unaided  eyes  seems  a  single 


STRUCTURE   OF  THE  EARTHWORM.  191 

cord.  As  the  segments  are  very  short,  the  limits  of  the 
successive  pairs  of  ganglia  are  not  very  evident,  especially 
in  the  anterior  region,  but  they  are  plain  enough  on  a  small 
portion  of  the  cord  examined  with  the  microscope,  when  it 
may  also  be  seen  that  each  ollhe  pairs  of  ganglia  gives  off 
nerves  to  the  walls  of  the  body.  Anteriorly,  just  behind 
the  mouth,  the  halves  of  the  cord  diverge  and  ascend, 
forming  a  ring  around  the  pharynx.  They  unite  above 
in  two  dorsal  or  cerebral  ganglia.  These  form  the  earth- 
worm's "  brain,"  and  give  off  nerves  to  the  adjacent  pre-oral 
segment  or  prostomium,  on  which  are  numerous  sensitive 
cells.  These,  coming  in  contact  with  many  things,  doubtless 
receive  impressions,  which  are  transmitted  by  the  associated 
nerves  to  the  "brain."  As  Mr.  Darwin  observed  that 
earthworms  seized  hold  of  leaves  in  the  most  expeditious 
fashion,  taking  the  sharp  twin  leaves  of  the  Scotch  fir  by  their 
united  base,  we  may  credit  the  earthworms  with  some  power 
of  profiting  by  experience ;  moreover,  as  they  deal  deftly 
with  leaves  of  which  they  have  no  previous  experience,  we 
may  even  charitably  grant  them  a  modicum  of  intelligence. 
From  the  nerve  collar  uniting  the  dorsal  ganglia  with  the 
first  pair  on  the  ventral  cord,  nerves  are  given  off  to  the 
pharynx  or  gut,  forming  what  is  called  a  "  visceral  system." 
The  earthworm  has  no  special  sense  organs,  but  we  have  just 
mentioned  sensitive  cells,  which  are  particularly  abundant 
on  the  head  end  of  the  worm.  By  them  the  animal  is  made 
aware  of  the  differences  between  light  and  darkness,  and  of 
the  approaching  tread  of  human  feet,  not  to  speak  of  the 
hostile  advances  of  a  hungry  blackbird.  The  sense  of  smell 
is  also  developed.  The  afferent  or  sensory  nerve  fibres  from 
the  nervous  cells  of  the  skin  enter  the  nerve  cord  and 
bifurcate  into  longitudinal  branches,  which  end  freely  in  the 
nearest  ganglia.  In  this  the  earthworm's  nervous  system 
suggests  that  of  Vertebrates. 

Two  facts  in  regard  to  minute  structure  deserve  attention.  The 
nerve  cells,  instead  of  being  confined  to  special  centres  or  ganglia,  as 
they  are  in  Arthropods,  occur  diffusely  along  with  the  nerve  fibres 
throughout  the  course  of  the  cord.  Along  the  dorsal  surface  of  the 
ventral  nerve  cord  there  run  three  peculiar  tubular  fibres,  with  firm 
walls  and  clear  contents.  These  "  giant  fibres,"  which  do  not  seem  to 
be  nervous,  but  are  rather  supporting  elements,  have  been  dignified  by 
the  name  of  neurochord. 


192  SEGMENTED    WORMS   OR  ANNELIDA. 

Alimentary  System. 

Earthworms  eat  the  soil  for  the  sake  of  the  plant 
debris  which  it  may  contain,  and  also,  indeed,  because 
they  must  swallow  as  they  tunnel.  In  eating  they  are 
greatly  helped  by  the  muscular  nature  of  the  pharynx, 
whence  the  soil  passes  down  the  gullet  or  oesophagus,  first 
into  a  swollen  crop,  then  into  a  strong  walled  grinding 
gizzard,  and  finally  through  a  long  digestive  and  absorptive 
stomach  intestine.  On  the  gullet  are  three  pairs  of  ceso- 
phageal  or  calciferous  glands — the  products  of  which  are 
limy  and  able  to  affect  the  food  chemically,  probably 
counteracting  the  acidity  of  the  decaying  vegetable  matter. 
The  long  intestine  has  its  internal  surface  increased  by  a 
dorsal  fold,  which  projects  inwards  along  the  whole  length. 
In  this  "  typhlosole,"  and  over  the  outer  surface  of  the  gut, 
the  yellow  cells  are  crowded.  There  is  no  warrant  for 
calling  these  hepatic  or  digestive.  Structurally  they  are 
pigmented  cells  of  the  peritoneal  epithelium,  which  here,  as 
in  most  other  animals,  lines  the  body  cavity  and  the  outside 
of  the  gut.  As  to  their  function  we  know  that  they  absorb 
particles  from  the  intestine,  and  go  free  into  the  body  cavity, 
whence,  as  they  break,  up,  4heir  debris  may  pass  out  by  the 
excretory  tubes.  When  a  worm  has  been  made  to  eat 
powdered  carmine,  the  passage  of  these  useless  particles 
from  gut  to  yellow  cells,  from  yellow  cells  to  body  cavity, 
and  thence  out  by  the  excretory  tubes,  has  been  traced. 
Various  ferments  have  been  detected  in  the  gut,  a  diastatic 
ferment  turning  the  starchy  food  into  sugars,  and  others — 
peptic  and  tryptic — not  less  important.  The  wall  of  the 
stomach  intestine  from  without  inwards,  as  may  be  traced 
in  sections,  is  made  up  of  pigmented  peritoneum,  muscles, 
capillaries,  and  an  internal  ciliated  epithelium.  In  the  other 
parts  of  the  gut  the  innermost  lining  is  not  ciliated,  but 
covered  with  a  cuticle. 

Vascular  System. 

The  fluid  of  the  blood  is  coloured  red  with  haemoglobin, 
and  contains  small  corpuscles.  Along  the  median  dorsal 
line  of  the  gut  a  prominent  blood  vessel  extends, 
another  (supra-neural)  runs  along  the  upper  surface 


STRUCTURE   OP   THE  EARTHWORM.  193 

of  the  nerve  cord,  another  (infra-neural)  along  the  under 
surface,  while  two  small  lateral-neurals  pass  along  each  side 
of  this  same  cord.  All  these  longitudinal  vessels,  of  which 
the  first  three  are  most  important,  are  parallel  with  one 
another  ;  the  first  three  meet  in  an  anterior  network  on  the 
pharynx  ;  the  dorsal  and  the  supra-neural  are  linked  together 
in  the  region  of  the  gullet  by  five  or  six  pairs  of  contractile 
vessels  or  "  hearts."  The  precise  path  of  the  blood  is  not 


V 


c.m 
-l.rti 


c.ce 
1 11.  c 


FIG.  61. — Transverse  section  of  Earthworm. 
(After  CLAPAR&DE.) 

r.,  Cuticle  ;  e.,  epidermis:  c.m.,  circular  muscles  ;  /.;«.,  longi- 
tudinal muscles  ;  s.,  a  seta;  c.ae.,  ccelome  ;  y.c.,  yellow  cells  ;  T., 
typhlosole ;  z/.e/.,  ventral  blood  vessel;  s.n.v.,  sub-neural  vessel 
below  nerve  cord  ;  d.v.,  dorsal  vessel. 

known,  but  the  distribution  of  vessels  to  skin,  nephridia, 
and  alimentary  canal  is  readily  seen. 

Respiration  is  effected  by  the  distribution  of  blood  on  the 
general  surface  of  the  skin. 

13 


194  SEGMENTED    WORMS   OR  ANNELIDA. 

Excretory  System* 

As  we  have  mentioned,  small  particles  may  pass  from  the 
gut  to  the  body  cavity,  and  thence  to  the  exterior  by  the 
excretory  tubes.  There  is  a  pair  of  these  little  kidneys, 
nephridia  or  segmental  organs,  in  each  segment  except  the 
first  four.  Each  opens  internally  into  the  segment  in  front 
of  that  on  which  its  other  end  opens  to  the  exterior.  They 
remove  little  particles  from  the  body  cavity,  but  probably 
get  finer  waste  products  from  the  associated  blood  vessels. 
Nephridia  occur  in  many  animals,  in  most  young  Vertebrates 
as  well  as  among  Invertebrates,  but  they  are  never  seen 
more  clearly  than  in  the  earthworm.  When  a  nephridium 
is  carefully  removed,  along  with  a  part  of  the  segment 
septum  through  which  it  passes,  and  examined  under  the 
microscope,  the  following  three  parts  are  to  be  seen  :  (a)  an 
internal  ciliated  funnel,  (b)  a  trebly  coiled  ciliated  tube,  at 
first  transparent  then  glandular  and  granular,  and  (c)  a 
muscular  duct  opening  to  the  exterior.  Minute  particles 
swept  into  the  ciliated  funnel  pass  down  the  ciliated  coils  of 
the  tube,  and  out  by  the  muscular  part  which  opens  just 
outside  of  the  ventral  bristles.  The  coiled  tube  consists  in 
part  at  least  of  a  series  of  intracellular  cavities,  that  is  to 
say,  it  runs  through  the  middle  of  the  cells  which  compose 
it ;  the  external  muscular  portion  arises  from  an  invagina- 
tion  of  skin. 

Reproductive  System 

The  earthworm  is  hermaphrodite,  and  its  reproductive 
organs  are  somewhat  difficult  to  demonstrate  with  com- 
pleteness. To  see  them  it  will  be  necessary  to  dissect 
several  earthworms  with  special  attention  to  individual  parts. 

(a)  The  Male  Organs  consist  of  two  pairs  of  testes,  three 
pairs  of  seminal  vesicles,  and  a  paired  vas  deferens. 

(1)  The  testes  lie   near  the  nerve  cord   on  the  septa 
between  segments  10  and  n  ;  each  is  "a  white  translucent 
body  of  irregular  quadrangular  form,  rarely  more  than  one- 
tenth  of  an  inch  in  diameter."     (Fig.  62,  T.) 

(2)  Mother  sperm  cells,  which  give  rise  by  division  to 
young   spermatozoa,    pass   from   the   testes    to    the   much 
lobed  semina   vesicles,  where  the  spermatozoa  are  matured. 


STRUCTURE   OF   THE  EARTHWORM.  195 

These  vesicles  (Fig.  62,  s.v.)  are  very  prominent,  and  seem  to 
be  outgrowths  of  the  septa  between  segments  nine  to  twelve. 
Among  the  spermatozoa  there  are  parasitic  Gregarines 
(Monocystis)  in  various  stages  of  development. 

(3)  The  spermatozoa  pass  from  the  seminal  vesicles  into 
two  vasa  deferentia  or  male  ducts.  These  open  to  the 
exterior  on  the  i5th  segment.  Each  vas  deferens  bears 
two  ciliated  funnels,  which  collect  spermatozoa  in  segments 
10  and  n,  and  soon  unite  in  one  duct. 

(b]  The  Female  Organs  consist  of  two  ovaries,  and 
two  oviducts  each  of  which  has  a  side  receptacle  for  the 
eggs. 

(i)  The  two  ovaries  are  small  bodies  situated  near  the 


if 

*VIII 

fly 

r  —  Tx^-5 

H-i 

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:^-x-i-5 

XII 

„:„__,  T;.O 

XIII 

1 

XIV 

1 

iXV 

FIG.  62. — Reproductive  organs  of  Earthworm. 
(After  HERING.) 

N.t  Nerve  cord;  J1.,  anterior  testes ;  S.,  sacs  of  setae  ;  R.S.,  recep- 
tacula  seminis  ;  s.,  seminal  funnels;  v.o.,  vas  deferens;  ovd., 
oviduct;  ov.,  ovary;  s.v.,  seminal  vesicles;  VII1.-XV.)  segments. 

nerve  cord  on  the  septum  between  segments  12-13.  Each 
is  pear  shaped,  the  stalk  of  the  pear  being  a  string  of  ripe 
ova.  They  are  more  likely  to  be  seen  than  the  testes. 

(2)  The  two  oviducts  open  internally  on  the  anterior  face 
of  the  septum  between  13-14,  and  externally  on  the 
ventral  surface  of  segment  14.  Into  the  wide  ciliated  in- 
ternal mouths,  which  lie  opposite  the  ovaries,  the  ripe  eggs 
pass. 


196  SEGMENTED    WORMS   OR  ANNELIDA. 

(3)  The  egg  sac  or  receptaculum  ovorum,  near  the  internal 
mouth  of  each  oviduct,  is  a  posterior  diverticulum  of  the 
septum  between  segments  13-14.  Within  it  a  few  mature 
ova  are  stored. 

(c)  Two  pairs  of  receptacula  seminis  or  spermathecae 
receive  spermatozoa  from  another  earthworm,  and  liberate 
them  to  fertilise  the  eggs^f  this  one.  They  are  white 
globular  sacs,  opening  in  the  grooves  between  segments  9-10 
and  10- 1 1.  According  to  some,  these  spermathecae  not 
only  receive  and  store  spermatozoa,  but  make  them  into 
packets  or  spermatophores.  Others  say  that  the  glands  of 
the  clitellum  make  these  packets.  At  any  rate  minute 
thread-like  packets  of  spermatozoa  are  formed,  and  a  pair 
of  them  may  often  be  seen  adhering  to  the  skin  of  the 
earthworm  about  the  saddle  region. 

When  two  worms  unite  sexually  they  lie  apposed  in 
opposite  directions,  the  head  of  the  one  towards  the  tail  of 
the  other.  What  happens  is  that  spermatozoa  of  the  one 
pass  into  the  receptacula  of  the  other. 

When  the  eggs  of  an  earthworm  are  liberated  they  are 
surrounded  by  a  sheath  of  gelatinous  stuff  said  by  some  to 
be  secreted  by  the  saddle.  As  this  is  peeled  off  towards 
the  head  a  spermatophore  is  also  enclosed. 

Development  of  the  Earthworm. 

Many  cocoons  are  made  about  the  same  time,  and  each 
contains  numerous  ova,  and  also  packets  of  sperms,  so  that 
fertilisation  takes  place  outside  the  body.  These  cocoons 
are  buried  in  the  earth  a  few  inches  below  the  surface. 
They  measure  about  a  quarter  of  an  inch  in  length. 

The  favourite  time  for  egg  laying  is  during  the  spring 
and  summer,  though  it  may  be  continued  throughout  the 
whole  year.  The  earthworm  of  the  dung  heap  (L.  fatidus) 
makes  this  a  habit,  induced  probably  by  the  warmth  of  its 
habitat. 

Of  the  many  ova  of  the  earthworm  L.  terrestris,  only 
one  comes  to  maturity,  while  of  L.  fatidus  a  few,  and  of 
L.  communis  two  may  do  so.  But  in  the  last  species  the 
two  embryos  are  often  twins  formed  from  one  ovum,  separa- 
tion taking  place  at  the  gastrula  stage. 


DEVELOPMENT  OF   THE  EARTHWORM.  197 

The  whole  process  of  growth,  until  leaving  the  egg,  lasts 
from  two  to  three  weeks,  the  time  varying  however  with  the 
temperature. 

The  ovum  is  surrounded  by  a  vitelline  membrane,  and  is 
laden  with  yolk  granules.  It  seems  that  several  polar  cells 
are  formed,  probably  by  division  of  the  two  primary  ones 
separated  from  the  ovum.  Segmentation  is  slightly  unequal, 
and  exhibits  considerable  variation  even  within  the  limits  of 
a  species. 

In  about  twenty-four  hours,  a  nearly  spherical,  one  layered 
blastosphere  or  blastula  is  formed.  It  consists  of  only  about 
thirteen  cells.  During  the  next  twenty-four  hours  the  cells 
increase  in  number  rapidly,  but  the  blastula  remains  one 
layered.  Two  cells  lying  together  do  not  take  part  in  this 
division ;  they  are  rather  larger  than  the  rest,  and  their 
inner  ends  project  into  the  cavity  and  are  soon  cut  off. 
Gradually  these  large  cells  begin  to  sink  in,  giving  rise  to 
more  daughter  cells,  and  at  last  are  quite  included  in  the 
cavity.  Thus  there  arise  two  parallel  rows  of  cells  within 
the  blastula,  and  these  define  the  longitudinal  axis  of  the 
embryo.  This  is  the  beginning  of  the  mesoblast  which  will 
form  all  the  muscles  of  the  trunk,  and  which  thus  takes 
origin  from  two  primary  mesoblasts. 

After  five  to  six  pairs  of  secondary  mesoblasts  have  been 
formed,  the  blastula  begins  to  flatten,  and  to  elongate, 
becoming  an  oval  disc.  The  cells  of  the  lower  surface 
become  clearer,  and  the  hypoblast  is  thus  defined.  The  cells 
of  the  upper  surface  are  smaller,  and  become  very  much 
flattened ;  they  compose  the  epiblast.  The  mesoblasts  lie 
side  by  side  near  one  end,  forming  two  rows  extending 
forwards  and  downwards,  but  divergent,  because  of  the 
flattening  of  the  blastula.  The  hypoblast  now  becomes 
concave,  and  thus  the  blastopore  arises,  occupying  the 
whole  of  the  lower  surface.  The  sides  close  in  and  the 
blastopore  becomes  a  slit,  which  further  closes  from  behind 
forwards  leaving  only  a  small  opening, — the  future  mouth. 
During  these  processes  the  cells  at  the  anterior  tip  of  the 
blastopore,  which  will  give  rise  to  the  praeoral  lobe,  undergo 
no  change,  but  the  mesoblast  has  been  active. 

As  gastrulation  proceeds,  the  mesoblast  rows  grow  forwards 
and  upwards  until  they  come  near  each  other  above  the 


I98 


SEGMENTED    WORMS   OR  ANNELIDA. 


rp.c. 


AI-. 


fcEn 


anterior  tip  of  the  blastopore,  while  their  middle  portions 
are  carried  downwards  until  they  lie  on  the  ventral  sur- 
face. Over  them  the  epi blast  is 
thickened  in  two  bands.  Two 
longitudinal  rows  of  epiblast 
cells  near  the  anterior  end,  and 
ending  behind  in  large  cells, 
sink  in  just  as  the  primary 
mesoblasts  did.  The  thickening 
now  extends  ventrally  until  the 
two  bands  meet,  and  passing 
into  the  blastopore  forms  the 
stomatodaeum.  Even  before 
this  the  embryo  has  begun  to 
swallow  the  albumen  in  which 
it  floats. 

There  are  now  two  lateral 
bands  of  cells  called  the  germ 
bands,  composed  of  three  layers : 
outside  is  the  thickened  epi- 
blast, next,  the  rows  of  cells 
which  sank  in,  and  inmost  the 
mesoblast  rows.  The  mesoblast 
rows  have  met  in  the  middle 
line  by  dividing  and  widening 
out  into  a  pair  of  flattened 
plates,  but  they  still  end  behind 
in  the  two  primary  mesoblasts. 
Ccelomic  cavities  develop  in  the 
plates,  and  the  anterior  ends 
meet  above  the  mouth.  The 
epiblastic  rows  which  sank  in 
(there  were  eight  of  them,  four 
on  each  side  of  the  median  line 
and  each  ending  in  a  large 
mother  cell)  go  on  growing. 
The  mother  cells  are  apparently 
carried  backwards  as  the  em- 
bryo lengthens,  leaving  a  trail  of 
daughter  cells  behind  them.  The  cells  so  formed  also 
divide,  the  embryo  rapidly  lengthening  and  finally  becoming 


FIG.  63. — Stages  in  the 
development  of  Earthworm. 
(After  WILSON.) 

1.  2-celled     stage,     p.c,     polar 
bodies. 

2.  Blastula,  m,  a  primary  meso- 
blast. 

3.  Gastrula  stage,  EC,  ectoderm 
or    epiblast ;     En,    endoderm    or 
hypoblast ;  in,  mesoblast  cells. 

4.  Longitudinal  section  in  late 
gastrula  stage,  ec,  ectoderm  ;  en, 
endoderm ;  M,  mouth  ;  st,  stoma- 
todaeum  ;  m,  primary  mesoblasts  ; 
nb,  neuroblasts ;   nc,  nerve  cord  J 
n,  nephridioblasts. 


DEVELOPMENT  OF   THE   EARTHWORM. 


199 


vermiform.  The  two  inner  rows  (neuroblasts)  give  rise 
to  the  nervous  system,  the  next  two  rows  on  either  side 
(nephridioblasts)  form  parts  of  the  nephridia,  while  of  the 
fourth  row  nothing  definite  is  known.  Each  row,  ending 
behind  in  a  single  cell,  widens  out  and  deepens  as  it  is 
traced  forwards,  the  neuroblasts  are  much  further  forwards 
than  the  mesoblasts,  with  the  nephridioblasts  just  behind 
them.  The  neural  and  mesoblastic  rows  can  be  traced 
round  the  mouth  and  help  to  form  the  prostomium,  the 
others  fade  away  at  the  sides  of  the  stomatodaeum.  The 
mesoblast  rows  grow  to  meet  one  another  on  the  median 
dorsal  line. 

Let  us  sum  up  this  complex  history  : — 


Fertilised 
ovum. 


Blastosph 

or 
blastuh 


with  primitive 
mesoblasts. 


Epiblast 

or 
Ectoderm 


'(a)   The   original   outer  layer 

becomes  the  epidermis. 
The  secondary  inner  strat- 
um consists  of  two  rows  of 
neuroblasts  which  form 
the  nervous  system,  and 
of  four  rows  of  nephridio- 
blasts which  form  parts 
of  the  nephridia. 


Mesoblast 


"  mesoblasts." 


Lining  of 


General  Development  of  the  Organs.  —  Though  it  will  involve  a  slight 
repetition,  we  shall  now  describe  the  origin  of  the  various  organs. 

The  skin  arises  from  the  original  outer  wall  of  the  gastrula.  The 
"setigerous  glands,"  within  which  the  setts  develop,  and  from  which 
they  push  their  way  to  the  exterior,  arise  partly  from  the  rows  of  cells 
started  by  the  nephridioblasts,  and  partly  in  all  probability  from  the 
outermost  of  the  four  cell  rows  previously  mentioned.  The  double 
ventral  nerve  cord  arises  from  the  neuroblasts.  The  two  cerebral  ganglia 
originate,  according  to  Kleinenberg,  independently  of  the  ventral  cord 
from  a  median  unpaired  apical  plate  of  ectoderm,  while  according  to 
Wilson  they  arise  along  with  the  ventral  cord,  and  have  their  founda- 
tions in  the  thickened  anterior  end  of  each  of  the  two  neural  rows. 

The  history  of  the  excretory  system  is  complex,  (a)  At  the  anterior 
end  of  young  embryos,  a  group  of  ectoderm  cells,  dorsal  in  position, 
forms  a  larval  excretory  organ,  which  wholly  disappears  in  later  stages. 
(b]  Next  appear  two  ciliated  canals  in  the  anterior  region,  closed  inter- 


200  SEGMENTED    WORMS   OR  ANNELIDA. 

nally,  opening  on  the  head.  These  are  known  as  "provisional  nephridia  " 
or  "head  kidneys."  They  degenerate  as  the  permanent  excretory 
organs  develop,  (c)  The  numerous  permanent  nephridia  are  for  the 
most  part  ectodermic,  arising  from  the  rows  of  nephridial  cells  already 
described.  Two  parts  of  each  nephridium,  however,  have  a  meso- 
blastic  origin,  viz.,  the  innermost  part  or  the  ciliated  funnel,  and  the 
peritoneal  investment  which  ensheaths  the  whole  organ. 

By  the  invagination  of  the  blastosphere,  a  globular  gastrula  cavity  is 
formed.  This  forms  the  archenteron, — the  future  mid  gut, — and  elon- 
gates with  the  growth  of  the  embryo.  To  the  completion  of  the  entire 
alimentary  canal,  however,  two  other  processes  are  necessary,  an  in- 
tucking  of  ectoderm  from  in  front — the  stomatodaum  or  "fore  gut  " — 
which  pushes  the  archenteron  backwards  and  forms  the  future  pharynx, 
and  a  similar  in-tucking  of  ectoderm  from  behind — the  proctod&um  or 
<; hind  gut" — which  meets  and  fuses  with  the  archenteron,  and  forms 
the  anus  and  a  small  portion  of  the  posterior  gut. 

The  mesoderm  begins  with  the  two  primary  mesoblasts  already 
described.  These  multiply  and  form  mesoderm  bands,  which,  insinua- 
ting themselves  between  ectoderm  and  endoderm,  proceed  to  surround 
the  gut.  At  the  same  time,  some  of  the  mesoderm  cells  become 
migratory,  wander  on  to  the  head,  and  also  surround  the  gut,  before  the 
final  trunk  musculature  is  completed.  The  migratory  mesoblasts  of  the 
trunk  appear  to  form  a  special  larval  musculature  precociously  devel- 
oped, in  order  to  enable  the  embryo  to  manage  the  enormous  mass  of 
albumen  (absorbed  from  the  capsule)  with  which  its  body  is  distended. 
The  mesoderm  bands  grow  in  strength,  and  form  a  complete  ring 
encircling  the  archenteron. 

Origin  of  the  body  cavity. — The  mesoderm  bands,  growing  in  strength, 
become  twro  layered.  These  two  layers  separate,  the  inner  (splanchnic) 
cleaving  to  the  gut,  the  outer  (somatic)  clinging  to  the  body  wall.  The 
space  between  them  is  the  body  cavity  or  cctlome.  But  as  the  separa- 
tion of  somatic  and  splanchnic  layers  takes  place,  partitions  are  also  ' 
formed  transversely,  to  become  the  septa  which  partition  off  the  body 
cavity  into  a  series  of  segments.  The  cavity  of  the  pre-oral  segment  or 
prostomium  differs  somewhat  from  that  of  the  others,  being  from  the 
first  unpaired,  instead  of  including  two  lateral  cavities  one  on  each  side 
of  the  gut. 

As  to  the  blood  vessels,  the  ventral  or  sub-intestinal  appears  first,  as  a 
space  between  the  wall  of  the  archenteron  and  the  underlying  meso- 
derm ;  the  dorsal  vessel  has  a  double  origin,  arising  from  the  fusion  of 
two  lateral  vessels  which  develop  like  the  ventral.  The  important 
point  is,  that  the  blood  vessels  are  at  first  long  lacunar  spaces,  which 
gradually  acquire  definite  walls.  By  and  by  the  "  hearts  "  and  other 
complications  in  the  vascular  system  appear. 

The  reprodtictive  organs,  though  probably  arising  from  cells  which 
have  kept  to  some  extent  apart  from  the  formation  of  the  embryo, 
certainly  appear  in  association  with  the  mesoderm. 

The  above  is  the  account  of  the  development  of  Lumbricus  given  by 
Wilson ;  another  investigator,  Bergh,  differs  from  Wilson  in  several 
important  points.  First,  with  regard  to  the  nomenclature  of  the  con- 
stituents of  the  germ  bands. 


STRUCTURE   OF  ARENICOLA. 


201 


WILSON. 

Two    inner    primitive    Neuroblasts. 
cells. 


Next    three    cells    on 
either  side. 

Posterior   cell  of  each 
side. 


/Two  nephridioblasts, 
\     one  lateral  cell, 

Primitive  mesoblast. 


BERGH. 

Neuroblasts. 


f  Outer     or     anterior 
\     myoblasts. 

Inner      or       posterior 
myoblast. 


According  to  Bergh  the  germ  band  consists  originally  of  three  cells 
on  each  side,  the  neuroblast,  the  primary  inner  myoblast,  the  outer 
myoblast.  The  primary  inner  myoblast  later  gives  origin  to  the  three 
inner  myoblasts,  nephridioblasts,  and  lateral  cell  of  Wilson. 

Further,  Bergh  states  that  at  an  early  stage  a  "nerve  plexus"  arises 
on  the  mid-ventral  line,  probably  from  the  ectoderm,  and  that  this 
unites  with  the  neuroblastic  rows  to  produce  the  nervous  system  of  the 
adult. 


Type  of  POLYCH^TA.     The  Lob  Worm  (Arenicola 
piscatoruni). 

Habits. 

On  the  flat  sandy  beach  uncovered  at  low  tide,  the 
" castings"  of  the  lob  worm  are  very  numerous.  There 
the  fishermen  seek  the  worms  for  bait,  and  have  to  dig 
deep,  for  the  burrowers  rapidly  retreat  far  into  the  sand. 
The  burrows  of  the  lob  worm  are  cylindrical  tubes,  lined  by 
a  yellowish  green  secretion,  and  the  surrounding  sand  is 
often  discoloured  by  some  change  in  which  the  organic 
juices  reduce  the  iron  constituents  to  lower  oxides.  The 
tubes  are  at  first  vertical,  and  afterwards  oblique  or  hori- 
zontal. 

The  lob  worm  burrows  like  the  earthworm — eating  the 
sand  for  the  sake  of  the  organic  particles  or  small  organisms 
which  it  contains.  The  sandy  castings,  which  pass  from 
the  end  of  the  food  canal,  and  are  got  rid  of  at  the  mouth 
of  the  tube,  fall  into  spiral  coils.  When  getting  rid  of  the 
castings,  the  worm  lies  with  its  tail  upwards  and  its  head 
downwards,  or  with  its  body  bent  like  a  bow;  when  the 


202  SEGMENTED    WORMS   OR  ANNELIDA. 

tide  comes  in,   the  mouth  may  protrude.     The  animal  is 
able  to  turn  in  its  burrow. 

The  young  are  pelagic,  and  it  is  said  by  some  that  the 
adults  are  active  and  swim  about  at  certain  seasons. 

External  Appearance. 

The  lob  worm  varies  in  length  from  eight  inches  to  a 
foot,  and  at  its  thickest  part  is  about  half  an  inch  in 
diameter.  There  are  three  regions  in  the  body : — (a)  the 
anterior  seven  segments,  of  which  all  but  the  first  have 
bristles ;  (b}  the  gill  bearing  region  of  thirteen  segments ; 
(c)  the  thinner  posterior  part  of  variable  length,  without  either 


FIG.  64. — Arenicola  piscatorum.     (After  CUNNINGHAM 
and  RAM  AGE.) 

Note  anterior  protrusion  of  mouth  ;  setae  on  anterior  region ; 
setae  and  gills  on  median  region ;  thinner  tail  region  often  longer 
than  shown. 

bristles  or  gills.     The  head  lobe  is  very  smallj;  there  are  no 
tentacles  or  eyes.     Anteriorly  a  soft  proboscis  is  protruded. 

Skin  and  Muscles. 

Each  segment  is  marked  by  several  rings ;  there  are 
numerous  warts  on  the  posterior  region.  Most  externally 
lies  the  cuticle,  then  the  pigmented  epidermis,  then  the 
circular  and  the  longitudinal  muscle  fibres. 

Appendages. 

Unlike  many  of  the  marine  Annelids  which  have  on  each 
segment  well-developed  outgrowths  or  parapodia,  divided 
into  a  dorsal  notopodium  and  a  ventral  neuropodium, 
Arenicola  has  very  rudimentary  appendages.  This  reduc- 
tion of  appendages  must  be  associated  with  the  animal's 
mode  of  life;  the  same  is  true  of  many  tube  inhabiting 
worms.  The  first  segment  has  no  trace  of  appendages, 


STRUCTURE   OF  A  RE  NICOLA.  203 

the  next  nineteen  have  rudiments.  The  dorsal  part  con- 
sists of  a  tuft  of  bristles,  whose  bases  are  enclosed  in  a 
sac ; — the  ventral  part,  separated  by  a  short  interval,  bears 
several  hooks. 

Nervous  System. 

The  nervous  system  is  in  its  general  features  like  that  of 
the  earthworm,  but  ganglia  are  not  developed.  In  the 
ventral  nerve  cord,  the  ring  round  the 
gullet,  and  the  slight  cerebral  enlarge- 
ment which  represents  a  brain,  nerve 
cells  occur  diffusely  scattered  among 
the  nerve  fibres.  Along  the  dorsal  sur- 
face of  the  nerve  cord  run  two  "giant 
fibres  "  like  those  in  the  earthworm. 

In  some  species  at  least,  the  head  lobe  is 
distinctly  sensory  and  there  are   two   ciliated 

FIG.  65. Anterior       "neck  organs."      Otherwise  sense  organs  are 

part  of  nervous  system  represented  only  by  a  pair  of  otocyst  sacs,  one 
in  Arenicola.  (After  on  each  side  of  the  oesophageal  nerve  ring. 
VOGT  and  YUNG.)  These  sacs,  like  those  which  occur  in  many 

other  Invertebrates,  seem  to  have  to  do  rather 

c.,  Cerebral   part   on  -,     ,          ,.       i.  r  ,_,  .       ,, 

dorsal    surface ;    ee.r.         with  tne  direction  of  the  animals  movements 

oesophageal   ring;    g.\       than  with  hearing.     Prof.  Ehlers  notes  an  in- 

nervet;  rd'*'f '  7^™!       teresting  series  :— In  A.  Claparedii,  there  are 

nervesT^.',  otocyst!*          simply  two  open  grooves ;  in  A.  marina,  the 

sacs    have    open    necks   and    contain   foreign 

particles  ;  in  A.  Grubii  and  A.  antillensis,  the  sacs  are  closed  and  con- 
tain intrinsic  otoliths  of  lime. 

Food  Canal. 

The  mouth  is  at  the  end  of  a  protrusible  cup-like 
proboscis;  the  gullet  has  smooth  walls,  and  bears  an  an- 
terior and  a  larger  posterior  pair  of  glands  which  secrete 
a  yellowish  fluid  perhaps  digestive;  the  succeeding  part 
of  the  gut  is  covered  with  yellow  cells  and  many  blood 
vessels,  and  is  divided  into  rings ;  the  terminal  portion  is 
full  of  sand  from  which  the  nutritive  matter  has  been 
absorbed ;  the  anus  is  at  the  very  end. 

The  Body  Cavity. 

The  body  cavity  is  spacious,  except  in  the  tail  region, 
and  contains  a  fluid.  Anteriorly  there  are  three  transverse, 


204  SEGMENTED    WORMS   OR  ANNELIDA. 

partly  muscular,  partitions  or  mesenteries  which  moor  the 
gullet ;  in  the  tail  region  there  are  many  such ;  the  median 
part  of  the  gut  swings  freely.  Posteriorly  there  are  also 
oblique  partitions  which  divide  the  segments  into  a  median 
and  two  lateral  chambers. 

The  Vascular  System. 

The  blood  has  a  bright  red  colour.     It  flows  forward  in  a 
dorsal   vessel,   running  along   the   mid-dorsal   line   of  the 


b  s 

,/> 


FIG.  66. — Dissection  of  anterior  region  of  Arenicola. 
(After  COSMOVICI.) 

M.2.,  m.3.  Second  and  third  mesenteric  septa ;  #.,  ventral  nerve- 
cord  ;  b.s.,  bristle  sac  ;  z/.z/.,  ventral  vessel ;  n.6.,  sixth  nephridium ; 
z.,  intestine;  l.v.,  lateral  vessel;  d.v.^  dorsal  vessel;  ^.,  heart; 
£•/.,  one  of  the  two  larger  oesophageal  glands ;  a*.,  oesophagus. 

gut,  backward  in  a  ventral  vessel  below  the  gut.  Two 
sub-intestinal  vessels  lie  between  the  ventral  vessel  and  the 
gut,  and  receive  tributaries  from  the  anterior  gills.  (Some 
deny  that  the  sub-intestinal  vessel  is  double.)  On  each  side 
of  the  digestive  part  of  the  gut  there  is  a  lateral  vessel. 

Just  behind  the  posterior  pair  of  cesophageal  glands  lies  a 
very  contractile  heart.     It  consists  of  two  lateral  chambers 


STRUCTURE   OF  ARENICOLA.  205 

or  ventricles,  each  of  which  receives  blood  from  the  dorsal 
vessel,  from  a  sub-intestinal  vessel,  and  from  a  lateral  vessel, 
and  drives  blood  into  the  ventral  vessel.  Each  of  the  lateral 
vessels  before  entering  the  heart  expands  into  a  kind  of 
auricle. 

The  longitudinal  vessels  are  all  connected  by  transverse 
branches.  From  the  ventral  vessel  arise  afferent  branchial 
vessels.  From  the  seven  posterior  gills  efferent  branches 
enter  the  dorsal  vessel ;  while  those  from  the  six  anterior 
gills  join  the  sub-intestinals.  Each  efferent  vessel  gives  off 


FIG.  67. — Cross  section  of  Arenicola.     (After  COSMOVICI.) 

E.  Epidermis;  c.m.,  circular  muscles  ;  /.?«.,  longitudinal  muscles; 
b.c.,  body  cavity ;  gl.,  gill;  s.,  setae;  «./.,  nephridial  pore;  a.br., 
afferent  branchial ;  e.lr.,  efferent  branchial;  «.,  ventral  nerve-cord 
with  blood  vessels  above;  d.v.,  dorsal  vessel;  /.#.,  lateral  vessel; 
s.z'.v.,  sub-intestinal  vessel ;  v.v.,  ventral  vessel ;  £-.,  gut. 

a  branch  to  the  skin,  while  the  dorsal  and  sub-intestinal 
vessels  give  off  numerous  branches  to  the  walls  of  the  gut. 
It  seems  that  the  flow  of  the  blood  is  not  always  quite  the 
same. 

Respiratory  Sy stern. 

There   are  thirteen   pairs  of  gills.      Each   is   a   tuft    of 
thread-like  branches,  through  the  thin  walls  of  which  the 


206  SEGMENTED    WORMS   OR  ANNELIDA. 

red  blood  shines.  As  the  papillae  on  the  proboscis  are 
hollow  and  contain  vessels,  they  are  doubtless  of  respira- 
tory significance.  Indeed,  the  gills  may  be  regarded  as 
exaggerated  papillae. 

Excretory  System. 

In  the  anterior  region,  from  the  fifth  to  the  tenth  seg- 
ments, there  are  six  pairs  of  nephridia.  Each  consists 
of  three  parts — a  funnel  opening  into  the  body  cavity,  a 
glandular  portion,  and  a  bladder  communicating  with  the 
exterior. 

Reproductive  System. 

The  sexes  are  separate  and  similar.  The  reproductive 
organs  are  very  simple,  and  lie  on  the  peritoneal  mem- 
brane of  the  body  cavity.  They  are  developed  in  close 
association  with  the  nephridia.  The  reproductive  cells  are 
liberated  into  the  body  cavity,  and  there  matured.  In 
August  and  September  they  pass  out  by  the  nephridia. 
Little  is  known  in  regard  to  the  development,  beyond 
the  fact  that  the  young  are  for  a  time  free  swimming 
pelagic  forms. 

Development  of  Polychceta. 

As  an  example  of  the  development  of  the  marine  Chsetopods,  we  may 
take  Eupomatus,  which  has  been  investigated  by  Hatschek.  Here 
segmentation  is  complete  but  somewhat  unequal,  and  results  in  the 
formation  of  a  blastula,  with  its  upper  hemisphere  composed  of  small 
(ectodermic)  cells,  and  the  lower  of  large  (endodermic)  cells.  Among 
these  latter  are  two  spherical  cells — the  primitive  mesoblasts.  Invagina- 
tion  takes  place  in  the  usual  way  to  form  a  gastrula,  the  primitive  meso- 
blasts divide  and  form  mesoblastic  bands.  During  these  processes  the 
external  form  has  altered  considerably.  The  apical  (aboral)  region  of 
the  gastrula  becomes  tilted  forward,  an  ectodermic  invagination  arises 
posteriorly,  and  uniting  with  the  archenteron  produces  hind-gut  and 
anus,  while  a  similar  insinking  anteriorly,  in  the  region  of  the  blasto- 
pore,  forms  fore-gut  and  mouth.  The  larval  gut  so  formed  has  a 
distinct  ventral  curve.  Cilia  appear  on  the  surface  at  an  early  stage,  and 
now  form  a  distinct  pre-oral  ring,  and  also  a  less  constant  post-oral  ring. 
At  the  apex  of  the  pre-oral  region  an  ectodermic  thickening  takes  place, 
this  gives  rise  to  an  apical  ganglion  with  which  sensory  structures  are 
often  associated.  The  mesodermic  bands  give  rise  to  muscle  cells  used 
in  swimming,  and  also  to  the  "  head  kidneys " — a  pair  of  larval  ex- 
cretory tubes.  The  larva  so  formed  is  a  typical  Trochosphere,  such  as 
occurs  in  the  great  majority  of  Polychseta,  in  a  more  or  less  modified 


It 

FIG.  68. — Development  of  Polygordius.     (After  FRAIPONT.) 

#.,  Mother  sperm  cell  ;  3.,  c.,  sperm  morulae  ;  d.,  spermatozoa. 

i.  Ovum  with  large  nucleus ;  2.  2-cell  stage ;  3.  4-cell  stage ;  4.  blastosphere ; 
5.  gastrula  ;  ac.,  archenteron  ;  6.  closure  of  gastrula  mouth  or  blastopore ;  7.  for- 
mation of  stomatodaeum^z1.),  and  proctodaeum  (/r.),  invaginated  to  meet  archenteron 
(#£.)  j  8.  complete  gut  formed;  9.  elongation  of  larva;  ap.  sfi.,  apical  spot;  «"/., 
ciliated  ring;  neph.,  primitive  nephridia ;  10.  formation  of  posterior  segments; 
ii.  Form  of  adult  Polygordius. 


208  SEGMENTED    WORMS   OR  ANNELIDA. 

guise  in  many  other  worm-types,  and   also   in   Molluscs.      We   may 
summarise  the  chief  characters  of  the  Trochosphere  thus  : — 

(1)  There  is  a  prominent  pre-oral  region  with  an  apical  ganglion  and 
a  ring  of  cilia. 

(2)  The  gut  has  a  distinct  ventral  curve  and  a  threefold  origin. 

(3)  The   larval   body  cavity  is   simply  the    persistent   segmentation 
cavity,  and  in  it  posteriorly  lie  the  primitive  mesoblasts. 

The  Trochosphere  is  a  free  swimming  pelagic  larva  which,  among 
worms,  corresponds  largely  to  the  future  head  region  of  the  adult.  Its 
metamorphosis  into  the  adult  probably  takes  place  in  the  most  primitive 
fashion  in  the  little  worm  Polygordius.  We  shall,  therefore,  follow  it 
there  (Fig.  68). 

In  the  larva,  which  is  a  typical  Trochosphere,  the  first  sign  of 
segmentation  appears  in  the  bands  of  mesoblast.  These  become  divided 
into  successive  segments,  while  at  the  same  time  the  posterior  region  of 
the  larva  elongates  greatly,  carrying  the  larval  gut  backwards  with  it. 
Meanwhile,  a  cavity  appears  in  each  of  the  mesoblastic  segments.  These 
cavities,  taken  together,  form  the  adult  body  cavity ;  the  outer  and 
inner  walls  form  the  somatic  and  splanchnic  layers ;  the  posterior  and 
anterior  walls  of  adjacent  segments  fuse  to  form  the  septa  of  the  adult 
worm  ;  the  inner  (splanchnic)  walls  of  the  primitive  segments  on  each 
side  fuse  above  and  below  the  gut  to  form  the  dorsal  and  ventral  sup- 
porting mesenteries  of  the  gut.  The  head  region  is  at  first  dispropor- 
tionately large,  but  later  by  an  independent  process  of  growth  becomes 
reduced.  The  larva  abandons  its  pelagic  life  and  becomes  adult. 

Comparing  the  development  of  Polychseta  with  this,  we  find  that  the 
Trochosphere  is  often  modified,  and  that  segmentation  tends  constantly 
to  appear  at  an  earlier  stage.  As  a  further  step  in  the  same  direction, 
we  may  note  that  in  some  Polychreta  the  Trochosphere  stage  is  no  longer 
recognisable  as  such. 

A  general  contrast  of  the  modes  of  Development  in  different 

Annelids. 

A.  B. 

"  Larval "  Types  "  Fcetal  "  Types 

as  in  as  in 

marine  ChiX3topods,  Earthworm,  Leech,  £c. 
Polygordius,  &c. 

Development  indirect.  Development  direct,  within  egg 

A  free  swimming  Trochosphere  capsule  ;  Trochosphere  stage  almost 

stage,  with  trunk  almost  or  wholly   or  wholly  suppressed. 

suppressed,     with     head     region 

greatly   developed,    with    adapta-   ^— A N 

tions  to  free  marine  life.  Lumbrtcus  type  Clepsine  type 

with  little  nutri-  with  much  nutri- 
tive material  in  tive  material  in 
ovum,  with  gas-  ovum,  with  gas- 
trula  formed  by  trula  therefore 
invagination  (em-  formed  by  over- 
bolic).  growth  (epibolic.) 


CLASSIFICATION  OF  CH&TOPODA.  209 

General  Survey  of  Chcetopoda. 

I.  Oligochccta. 

Of  Lumbricus  there  are  many  species,  e.g.,  the  common  earthworm 
L.  terrestris,  the  dunghill  worm  L.  fcetidus,  and  L.  com  munis  or 
trapezoides,  whose  ova  usually  form  twins.  We  may  conveniently 
include  under  the  title  "  earthworms  "  a  great  array  of  animals  more  or 
less  like  Lumbricus,  and  usually  described  as  terricolous  Oligocketa. 
These  are  arranged  by  Beddard  in  four  main  groups— LUMBRICINI, 
GEOSCOLEC.INI,  ACANTHODRILINI,  and  EUDRILINI,  with  a  divergent 
group  MONILIGASTRES.  It  is  enough  for  us  to  notice  here  that  the 
modern  classification  is  chiefly  based  on  the  modifications  of  the 
excretory  system.  The  largest  "  earthworm  "  is  a  Tasmanian  species — 
Megascolides  gippslandicus — measuring  about  six  feet  in  length,  said  to 
make  a  gurgling  noise  as  it  retreats  underground. 

To  these  must  then  be  added  a  number  of  families,  Tubificida, 
Enchytraidce,  &c.,  which  live  in  mud  and  water,  and  are  often  called 
limicolous  Oligochoeta.  Of  these  a  very  common  representative  is  the 
little  river  worm  Tubifex  rivulorum,  often  found  in  the  mud  of  brooks, 
and  well  suited  in  its  transparency  and  small  size  for  microscopic 
examination.  Also  notable  is  the  fresh  water  Nats,  with  remarkable 
powers  of  asexual  budding.  Another  interesting  ally  of  Tubifex  is 
Branchiura,  which  has  paired  contractile  gills  on  each  of  the  posterior 
segments  of  its  body,  thus  resembling  a  Polychaete. 

The  leech-like  Branchiobdella,  which  is  parasitic  on  the  crayfish,  is 
apparently  an  abnormal  Oligochaete. 

II.  Polychata. 

Living  in  surroundings  usually  very  different  from  those  of  the  more 
or  less  subterranean  earth-  and  mud-worms,  the  marine  Polychoeta  have 
a  richer  development  of  external  structures,  and  a  more  complex  life 
history.  From  the  sides  of  the  body  rings  distinct  outgrowths  form 
the  first  genuine  legs.  These,  known  as  parapodia,  bear  bundles  of 
firm  bristles,  and  are  typically  divisible  into  a  ventral  neuropodium  and 
a  dorsal  notopodium.  Each  of  these  is  usually  furnished  with  a  probably 
tactile  process,  the  two  being  known  respectively  as  the  notopodial 
cirrus  and  the  neuropodial  cirrus.  With  the  notopodium  the  first  true 
gills,  which  contain  prolongations  of  the  body  cavity,  are  often  associated, 
but  the  respiratory  plates  which  occur  in  the  sea  mouse,  &c. ,  are  probably 
metamorphosed  dorsal  cirri.  Parapodia  are  absent  from  the  anterior 
region,  but  this  is  frequently  well  furnished  with  tactile  cirri,  as  well  as 
with  eyes,  "  ear  sacs,"  and  other  sensitive  structures.  The  eyes  show  an 
interesting  series  of  gradations  from  simple  pigment  spots  to  very  com- 
plicated structures  (e.g.,  Alciope],  exhibiting  cornea,  crystalline  lens, 
retina,  &c.  In  many  cases  in  these  marine  worms  the  blood  is  red  as  it 
is  in  most  Oligochsetes,  but  it  may  be  colourless  (Aphrodite],  green 
(Sabella),  or  yellow.  The  pigment  is  usually  dissolved  in  the  plasma, 
and  its  variations  in  character  and  amount  among  nearly  allied  forms  are 
of  great  interest  to  the  comparative  physiologist.  The  gut  is  frequently 
branched  and  of  large  calibre.  In  some  cases  (CapitellidDe)  it  possesses 
an  accessory  communicating  tube  (Nebendarm)  which  is  of  interest, 
U 


SEGMENTED    WORMS   OR  ANNELIDA. 


having  been  compared  to  the  notochord  of  Ve-rtebrates.  The  sexes  are 
usually  separate,  the  reproductive  organs  simple  and  devoid  of  accessory 
structures.  The  nephridia  function  as  genital  ducts.  There  is  a 
metamorphosis  in  development. 

(a)  Some   of  these  marine  Polychcetes  lead  a  free  and  more  or  less 
active  life,  crawling  between  tidemarks  or  on  the  sea  bottom,  burrowing 
in  the  sand,  or  swimming  in  the  open  water.    These  Errantia  have  well- 
developed  appendages,  and  a  large  pre-oral  segment,  and  are  generally 
furnished   with   eyes  and   well-developed  antennre.      Gills  are  usually 
associated  with  the  dorsal  parts  of  the  parapodia.      Most  of  them  feed 
on  other  animals,  and  have  sharp  "horny  jaws,"  while  the  anterior  part 
of  the  gut  is  protrusible  as  a 

proboscis. 

Nereis  and  Nephthys  are 
two  common  genera,  species 
of  which  may  be  unearthed 
by  digging  in  the  sand  close 
to  rocks,  though  at  times 
these  or  other  species  are 
seen  swimming  freely.  The 
sea  mouse,  Aphrodite,  has 
irridescent  bristles,  a  feltwork 
of  matted  hair  covering  large 
gill  plates  which  lie  along  its 
back,  a  very  large  muscular 
pharynx,  and  a  gut  with 
numerous  irregular  branches 
extending  throughout  the 
body.  A  very  common 
shore  form,  a  little  like  a 
small  Aphrodite,  is  Polynoe. 
As  an  actively  errant  worm, 
with  well  developed  eyes, 
Alciope  may  be  noted,  and 
the  family  of  Syllicke  is  re- 
markable for  the  unusually 
prolific  asexual  budding, 
which  sometimes  results  in 
a  chain  or  even  an  irregular 
branched  aggregate  of  individuals.  As  the  cuticle  is  often  irridescent, 
and  as  the  red  blood  may  shine  through  the  skin,  these  marine  worms 
are  frequently  beautiful.  The  list  of  nymphs  and  goddesses  has  been  the 
source  of  such  titles  as  Nereis,  Aphrodite,  Eunice,  and  Hermione,  and 
one  can  almost  believe  the  legend,  according  to  which  a  specialist  on 
Errantia  christened  his  daughters  after  his  seven  favourites. 

(b]  Other  marine  Polycheeta,  however,  lead  a  more  sluggish  life  within 
various  kinds  of  tubes,  limy,  sandy,  papery,  or  gelatinous.     As  one  would 
expect,  their  parapodia  are  minute,  apt  to  degenerate,  and  often  used 
solely  for  clambering  within  the  tube.     The  pre-oral  region  is  small,  but 
the  anterior  rings  usually  bear  gills,  cirri,  and  tentacles,  often  in  rich 
profusion.     These  Sedentaria  rarely  have  a   protrusible  pharynx,  and 


FIG.  69. — Parapodium  of  a  Marine 
Polychrete,       Heteronereis.        (From 

QUATREFAGES. ) 

A,  Notopodium  ;  B,  neuropodium  ;  a, 
notopodial  cirrus ;  f,  neuropodial  cirrus ; 
b,  c,  g;  gill  plates  ;  e,  i,  tufts  of  bristles. 


CLASSIFICATION  OF  CHALTOPODA.  211 

never  "  jaws."     Most  of  them  feed  on  minute  Algse  swept  in  by  the  cilia 
on  the  tentacles  and  other  structures  about  the  mouth. 

The  fisherman's  lob-worm  (Arenicola)  burrows  on  the  sandy  shore 
like  Lumbricus  in  the  fields.  Common  also  on  the  shore  within  a  tube 
of  glued  sand  particles  is  Terebella  or  Lanice  conchilega^  where  the  ex- 
cretory tubes  are  partly  united  by  a  longitudinal  tube  in  a  manner  sug- 
gestive of  the  segmental  duct  which  connects  the  nephridia  of  a  young 
Vertebrate.  The  twisted  limy  tubes  of  Serpula  are  common  outside 
shells  and  all  sorts  of  marine  objects,  and  the  animal  bears  a  stopper 
or  operculum,  with  which  it  closes  the  mouth  of  its  tube,  but  through 
which  it  probably  at  the  same  time  breathes.  In  deep  water,  within 
a  yellow  parchment -like  tube,  Chatopterus  may  be  dredged,  perhaps 
the  strangest  form  of  all. 

III.  Echiuridce. 

In  holes  in  the  rocks  on  some  of  the  warmer  European  coasts  lives  a 
curious  "worm" — Bonellia  viridis,  of  a  beautiful  green  colour,  with  a 
globular  body  and  a  long,  grooved,  anteriorly  forked,  pre-oral  protru- 
sion. Such,  at  least,  is  the  female,  but  the  male  is  microscopic  in  size, 
hopelessly  degenerate,  living  parasitically  in  or  on  his  mate.  The 
male  resembles  in  some  ways  a  Turbellarian,  is  mouthless  and  gutless, 
and  little  else  than  a  migratory  spermatophore.  By  means  of  cilia,  it 
moves  from  one  part  of  the  female  to  another,  and  fertilises  the  eggs  in 
a  modified  excretory  tube,  which  serves  the  female  Bonellia  as  a  uterus. 
Here  illustrated  in  extreme,  we  see  the  usual  inequality  (in  size)  between 
the  sexes. 

Less  abnormal  than  Bonellia  are  the  genera  Echiurus  and  Thal- 
assenia. 

In  this  small  sub-order  the  adults  have,  at  most,  indistinct  traces  of  the 
segments  which  the  young  forms  exhibit.  Nor  are  there  parapodia, 
cirri,  or  gills,  but  setae  are  always  represented  (except  in  the  male 
Bonellia}  by  two  anterior  bristles,  and  in  Echiurus  by  posterior  spines 
as  well.  The  nerve  cord  is  unsegmented,  and  there  is  but  a  slight 
anterior  ring  without  a  brain.  The  anterior  part  of  the  body  forms  a 
muscular,  well-innervated,  ciliated  proboscis,  with  the  mouth  deeply 
situated  at  its  base  ;  the  gut  is  much  coiled,  bears  a  curious  adjacent  tube 
known  as  the  "collateral  intestine,"  and  a  pair  of  excretory  "anal 
glands  "  opening  into  the  body  cavity  by  ciliated  funnels.  There  is  a 
terminal  anus.  There  are  dorsal  and  ventral  blood  vessels,  and  two  or 
three  pairs  of  nephridia,  one  or  more  of  which  function  as  reproductive 
ducts.  The  sexes  are  separate,  and  the  reproductive  elements  are 
formed  on  the  walls  of  the  body  cavity,  into  which  they  are  liberated. 
There  is  a  metamorphosis  in  development,  the  larvse  differing  from  the 
adults  in  many  ways,  e.g.,  in  being  segmented. 

Appendix  (i)  to  Chcetopoda. 

PRIMITIVE  CHyETOPODS  AND  ANNELIDS  (Archi-Choetopoda  and 
Archi-Annelida). 

An  aberrant  Chsetopod  type  is  represented  by  Saccocirrus,  a  small 
marine  "worm"  with  many  primitive  characteristics.  The  body  is 


212  SEGMENTED    WORMS   OR  ANNELIDA. 

segmented,  and  very  uniform  throughout  ;  the  pre-oral  region  is  small, 
but  the  mouth  segment  is  large  ;  there  are  bundles  of  setae  on  the  rings  ; 
the  nervous  system  remains  embedded  in  the  epidermis. 

More  primitive,  however,  are  the  Archi-Annelida  represented  by 
Polygordius,  Protodrilus,  and  Histriodrihis — all  marine.  The  small 
body  is  segmented  and  uniform  ;  there  are  no  setre,  parapodia,  cirri, 
or  gills,  but  the  head  bears  a  few  tentacles ;  as  in  Saccocirrus,  the 
pre-oral  region  is  small,  and  the  segment  around  the  mouth  is  large  ; 
the  very  simple  nervous  system  is  retained  in  the  epidermis. 

Polygordius  is  a  thin  worm,  an  inch  or  more  in  length,  living  at 
slight  depths  in  sand  or  fine  gravel,  often  along  with  the  lancelet.  It 
has  a  few  external  cilia  about  the  mouth  in  a  pair  of  head-pits,  and 
sometimes  on  the  body ;  it  moves  like  a  worm,  but  has  no  bristles.  It 
feeds  like  an  earthworm,  or  sometimes  more  discriminatingly  on  uni- 
cellular organisms.  The  females  are  usually  larger  than  the  males,  and 
in  some  species  break  up  at  sexual  maturity.  The  development  includes 
a  metamorphosis,  and  the  larvae  seem  to  throw  some  light  on  the  nature 
of  the  ancestral  Annelids.  They  are  ciliated,  free  swimming,  light 
loving,  surface  animals,  feeding  on  minute  pelagic  organisms,  seeking  the 
depths  as  age  advances.  According  to  some,  the  larva  represents  a 
primitive  unsegmented  ancestral  Annelid  with  medusoid  affinities ; 
according  to  others,  the  larval  characteristics  are  adaptive  to  the  mode 
of  life,  and  without  historic  importance. 

Protodrilus  is  even  smaller  than  Polygordius,  with  more  cilia,  mobile 
tentacles,  and  two  fixing  lobes  on  the  posterior  extremity  ;  the  move- 
ments are  Turbellarian-like,  the  reproductive  organs  hermaphrodite, 
the  development  direct.  Histriodrihis  is  parasitic  on  the  eggs  of  the 
lobster. 

Appendix  (2)  to  Chatopoda. 
PARASITIC  AND  DEGENERATE  CH^TOPODS.     MYZOSTOMATA. 

The  remarkable  forms  (Myzostoma)  included  in  this  small  class,  live 
parasitically  on  feather  stars,  on  which  they  form  galls.  They  are 
regarded  as  divergent  offshoots  from  primitive  Annelids,  the  larval  form 
showing  some  distinctly  Chaetopod  characters.  The  minute  disc-like 
body  is  unsegmented,  and  bears  five  pairs  of  parapodia,  each  with  a 
grappling  hook,  with  which  five  pairs  of  suckers  usually  alternate. 
There  are  also  abundant  cirri.  The  skin  is  thick,  the  body  muscular, 
the  nervous  system  is  concentrated  in  a  ganglionic  mass,  which  encircles 
the  gullet,  and  gives  off  abundant  branches.  There  is  a  protrusible 
proboscis  and  a  branched  gut ;  the  mouth  and  anus  are  ventral.  The 
ova  arise  in  the  reduced  body  cavity,  and  pass  by  three  meandering 
oviducts  to  the  anal  aperture.  The  testes  are  paired,  branched,  and 
ventral,  with  associated  ducts,  which  open  anteriorly  on  the  side  of  the 
body.  The  sexual  relations  are  interesting,  for  one  species  is  herma- 
phrodite and  another  unisexual,  between  which  there  is  an  intermediate 
species  with  ovaries  and  rudimentary  testes.  The  hermaphrodite  form 
may  bear  on  its  body  dwarfish  males,  analogous  to  the  complemental 
pigmies  on  some  hermaphrodite  barnacles. 


HABITS   OF  LEECHES.  213 


Class  DISCOPHORA  or  HIRUDINEA.     Leeches. 

This  class  includes  forms  in  which  the  body  is  elongated 
or  flattened,  devoid  of  appendages  or  bristles,  and  marked 
externally  by  rings,  which  are  much  more  numerous  than  the 
segments  as  displayed  in  the  internal  structure.  The  body 
cavity  is  much  reduced  and  communicates  with  the  well- 
developed  vascular  system.  The  nephridia  are  numerous  and 
segmentally  arranged.  There  is  always  a  posterior  sucker,  and 
the  mouth  is  frequently  suctorial  also.  Almost  all  are  herma- 
phrodite, the  male  organs  are  numerous  and  usually  segmentally 
arranged.  The  nephridia  do  not  function  as  ducts  for  the 
reproductive  organs. 

Leeches  show  several  very  distinct  Annelid  characters, 
but  on  the  other  hand  differ  from  ringed  worms  and  agree 
with  flat  worms  in  having  suckers,  in  the  absence  of  bristles, 
in  the  frequently  flattened  form  and  other  features.  It  is 
impossible  to  say  how  far  these  resemblances  with  flat  worms 
are  due  to  the  adoption  of  a  peculiar  mode  of  nutrition,  but 
the  evidence  on  the  whole  seems  to  be  in  favour  of  Annelid 
affinities. 

Most  leeches  are  worm-like  aquatic  animals,  with  blood 
sucking  propensities,  but  some  live  in  moist  soil,  and  others 
keep  to  the  open  surface,  while  the  parasitic  "vampire" 
habit,  familiarly  illustrated  by  the  apothecary's  ancient 
panacea,  is  in  many  cases  replaced  by  carnivorous  habits 
and  predatory  life.  The  medicinal  leech  (Hirudo)  is 
typical  of  the  majority,  for  it  lives  in  ponds  and  marshes, 
and  sucks  the  blood  of  snails,  fishes,  frogs,  or  of  larger 
available  victims.  The  giant  leech  (Macrobdella  valdiviana\ 
said  to  measure  2\  feet  in  length,  is  subterranean  and 
carnivorous,  while  the  wiry  land  leeches  (Hcemadipsa,  &c.), 
of  Ceylon  and  other  parts  of  the  East  move  in  rapid  somer- 
saults along  the  ground,  fasten  on  to  the  legs  of  man  or 
beast,  and  gorge  themselves  with  blood.  By  attaching  the 
head  end  by  the  mouth  and  loosening  the  tail  sucker,  then 
fixing  the  tail  and  extending  the  anterior  region,  many  leeches 
move  very  quickly  and  deftly,  while  at  other  times,  or  in 
other  forms,  the  mode  of  locomotion  is  by  graceful  serpent- 
like  swimming,  or  by  gentle  gliding  after  the  manner  of 
snails.  The  hungry  horse  leeches,  "whose  daughters  cry 


214  SEGMENTED    WORMS   OR  ANNELIDA. 

Give,  Give,"  are  species  of  Hcemopis,  greedily  suctorial 
though  their  teeth  are  too  small  to  be  useful  in  blood- 
letting ;  but  the  popular  name  is  also  applied  to  species  of 
the  common  genus  Aulastoma,  whose  members  are  car- 
nivorous. Other  common  leeches  are  species  of  Nephelis, 
predacious  forms  with  indiscriminating  appetites,  and  the 
little  Clepsine,  also  common  in  our  ponds,  notable  for  its 
habit  of  carrying  its  young  about  on  its  belly.  Numerous 
marine  forms  prey  upon  fishes  and  other  animals,  e.g.,  the 
"  skate  sucker  "  Pontobdella,  with  a  leathery  skin  rough  with 
knobs,  and  Branchellion  on  the  Torpedo,  remarkable  for 
numerous  leaf-like  respiratory  plates  on  the  sides  of  its 
body.  Perhaps  the  strangest  habitat  is  that  of  Lophobdella, 
which  lives  on  the  lips  and  jaws  of  the  crocodile. 

Type.     The  Medicinal  Leech  (Hirudo  medirinalis). 

This  is  the  commonest  and  most  familiar  of  leeches,  once 
so  constantly  used  in  the  practice  of  medicine  that  leech 
became  synonymous  with  medical  practitioner.  It  lives  in 
ponds  and  sluggish  streams,  and  though  not  common  in 
Britain,  is  very  abundant  in  many  regions  of  the  Continent, 
where  leech  farms,  formerly  of  great  importance,  are  still  to 
be  seen.  Leeches  feed  on  the  blood  of  fishes,  frogs,  and 
the  like,  and  are  still  caught  in  the  old  fashion  on  the  bare 
legs  of  the  callous  collector.  As  animals  are  naturally  averse 
to  bloodletting  and  hard  to  catch,  leeches  make  the  most 
of  their  opportunity,  and  feed  very  greedily.  They  gorge 
themselves  with  blood  and  keep  on  slowly  digesting  it 
for  many  months,  it  may  be  indeed  for  a  year.  Watched  in 
a  glass  jar,  the  leech  will  be  seen  to  move  by  alternately 
fixing  and  loosening  its  oral  and  posterior  suckers,  while 
some  slight  provocation,  such  as  some  drops  of  chloroform 
or  alcohol,  will  induce  the  animal  to  swim  about  both 
actively  and  gracefully.  At  times  it  may  also  be  seen  to 
cast  off  from  its  skin  thin  transparent  shreds  of  cuticle, — a 
process  which,  in  natural  conditions,  usually  occurs  after  a 
heavy  meal,  when  the  animal  as  if  in  indigestion  spasmodi- 
cally contracts  its  body,  or  rubs  itself  on  the  stems  of  water 
plants.  Numerous  eggs  are  laid  together  in  cocoons  in  the 
damp  earth  near  the  edge  of  the  pool.  Thence  after  a 


STRUCTURE   OF   THE  LEECH.  215 

direct  development,  young  leeches  emerge  and   make  for 
the  water. 

External  Features. — The  leech  usually  measures  from  two  to  six 
inches  in  length,  and  appears  cylindrical  or  strap-like,  according  to  its 
state  of  contraction.  The  slimy  body  shows  over  a  hundred  skin  rings, 
its  dorsal  surface  is  beautifully  marked  with  longitudinal  pigmented 
bands,  while  the  ventral  surface  is  mottled  irregularly  ;  the  suctorial 
mouth  is  readily  distinguished  from  the  unperforated  hind  sucker,  above 
which  on  the  dorsal  surface  the  alimentary  canal  may  be  seen  to  end. 

It  is,  however,  necessary  to  consider  the  external  characters  in  greater 
detail.  As  already  noted,  the  rings  of  the  body  are  merely  superficial 
wrinkles  ;  it  is  therefore  not  difficult  to  realise  that  there  may  be  doubt 
as  to  their  exact  limits,  and  that  the  apparent  number  may  differ  accord- 
ing as  they  are  counted  from  the  dorsal  or  ventral  surface.  According 
to  Whitman's  precise  investigations,  102  skin  rings  in  all  are  represented, 
and  these  correspond  to  26  somites  or  true  segments.  These  segments 
may  be  recognised  externally  by  conspicuous  pigment  spots  ("segmental 
papillae"),  which  in  the  middle  region  of  the  body  occur  on  every  fifth 
ring.  In  type,  therefore,  five  rings  correspond  to  a  segment,  but  at 
either  end  of  the  body  the  number  of  rings  is  abbreviated.  In  the  head 
region  a  pair  of  "eyes"  occurs  on  each  of  the  1st,  2nd,  3rd,  5th,  and 
8th  rings  ;  these  are  homologous  with  "  segmental  papillae,"  and  there- 
fore in  this  region  eight  rings  correspond  to  five  segments. 

Careful  examination  of  the  surface  of  the  body  will  show  further,  the 
swollen  protrusion  of  the  male  organs  on  the  middle  ventral  line  between 
rings  30  and  31,  the  aperture  of  the  female  organs  five  rings  further 
back,  and  also  on  the  ventral  surface  seventeen  pairs  of  small  lateral 
apertures,  through  which  a  whitish  fluid  may  be  squeezed — the  apertures 
of  the  excretory  organs.  The  skin  of  segments  9-11  is  especially 
glandular,  and  forms  the  so-called  clitellum  or  saddle,  the  secretion  of 
which  forms  the  cocoon  for  the  eggs. 

The  Skin. 

The  skin  is  so  closely  connected  with  the  connective  and 
muscular  tissue  lying  beneath  that  little  can  be  seen  of  its 
structure  except  in  sections.  Most  externally  lies  the  cuticle 
— a  product  of  the  epidermis — periodically  shed  as  we  have 
already  noticed.  In  this  shedding  some  of  the  genuine 
epidermis  cells  are  also  thrown  off.  These  are  somewhat 
hammer-like  units  with  the  heads  turned  outwards,  while  the 
spaces  between  the  thick  handles  contain  pigment  and  the 
fine  branches  of  blood  vessels.  As  the  latter  come  very 
near  the  surface  a  respiratory  absorption  of  oxygen  and  out- 
ward passage  of  carbonic  acid  is  readily  effected.  Opening 
between  the  epidermal  elements,  but  really  situated  much 
deeper,  are  numerous  long  necked,  flask  shaped  glandular 


216  SEGMENTED    WORMS  OR  ANNELIDA. 

cells,  the  contents  of  which  form  the  mucus  so  abundant  on 
the  skin.  Underneath  the  epidermis  there  is  much  con- 
nective tissue,  and  not  a  little  pigment,  yellow  and  green, 
brown  and  black  in  colour. 

The  Muscular  System. 

The  muscular  system  consists  of  spindle  shaped  cells 
arranged  externally  in  circular  bands  like  the  hoops  of  a 
barrel,  internally  in  longitudinal  strands  like  staves.  Besides 
these  there  are  numerous  muscle  bundles  running  diagonally 
through  the  body,  or  from  dorsal  to  ventral  surface,  and 
there  are  other  muscles  associated  with  the  lips,  tooth  plates, 
and  pharynx. 

The  Body  Cavity. 

The  body  cavity  is  almost  quite  obliterated  in  the  adult 
leech,  where  the  predominant  connective  tissue  has  filled  up 
nearly  every  chink  and  crevice.  It  is  to  be  seen  in  the 
embryo,  and  its  remnants  may  be  detected  here  and  there 
in  the  adult.  The  virtual  absence  of  the  body  cavity,  and 
the  spongy  compactness  of  the  whole  animal,  make  the 
leech  a  tedious  subject  to  dissect. 

Nervous  System. 

The  nervous  system  mainly  consists  of  a  pair  of  dorsal 
ganglia  lying  above  the  pharynx,  and  of  a  double  nerve  cord 
with  twenty-three  ganglia  lying  along  the  middle  ventral  line. 
The  dorsal  (or  supra  cesophageal)  ganglia  are  connected  with 
the  most  anterior  (or  sub-cesophageal)  pair  on  the  ventral 
chain,  by  a  narrow  nerve  ring  surrounding  the  beginning  of 
the  gut.  From  the  dorsal  centres  nerves  proceed  to  the 
"  eyes  "  and  anterior  sense  spots,  from  the  ventral  centres 
the  general  body  is  innervated,  and  from  the  beginning  of 
the  ventral  chain  special  nerves  supply  the  alimentary  canal, 
forming  what  is  called  a  visceral  system. 

The  Sense  Organs. 

The  sense  organs  of  the  leech  are  ten  so-called  "eyes,"  besides 
numerous  sense  spots  usually  occurring  on  every  fifth  skin  ring.  The 
eyes  are  arranged  round  the  edge  of  the  mouth,  and  look  like  little 
black  spots.  Microscopic  examination  shows  them  to  be  definite  cups, 
surrounded  by  connective  tissue  with  black  pigment,  and  containing 
clear  strongly  refracting  cells,  each  in  connection  with  a  fibre  of  the 
optic  nerve. 


STRUCTURE   OF   7^HE   LEECH. 


217 


It  has  been  shown  (Whitman)  that  the  eyes  of  leeches  are  serially 
homologous  with  the  segmental  sense  organs.  At  the  one  extreme  there 
are  purely  tactile  organs,  at  the  other  extreme  there  are  purely  visual 
organs,  and  between  these  there  are  compound  sense  organs,  in  part 
tactile  and  in  part  visual, — a  series  which  is  full  of  suggestiveness  in 
regard  to  the  evolution  of  sense  organs  (cf.  of  the  series  sensitive  setae 
in  the  crayfish.  The  visual  organs  of  the  leech  are  not  able  to  form 


FIG.  70. — Transverse  section  of  Leech.     (Simplified  from 
A.  G.  BOURNE.) 

c.,  Cuticle;  e.,  epidermis;  C.M.,  dermis  and  outer  muscles  (cir- 
cular and  oblique)  ;  /.;/*.,  longitudinal  muscles  (the  peculiar  connec- 
tive tissue  is  hardly  indicated)  ;  r.fn.^  radial  muscles  ;  /.#.,  lateral 
blood  vessel ;  d.s.,  dorsal  blood  sinus  ;  v.s.,  ventral  sinus  enclosing 
nerve  cord  (n)  ;  g~.,  median  part  of  crop,  with  lateral  pockets  (/)  ; 
f.,  testes  ;  f.,  nephridial  funnel  ;  v.d.,  vas  deferens. 

images  of  external  objects,  but  the  animals  are  exquisitely  sensitive  to 
alterations  of  light. 

The  Alimentary  System. 

When  the  leech  has  firmly  fastened  itself  to  its  prey  by 
the  hind  sucker,  it  brings  its  muscular  mouth  into  action, 
pressing  the  lips  tightly  on  the  skin,  and  protruding  three 
chitinous  tooth  plates  which  lie  within.  Each  of  these 


218  SEGMENTED    WORMS   OR  ANNELIDA. 

tooth  plates  is  worked  by  muscles,  and  is  like  a  semi- 
circular saw,  for  the  edge  bears  from  60  to  100  small 
teeth.  Rapidly  these  saws  cut  a  triangular  wound,  whence 
the  flowing  blood  is  sucked  into  the  mouth  by  the 
muscular  pharynx.  The  process  may  be  observed  and  felt 
by  allowing  a  hungry  leech  to  fasten  on  your  arm.  As  the 
blood  passes  down  the  pharynx,  it  is  influenced  by  the  secre- 
tion of  salivary  cells  which  lie  among  the  muscles,  and  exude 
a  ferment  which  prevents  the  usual  clotting.  The  blood 
greedily  sucked  in  gradually  fills  the  next  region  of  the  gut 
— the  crop — which  bears  on  each  side  eleven  storing  pockets. 
These  become  wider  and  more  capacious  towards  the  hind 
end,  the  largest  terminal  pair  forming  two  great  sacs  on  each 
side  of  the  comparatively  narrow  posterior  part  of  the  gut. 
As  all  the  pockets  point  more  or  less  backwards,  it  is  evident 
why  a  leech  to  be  emptied  of  the  blood  which  it  has  sucked 
must  be  pressed  from  behind  forwards.  The  pockets  filled, 
the  leech  drops  off  its  victim,  seeks  to  retire  into  more 
private  life,  and  digests  at  leisure.  The  digestion  does  not 
take  place  in  the  pockets,  but  in  a  small  area  just  above  the 
beginning  of  the  terminal  part  or  rectum.  This  rectum, 
running  between  the  two  last  pockets,  is  separable  from  the 
true  stomach  just  mentioned  by  a  closing  or  sphincter 
muscle.  It  ends  in  a  dorsal  anus  above  the  hind  sucker. 

The  Vascular  System. 

The  vascular  system  consists  of  four  main  vessels  running 
longitudinally,  one  above  the  gut,  one  round  about  and 
obscuring  the  nerve  cord,  and  one  on  each  side  of  the  body. 
These  are  all  connected  with  one  another  by  looping  vessels, 
and  all  give  off  numerous  branches  which  riddle  the  spongy 
body.  The  main  side  vessels  are  most  distinct,  are  con- 
tractile throughout,  and  give  off  to  the  skin,  gut,  and  excre- 
tory organs,  a  rich  supply  of  branches.  The  dorsal  and 
ventral  vessels,  though  quite  distinct,  are  less  definite, 
being  rather  blood  spaces  than  well-formed  vessels.  That 
the  lateral  vessels  do  most  of  the  work  of  circulation  is 
certain,  but  the  precise  course  of  the  blood  is  not  satis- 
factorily known.  The  blood  itself  is  a  red  fluid  with  floating 
colourless  cells  diverse  in  form. 


STRUCTURE   OF  THE  LEECH. 


219 


Excretory  System. 

The  excretory  system  includes  seventeen  pairs  of  excretory 
tubules  or  nephridia,  opening  laterally 
on  the  ventral  surface,  ending  blindly 
within  the  body,  but  extracting  waste 
products  from  the  blood  vessels  which 
cover  their  walls.  Each  consists  of  two 
parts,  a  twisted  horse  shoe  shaped 
glandular  region  where  the  actual  ex- 
cretory function  is  discharged,  and  a 
spherical,  internally  ciliated  bladder 
opening  to  the  exterior.  Within  the 
latter  there  is  a  whitish  fluid  in  which 
microscopic  examination  shows  numer- 
ous waste  crystals.  The  nephridia 
secrete  a  clear  fluid  which  helps  to 
keep  the  skin  moist,  and  thus  makes 
respiratory  diffusion  easier. 

The  Reproductive  System. 

The  leech,  like  many  other  Inverte- 
brates, is  hermaphrodite,  containing 
both  male  and  female  reproductive 
organs.  The  essential  male  organs  or 
testes  are  diffuse,  being  represented  by 
nine  pairs,  lying  on  each  side  of  the 
nerve  cord  in  the  middle  region  of  the 
body.  Each  is  a  firm  globular  body, 
within  which  mother  sperm  cells  divide 
into  balls  of  sperms.  The  spermatozoa 
pass  from  each  testis  by  a  short  canal 
leading  into  a  wavy  longitudinal  vas 
deferens.  This  duct  followed  towards 
the  head  forms  a  coil  (so-called  seminal 
vesicle)  as  it  approaches  the  ejaculatory 
organ  or  penis.  From  the  coil  on  each 
side  the  sperms  pass  into  a  swollen  sac 
at  the  base  of  the  penis,  where  by  the 
viscid  secretion  of  special  ("  prostate  ")  glands,  they  are  glued 
together  into  packets  or  spermatophores.  These  pass  up 


FIG.  71. —  Dissection 
of  Leech. 

rg.,  Nerve  ring  around 
oesophagus,  here  incom- 
plete ;  /.,  penis;  s.v., 
seminal  vesicle  ;  07'., 
ovary  ;  uf.,  uterus  ;  v.d., 
vas  deferens;  t.,  one  of 
the  testes;  «.,  nephridium 
with  bladder  (bl.)  ;  £-.,  a 
ganglion  on  ventral  nerve 
cord  ;  s.,  posterior  sucker. 


220  SEGMENTED    WORMS   OR  ANNELIDA. 

the  narrow  canal  of  the  muscular  penis,  pass  out  on  the 
middle  ventral  line  between  rings  thirty  and  thirty-one,  and 
are  transferred  in  copulation  to  the  female  duct  of  another 
leech. 

The  female  organs  are  more  compact.  The  two  small 
tubular  and  coiled  ovaries  are  enclosed  in  a  spherical  vesicle, 
the  walls  of  which  are  continued  as  two  oviducts  which  unite 
together  in  a  convoluted  common  duct.  This  is  surrounded 
by  a  mass  of  glandular  cells,  which  exude  a  glairy  fluid  into 
the  duct.  Finally,  the  duct  opens  into  a  relatively  large 
muscular  sac — the  "uterus,"  which  opens  through  a  sphincter 
muscle  on  the  middle  ventral  line  between  rings  thirty-five 
and  thirty-six. 

The  favourite  breeding  time  is  in  spring  Two  leeches 
fertilise  one  another,  uniting  in  reverse  positions  so  that  the 
penis  of  each  enters  the  uterus  of  the  other.  Spermatophores 
are  passed  from  one  to  the  other,  and  the  contained  sperms 
may  remain  for  a  long  time  within  the  uterus,  or,  liberated 
from  their  packets,  may  work  their  way  up  the  female  duct, 
meeting  the  eggs  at  some  point,  or  reaching  them  even  in 
the  ovaries. 

The  development  has  been  most  carefully  worked  out  for 
the  little  leech  Clepsine,  and  we  shall  follow  it  there. 

Development  of  Clepsine. 

The  eggs  are  laid  in  water,  and  surrounded  by  a  cocoon  ;  they  are  large, 
and  contain  much  food  yolk. 

Cleavage  is  complete  but  unequal.  At  the  four  cell  stage,  there  are 
three  sub-equal  smaller  cells  and  one  larger  posterior  cell,  which  marks 
the  future  hind  end  of  the  body.  From  each  of  these  cells  a  small  cell 
is  cut  off,  and  in  this  way  four  macromeres  and  four  micromeres  are 
produced.  The  number  of  micromeres  is  continually  increased  by  the 
splitting  off  of  cells  from  the  macromeres,  so  that  a  disc  of  small  cells  is 
formed.  Except  for  this  continued  splitting  off  of  small  cells,  three 
of  the  four  macromeres  remain  passive  for  a  considerable  period  ;  they 
contain  most  of  the  food  yolk,  and  serve  as  reservoirs  of  nutriment.  The 
other,  or  posterior,  macromere  divides  into  two  cells  of  unequal  size,  the 
larger  speedily  again  divides  into  two  primitive  mesoblasts,  the  smaller 
divides  into  eight  symmetrically  arranged  cells,  the  neuro-nephroblasts. 
At  this  time  free  nuclei  appear  in  the  other  three  macromeres  (ento- 
blasts)  without  any  corresponding  process  of  cell  division,  these  surround 
themselves  with  protoplasm,  and  form  the  endoderm  cells  lining  the  gut. 
The  disc  of  small  cells  is  now  spreading  over  the  surface  of  the  entoblasts, 
over  the  neuro-nephroblasts,  and  over  the  primitive  mesoblasts,  which 


DEVELOPMENT  OF.  CLEPSINE.  221 

have  sunk  slightly  inwards.  The  small  cells  are  ectodermic,  they  con- 
tribute to  the  formation  of  the  epidermis,  and  apparently  form  also  the 
ectoderm  of  the  head  region.  The  ectodermic  structures  of  the  body,  on 
the  other  hand,  are  formed  by  the  eight  neuro-nephroblasts.  These, 
together  with  the  two  mesoblasts  underlying  them,  undergo  continuous 
division  in  a  forward  direction,  and  so  produce  long  rows  of  cells — the 
germ  bands.  The  two  germ  bands  are  widely  separated  posteriorly, 
but  commence  to  unite  anteriorly,  the  union  travelling  backward.  As 
the  neuro-nephroblasts  must  be  regarded  as  ectodermic  in  origin,  we 
see  that  the  spreading  of  the  micromeres  over  the  surface  of  the  egg,  and 
the  union  of  the  germ  bands,  constitute  together  the  delayed  epibolic 
gastrulation. 

Each  germ  band  consists  of  three  layers,  first  a  thin  epidermic  stratum, 
then  the  layer  of  the  neuro-nephroblasts,  and  finally  the  mesodermic 
layer.  Of  the  neuro-nephroblasts,  the  inner  two  form  the  ventral  nerve 
chain,  the  next  two  on  either  side  the  nephridia,  while  the  fate  of  the 
outer  on  each  side  is  unknown.  The  mesoblast  rows  give  rise  to  the 
mesoderm,the  gut  is  formed  by  the  entoblasts,  and  an  anterior  ectodermic 
invagination  forms  the  pharynx.  At  this  stage  the  embryos  leave  the 
cocoon  and  attach  themselves  to  the  ventral  surface  of  the  mother. 
A  little  later  the  form  of  the  body  becomes  approximated  to  that 
of  the  adult,  and  an  anus  is  formed  by  the  fusion  of  ectoderm  and 
endoderm. 

The  most  interesting  point  about  this  development  is  that,  although 
the  method  of  gastrulation  differs  widely  from  that  of  Lumbricus,  the 
history  of  the  germ  bands  shows  very  marked  resemblances  in  the  two 
forms.  It  can  hardly  be  that  these  resemblances  are  due  to  adaptation, 
so  that  we  must  consider  that  development  confirms  the  view  which  is 
otherwise  probable,  that  the  leeches  are  true  Annelida.  In  Clepsine  the 
eight  neuro-nephroblasts  are  not,  as  in  Lumbricus^  obviously  ectodermic 
in  origin,  but  are  early  covered  over  by  the  ectodermic  micromeres. 
Both  analogy  and  the  future  course  of  development,  however,  prove  that 
they  do,  nevertheless,  belong  to  the  outer  layer,  and  that  their  position  is 
due  to  a  hastening  of  events. 

Classification. 

1.  Rhychobdellidae,  in  which  the  fore  part  of  the  pharynx  can  be 

protruded  as  a  proboscis.  There  is  an  anterior  as  well  as 
a  posterior  sucker.  The  blood  plasma  is  colourless.  The  ova 
are  large  and  rich  in  yolk  ;  the  embryos  are  hatched  at  an 
advanced  stage,  and  soon  leave  the  cocoon,  which  contains  no 
albuminous  fluid. 

e.g.,  Clepsine,  Pontobdella,  Branchellion. 

2.  Gnathobdellidne,  in  which  there  is  no  proboscis,  but  the  pharynx 

usually  bears  three  tooth  plates.  The  mouth  is  suctorial.  The 
blood  plasma  is  red.  The  ova  are  small  and  without  much  yolk  ; 
the  embryos  are  hatched  at  an  early  stage,  and  swim  about  in  the 
nutritive  albuminous  fluid  of  the  cocoon. 

e.g.,  Hirudo,  Hietnopis,  Hcemadipsa,  Aulastoinci)  Nephelis. 


222 


SEGMENTED    WORMS   OR  ANNELIDA. 


Appendix  (i)  to  Annelid  Series. 

Class  CH^TOGNATHA.     Arrow  Worms. 

There  are  two  little  marine  "  worms,"  Sagitta  and  Spadella,  which 
are  so  different  from  all  others,  that  they  have  been  placed  in  a  class  by 
themselves.  It  is  possible  to  regard  them  as  Annelids  with  three 
segments. 

The  translucent  body,  which  is  about  an  inch  long,  has  three  distinct 
regions,  —  a  head  bearing  a  ventral  mouth  with  spines  and  bristles 
(whence  the  name  Chsetognatha),  a  median  region  with  lateral  fins,  and 
a  trowel-like  tail. 

The  nervous  system  consists  of  a  supra-cesophageal  ganglion  in  the 
head,  a  sub-cesophageal  about  the  middle  of  the  body,  long  commissures 
between  them,  and  numerous  nerves  from  both.  There  are  two  eyes  and 
various  patches  of  sensitive  cells. 

The  food  canal  is  complete  and  simple  ;  it  lies  in  a  spacious  ciliated 
body  cavity,  which  arises  in  the  embryo  as  two  pockets  (coelome  pouches) 
from  the  primitive  gut  cavity  or  archenteron.  Corresponding  to  the 
external  divisions,  the  cavities  of  head,  body  and  tail  are  distinct. 


P^IG.  72.  —  Development  of  Sagilta  (after  O.  HERTWIG), 
illustrating  formation  of  a  body  cavity  by  pockets  from  the 
archenteron,  and  early  separation  of  reproductive  cells  (R.}. 

EC.,  Ectoderm  ;  En.,  endoderm  ;  ac.,  archenteron  ;  ft.,  reproduc- 
tive cells;  bl.,  blastopore  ;  cp.,  coelome  pouches;  ;;/.,  mouth; 
i.  section  of  gastrula  ;  2  and  3.  origin  of  coelome  pouches. 

There  is  no  vascular  system,  nor  are  there  any  certain  nephridia.  It 
is  possible  that  the  latter  may  be  represented  by  the  genital  ducts. 

The  animals  are  hermaphrodite,  and  the  simple  reproductive  organs 
lie  near  one  another  posteriorly.  The  two  ovaries  project  into  the  body 
cavity,  and  their  ducts  open  laterally  where  body  and  tail  meet.  The 
two  testes  project  into  the  cavity  of  the  tail  ;  and  their  ducts  have  internal 
ciliated  funnels,  and  open  on  the  tail.  It  is  interesting  to  know  that 
two  reproductive  cells  are  set  apart  at  a  very  early  stage,  and  that  each 
divides  into  the  rudiment  of  an  ovary  and  of  a  testis. 

The  development  is  very  regular.  The  eggs  undergo  complete  seg- 
mentation ;  a  gastrula  is  formed  by  the  invagination  of  a  hollow  ball  of 
cells  ;  the  body  cavity  arises  in  the  form  of  two  pockets  from  the  gastrula 
cavity  or  archenteron. 


ROTATORIA.  223 

Appendix  (2}  to  Annelid  Series. 

Class  ROTATORIA.      Rotifers. 

Rotifers  are  beautiful  minute  animals,  abundant  in  fresh  water,  also  found 
in  damp  moss,  and  in  the  sea.  They  owe  their  name  and  the  old-fashioned 
title  of  wheel  animalcules  to  the  fact  that  the  rapid  movements  of  cilia 
on  their  anterior  end  produce  the  appearance  of  a  rotating  wheel.  The 
food  seems  to  consist  of  small  organisms  and  particles  caught  in  the 
whirlpool  made  by  the  lashing  cilia.  The  little  animals  are  tenacious  of 
life,  and  can  survive  prolonged  drought.  If  they  are  left  dry  for  long, 
however,  they  die,  though  the  ova  may  survive  and  subsequently 
develop. 

The  body  is  usually  microscopic,  and  is  sometimes  (e.g.,  in  Melicerta 
and  Floscularid]  sheltered  within  an  external  tube.  There  is  no  internal 
segmentation,  but  there  are  sometimes  external  rings,  and  a  ventral  out- 
growth or  "  foot  "  is  sometimes  segmented.  The  anterior  end  bears,  on 
a  retractile  ridge,  the  ciliated  ring  or  "trochal  apparatus." 
/  The  nervous  system  is  a  single  dorsal  ganglion  with  a  few  nerves.  An 
unpaired  eye  and  some  tufts  of  sensory  hairs  are  usually  present.  <\ 

The  food  canal  extends  along  the  body  in  a  well-developed  ccelomg, 
and  the  fore  gut  contains  a  mill  in  which  two  complex  hammers  beat 
upon  an  anvil.  The  canal  ends  posteriorly  on  the  dorsal  surface  between 
the  body  and  the  foot,  and  as  the  terminal  portion  also  receives  the 
excretory  canals  and  the  oviduct,  it  is  called  a  cloaca. 

There  is  no  vascular  system,  but  a  nephridial  tube  of  a  primitive  type 
lies  on  each  side  of  the  body,  and  opens  posteriorly  into  the  cloaca. 

The  sexes  are  separate  ;  the  reproductive  organs  are  simple.  Except 
in  the  marine  parasite  Seison  and  two  other  forms,  the  males  are  dwarfed 
and  degenerate,  destitute  even  of  a  true  food  canal.  In  many  cases  at 
least,  sexual  union  (effected  by  a  penis)  seems  to  be  ineffective,  and  there 
is  no  doubt  that  many,  if  not  most,  Rotifers  are  parthenogenetic.  The 
females  lay  three  different  kinds  of  eggs,  according  to  their  conditions 
and  constitution — either  small  ova,  which  become  males,  or  thin  shelled 
"summer  ova,"  or  thick  shelled  " resting  or  winter  ova,"  the  two  last 
developing  into  females.  Many  species,  however,  are  viviparous.  We 
include  the  Rotifers  beside  the  Annelids  proper,  because  it  seems  possible 
to  regard  them  as  derived  from  ancestors  somewhat  like  Annelid  larvae. 

Rotifers  living  in  fixed  tubes  or  envelopes, — Melicerta,  Floscularia, 
Stephanoceros. 

Free  Rotifers, — Notommata,  Hydatina,  Brachiomis. 

Parasitic  on  the  marine  Crustacean  Nebalia, — Seison. 

Pedalion  occupies  a  unique  position  ;  it  has  hints  of  appendages  and 
a  peculiar  jumping  motion. 


Equally  incertce  sedis,  but  plausibly  regarded  as  a  specialised  Trocho- 
sphere,  is  the  genus  Dinophilus,  with  the  nature  of  which  advanced 
students  should  make  themselves  acquainted. 


At  this  stage  I  may  also  mention  that  there  are  several  sets  of  small 
worm-like  animals  of  whichjwe  know  very  little.     It  is  quite  possible 


224 


SEGMENTED    WORMS   OR  ANNELIDA. 


that  some  of  them  may  become  of  great  interest  to  the  systematic 
zoologist,  but  we  do  not  yet  understand  what  places  in  the  system  they 
should  occupy.  Moreover,  as  they  are  small,  unfamiliar,  and  unknown 
to  myself,  I  shall  simply  refer  the  curious  to  what  more  complete  works 
say  about  the  Gasterotricha,  Echinoderidce,  Demoscolecidce,  and 
Chcietosomidne. 


Appendix  (3)  to  Annelid  Series. 
Class  SIPUNCULOIDEA,  e.g.,  Sipmmilus. 

Marine  worms  usually  living  in  the  sand.  The  body  is  elongated  and 
apparently  unsegmented.  The  oral  or  anterior  region  can  be  invaginated 
by  special  muscles.  There  are  no  setae.  They  are  sometimes,  but 
perhaps  erroneously,  placed  beside  Echiuridce  as  Gephyrea  Achaeta.  The 
nervous  system  consists  of  an  oesophageal  ring,  and  a  median  ventral 
nerve  cord,  which  shows  slight  hints  of  segmentation.  There  is  a 
spacious  body  cavity. 


SlPUNCULID.4 


PKIAPULID^:. 


The  anus  is  dorsal  and  anterior,  and 
the  food  canal  is  usually  in  a  spiral  :  the 
mouth  is  surrounded  by  tentacles.  There 
is  a  closed  vascular  system,  with  branches 
to  the  tentacles. 

An  anterior  pair  of  nephridia  serve  also 
as  genital  ducts,  removing  the  repro- 
ductive cells  from  the  body  cavity.  The 
sexes  are  separate. 

Examples — Sipunculus. 

Phascolosoma. 


The  alimentary  canal  is  straight  or 
slightly  looped,  and  the  anus  is  dorsal 
and  posterior.  There  are  no  tentacles. 

There  is  no  vascular  system. 

No  anterior  nephridia,  but  a  pair  of 
tubes  open  beside  the  anus,  and  are  said 
to  be  excretory  in  the  young,  genital  in 
the  adult.  The  sexes  are  separate. 

Examples — Priapulus. 
Halicryptus. 


Appendix  (4)  to  Annelid  Series. 

Under  the  old  term  Molluscoidea  are  sometimes  included  the  three 
classes — Phoronoidea,  Polyzoa  or  Bryozoa,  and  Brachiopoda.  Prof. 
Lang  includes  them  along  with  Sipunculoids  in  the  provisional  group 
Prosopygii. 

The  Molluscoidea  are  characterised  by  the  presence  of  a  true  ccelome, 
formed  in  development  by  the  folding  off  of  pouches  from  the 
archenteron,  and  by  the  shortening  of  the  dorsal  region  of  the  body, 
which  results  in  the  close  approximation  of  mouth  and  anus.  The 
mouth  is  typically  furnished  with  ciliated  tentacles,  and  is  often  over- 
hung by  an  epistome ;  both  tentacles  and  epistome,  when  present,  contain 
spaces  which  are  part  of  the  body  cavity.  Except  in  Polyzoa,  two  or 
four  nephridia  are  present,  and  serve  also  as  genital  ducts.  There  is 
always  a  metamorphosis  in  development,  and  the  larvae  are  peculiar. 

Class  PHORONOIDEA. 

The  crown  of  tentacles  is  shaped  like  a  horse  shoe,  each  tentacle  is 
supported  by  an  internal  skeleton.  The  nervous  system  lies  in  the 


POL  YZOA—BRA  CHIOPODA.  225 

ectoderm,  and  consists  of  a  ring  round  the  mouth,  and  of  a  cord  down 
the  left  side  of  the  body.  There  is  a  closed  vascular  system  with 
nucleated  red  cells.  The  body  cavity  is  well-developed.  The  sexes  are 
united.  The  larva,  known  as  an  Actinotrocha,  is  a  much  modified 
trochosphere. 

Phoronis,  the  only  genus,  is  a  worm-like  marine  animal,  always  found 
enclosed  in  a  fixed  leathery  tube,  and  social  in  habit. 

Class  POLYZOA. 

As  usually  defined  the  class  includes  two  sub-classes,  the  Ectoprocta 
and  the  Entoprocta,  but  it  seems  doubtful  whether  the  Entoprocta 
should  not  be  raised  to  the  dignity  of  a  distinct  class. 

The  Ectoprocta  include  fresh  water  and  marine  forms  in  which  the 
anus  is  outside  the  basis  of  the  tentacles.  The  nervous  system  is 
represented  by  a  ganglion  placed  between  the  mouth  and  anus.  There 
is  no  vascular  system.  In  Cristatella,  at  least,  there  are  two  nephridia. 
All  are  colonial  and  bud  very  freely  ;  the  marine  forms  show  con- 
siderable division  of  labour  among  the  members  of  the  colony. 

(a)  Tentacles  in  a  crescent — Fresh  water,  Cristatella,  Lophopus,  etc. 

[b]  Tentacles  in  a  circle — Marine,  except  Paludicella  ;  Fhistra,  the 
common  sea-mat  ;  Membranipora,  encrusting  seaweed,  etc.  ;   Cellepora^ 
very  calcareous  ;  Alcyonidtum,  gelatinous. 

The  Entoprocta  include  the  colonial  Pedicel  Una,  with  a  few  allied 
genera,  and  Urnatella,  also  the  non-colonial  Loxosoma,  in  which  the 
buds  separate  as  soon  as  they  are  formed.  All  are  stalked  and  minute. 
The  anus  is  included  within  the  tentacular  circle.  In  the  metamorphosis 
of  Pedicellina,  there  is  an  elongation  of  the  dorsal  region  of  the  body, 
and  a  consequent  approximation  of  the  mouth  and  anus  on  the  shortened 
ventral  surface.  There  is  no  apparent  body  cavity  in  the  adult,  and  the 
mesoderm  arises  from  two  primitive  mesoblasts.  The  nephridia  are 
anterior,  minute,  and  do  not  serve  as  genital  ducts,  but  resemble  the 
"  head  kidneys  "  of  Annelid  trochospheres.  In  all  these  three  respects 
the  Entoprocta  differ  from  the  Ectoprocta,  and  from  the  Molluscoidea 
generally,  but  the  significance  of  this  is  uncertain,  more  especially  as  it 
is  possible  that  the  differences  may  in  part  arise  from  defective 
observation. 

Class  BRACHIOPODA. 

The  Brachiopods  or  Lampshells  are  quaint  marine  animals,  once 
very  numerous,  but  now  decadent.  The  body  is  enveloped  dorsally  and 
ventrally  by  two  folds  of  skin  or  mantle,  these  secrete  a  shell,  usually  of 
lime,  but  sometimes  organic.  The  development  of  this  shell  has  appar- 
ently modified  both  the  position  and  the  relations  of  the  organs.  There 
is  no  real  resemblance  between  a  Brachiopod  shell  and  that  of  a  bivalve 
Mollusc,  except  that  both  consist  of  two  valves.  In  Brachiopods  these 
lie  dorsally  and  ventrally,  in  Lamellibranchs  they  are  lateral  ;  moreover, 
in  Brachiopods  the  ventral  valve  is  usually  the  larger.  It  is  hardly 
necessary  to  say  that  the  Brachiopod  organism  is  not  the  least  like  a 
Mollusc. 
15 


226 


BRACHIOPODA. 


A  considerable  part  of  the  space  between  the  valves  of  the  shell  is 
rilled  up  by  two  long  "  arms,"  which  are  coiled  in  a  spiral,  and  often 
supported  by  a  calcareous  skeleton. 
These  arise  in  development  from  the 
specialisation  of  a  horse-shoe  shaped 
"  lophophore,"  such  as  is  characteristic 
of  the  Polyzoa.  The  mouth  is  placed 
between  the  arms,  and  opens  into  the 
ciliated  food  canal.  This  may  end 
blindly,  or  may  be  furnished  with  an 
anus  placed  near  the  mouth  ;  in  Crania 
the  anus  is  dorsal  and  posterior.  The 
muscular  system  is  well-developed,  the 
shell  is  both  opened  and  closed  by 
means  of  muscles.  There  is  a  nerve- 
ring  round  the  gullet,  with  a  slight 
brain  and  an  inferior  ganglion.  Sen- 
sory structures  in  many  cases  perforate 
the  valves.  Above  the  gut  lies  the 
heart,  which  is  connected  with  blood 
vessels.  Two  (or  more  rarely  four) 
nephridia  open  near  the  mouth,  and 
serve  also  as  genital  ducts.  The  pos- 
terior region  of  the  body  often  forms  a  stalk  by  which  the  shell  is 
moored,  but  in  many  this  stalk  is  absent,  and  the  animal  is  directly 
attached  to  the  substratum.  The  sexes  are  sometimes  separate,  but 
perhaps  some  are  hermaphrodite.  There  is  a  metamorphosis  in  the 
development,  and  the  larvae  resemble  those  of  Polyzoa.  Of  the  details 
little  is  yet  known. 


FIG.  73. — Interior  of  Bra- 
chiopod  Shell,  showing  cal- 
careous support  for  the 
"arms."  (After  DAVIDSON.) 


TESTICARDINES. 


The  valves  are  hinged. 
There  is  no  anus. 
Terebratula.     IValdheimia. 


ECARDINES. 


There  is  no  hinge. 
There  is  an  anus. 
Crania. 

Lingiila,  persistent   since   Palaeozoic 
ages. 


CHAPTER    XII 

ECHINODERMA. 


Class  I.  HOLOTHUROIDEA  (Scytoderma).     Sea  Cucumbers. 

2.  ECHINOIDEA.     Sea  Urchins 

3.  ASTEROIDEA.     Starfishes 

4.  OPHIUROIDEA.     Brittle  stars 

5.  CRINOIDEA.     Feather  stars  "I 

6.  BLASTOIDEA.     Extinct          VPELMATOZOA. 

7.  CYSTOIDEA.     Extinct  J 

IN  contrast  to  the  "worms,"  the  Echinoderms  form  a 
well-defined  series.  They  may  be  described  as  sluggish 
marine  animals,  generally  of  radiate  symmetry,  with  a 
tendency  to  form  limy  skeletons.  The  radial  symmetry  led 
the  older  zoologists  to  place  the  Echinoderma  near 
Ccelentera,  but  the  larval  Echinoderm  is  more  specialised 
than  most  of  the  larval  "worms,"  and  is  bilateral  in  its 
symmetry.  It  seems  likely  that  the  adult  radial  symmetry 
is  an  adaptation  to  sedentary  life,  and  that  the  Echinoderms 
represent  an  offshoot  of  some  "worm"  stock.  Yet  it  is 
interesting  to  notice  that  in  both  Ccelentera  and  Echino- 
derma the  nervous  system  shows  a  marked  absence  of 
centralisation,  which  may  be  connected  with  the  absence 
of  a  definite  head  region,  and  this  again  with  the  relatively 
sedentary  habit. 

GENERAL  CHARACTERS. — The  Echinoderms  include  forms 
in  which  the  bilateral  symmetry  of  the  larva  is  replaced 
in  the  adult  by  radial  symmetry.  In  addition  to  the 
dominant  radial  symmetry,  the  adults  show  to  a  varying 
extent  a  tendency  towards  a  bilateral  form,  but  this  is 
never  the  same  as  that  of  the  larva,  nor  is  it  equivalent 
in  the  different  types.  Lime  is  always  deposited  in  the 


228  ECHINODERMA. 

mesodermic  tissues  (mesenchyme),  and  in  consequence  there 
is  frequently  a  very  complete  skeleton.  From  the  primi- 
tive gut  of  the  larva,  pouches  grow  out  to  form  the 
usually  spacious  ccelome  and  the  characteristic  water  vas- 
cular system.  The  branches  of  this  system,  together  with 
the  nerves,  exhibit  in  most  cases  a  typical  five-rayed 
arrangement.  In  development  there  is  a  marked  distinc- 
tion between  mesoblast  derived  from  gut  pouches,  and  mesen- 
chyme produced  by  immigrant  amoeboid  cells.  There  is 
usually  a  very  striking  circuitousness  or  indirectness  in 
development. 

The   Echinoderms  are   all  marine.     By  reason  of  their 


FIG.  74.  — Pluteus  larva  with  rudiment  of  adult. 
(After  JOHANNES  MULLER.) 

durable  skeletons,  they  are  extremely  well  represented  as  fossils, 
yet  this  does  not  alter  the  fact  that  the  group  is  well-defined, 
and  shoivs  no  close  relation  to  any  other,  whether  in  its  living 
or  extinct  representatives. 

The  average  habit  is  sluggish,  and  this  may  be  correlated 
with  the  constant  development  of  lime  in  the  tissues.  This 
power  of  Jorming  skeletal  substance  is  indeed  so  deep-seated 
that  lime  may  appear  in  almost  any  of  the  organs  of  the 
body.  i£he  diet  is  vegetarian  (most  sea  urchins),  carnivorous^ 
( starfishes ),~or 'consists  of  the  organic  particles  found  in  sand 


STARFISH.  229 

and  mud,  the  Holothurians  in  particular  practising  this 
worm-like  mode  of  nutrition. 

Most  Echinoderms  have  to  a  remarkable  extent  the  power 
of  /V7C//W  off  ftfttf  rp.Pfine.ratit$g  fftrtin?1^  of  tfajr  hqsfo.  This 
power  is  frequently  reckoned  as  one  of  their  means  of  defence, 
but  they  often  mutilatej^M^dv^  vpp.rtfy  a*  a  Mnseque.^  qf 
unfavourable  conditions  of  life.  The  self-  mutilation,  or 
autotomy,  as  it  is  called,  seems  to  be  entirely  a  reflex  action, 
nof~  voluntary . 

The  peculiar  water  vascular  system  attains  great  develop- 
ment, and  has  usually  respiratory  or  locomotor  functions.  It 
is  possible  that  in  some  cases  it  may  also~Junctwn  as  an  organ 
of  excretion.  Well-defined  excretory  organs  are  conspicuously 
rare.  Soluble  waste  products  seem  generally  to  diffuse  out 
into  the  water,  ivhile  the  insoluble  are  here,  as  in  sea  squirts, 
stored  up  in  the  tissues  in  the  form  of  granular  masses. 

The  Holothurians  are  in  form  nearest  to  the  supposed  worm-like 
ancestor,  and  are  perhaps  primitive  forms,  which  do  not  lead  up  to  any 
of  the  other  classes.  From  primitive  unspecialised  Cystoids,  the 
Echinozoa,  and  Pelmatozoa  have  perhaps  taken  origin.  Of  the 
Echinozoa  the  Asteroidea  and  Ophiuroidea  are  very  closely  related,  and 
seem  to  be  connected  by  fossil  forms. 

In  our  survey  of  the  group  it  is  more  convenient  to  begin  with  the 
familiar  starfishes  than  with  the  more  primitive  forms.  The  general 
characters  of  each  class  may  be  read  from  the  synoptic  table  at  the  end. 


Class  ASTEROIDEA.     Starfish. 

The  description  applies  especially  to  the  common  five- 
rayed  starfish  (Asterias  or  Asteracanthion  rubens}.  It  is 
often  seen  in  shore  pools  exposed  at  low  water,  but  its 
haunts  are  on  the  floor  of  the  sea  at  greater  depths.  There 
it  moves  about  sluggishly  in  any  direction  by  means  of  its 
tube  feet. 

Form. — Each  of  the  five  arms  bears  a  deep  ventral  groove 
in  which  the  tube  feet  are  lodged.  The  mouth  is  in  the 
middle  of  the  ventral  surface,  the  food  canal  ends  about 
the  centre  of  the  dorsal  disc.  With  this  flat,  five-rayed 
form,  the  11-13  rayed  sun  star  (Solaster),  the  pincushion- 
like  Goniaster,  and  the  flat  pentagonal  Palmipes,  should  be 
contrasted. 


230  ECHINODERMA. 

Integument. — (a)  The  body  is  covered  by  a  ciliated  ectoderm.  This 
includes  supporting,  glandular,  and  sensory  cells,  and  beneath  it 
there  is  a  network  of  nerve  fibrils  with  ganglionic  cells. 

(b)  The  middle  layer  of  the  integument  consists  of  a  double  stratum 

of  ground  substance,  the  outer  part  of  which  contains  the  chief 
limy  structures  except  the  ambulacra!  ossicles  which  are  formed 
more  internally.  There  is  also  a  thin  muscular  layer.  The 
whole  of  this  middle  layer  is  formed  in  development  from  the 
mesenchyme  tissue. 

(c)  Internally  the  body  wall  is  lined  by  a  ciliated  epithelium,  derived 

in  development  from  the  wall  of  the  ccelomic  pouches.  (See 
Development.) 

Between  two  of  the  arms  lies  the  perforated  madreporic  plate,  the 
entrance  to  the  water  vascular  system,  thus  defining  the  bivium, 
while  the  other  three  arms  constitute  the  trivium. 


The  Calcareous  Skeleton. 

In  association  with  the  inner  mesodermic  layer  of  the 
integument,  there  is  developed  on  the  ventral  surface  of  each 
arm  a  double  series  of  sloping  plates.  These  two  series 
meet  dorsally,  like  rafters,  in  the  middle  line  of  the  arm, 
forming  an  elongated  shed.  The  rafter-like  plates  are  called 
ambulacral  ossicles ;  the  groove  which  they  bound  lodges 
the  nerve  cord,  the  blood  vessel,  the  water  vessel,  and  the 
tube  feet  of  each  arm. 

In  association  with  the  outer  mesodermic  layer  of  the 
integument,  numerous  smaller  plates  are  developed,  e.g.,  the 
adambulacrals,  which  articulate  with  the  outer  lower  ends  of 
ambulacrals.  The  dorsal  surface  bears  a  network  of  little 
ossicles,  and  many  of  these  bear  spines.  Peculiarly  modified 
spines,  known  as  pedicellarice,  look  like  snapping  scissor 
blades  mounted  on  a  single  soft  handle.  They  have  been 
seen  gripping  Algae  and  the  like,  and  probably  keep  the  sur- 
face of  the  starfish  clean. 

Muscular  System. 

A  starfish  is  not  very  muscular,  but  it  often  bends  its  arms 
upwards  by  means  of  the  muscular  layer  noted  above,  and 
may  sometimes  be  seen  tightly  embracing  an  oyster.  Other 
muscles  affect  the  size  of  the  ventral  grooves,  and  muscular 
elements  also  occur  on  the  protrusible  part  of  the  stomach, 
and  in  connection  with  the  water  vascular  system. 


SENSE    ORGjANS.  231 


Nervous  System. 

Underneath  the  ciliated  ectoderm  lies  a  network  of  nerve 
fibrils,  with  some  ganglionic  cells.  But  besides  these  diffuse 
elements  there  is  a  pentagon  around  the  mouth,  and  a  nerve 
along  each  arm.  The  system  is  not  separable  from  the 
skin. 

Sense  Organs. 

A  red  eye  spot,  sensitive  to  light,  lies  on  the  terminal 
ossicle  at  the  tip  of  each  arm,  and  is  usually  upturned.  It 


FIG.  75. — Alimentary  system  of  Starfish.     (After 
MULLER  and  TROSCHEL.) 

The  dorsal  surface  has  been  removed  ;  the  digestive  caeca,  the 
stomach,  &c.,  are  shown. 

is  a  modified  tentacle,  bearing  numerous  little  cups,  lined  by 
sensitive  and  pigmented  cells,  containing  clear  fluid,  and 
covered  by  cuticle.  The  skin  is  diffusely  sensitive.  The 
terminal  tube  foot  of  each  ray  seems  to  be  olfactory. 


232  ECHINODERMA. 

Alimentary  System. 

The  starfish  is  fond  of  young  oysters  and  other  bivalves, 
and  may  be  found  with  part  of  its  stomach  extruded  over 
them.  This  protrusible  or  cardiac  portion  of  the  stomach  is 
glandular  and  sacculated,  and  bulges  slightly  towards  the 
arms ;  it  is  followed  by  an  upper  or  pyloric  portion,  giving 
off  five  branches,  each  of  which  divides  into  two  large  diges- 
tive caeca,  a  pair  in  each  arm  (Fig  75.)  These  glands  con- 
tain a  yellowish  pigment  (enterochlorophyll)  and  secrete 
tryptic,  peptic,  and  diastatic  ferments.  From  the  short 
tubular  intestine  between  the  stomach  and  the  almost  central 
dorsal  anus  two  little  outgrowths  are  given  off,  perhaps  homo- 
logous with  the  "  respiratory  trees  "  of  Holothuroids.  Some 
parts  of  the  food  canal  are  ciliated. 

Body  Cavity. 

The  coelome  is  distinct,  though  not  much  of  it  is  left 
unoccupied  either  in  the  disc  or  in  the  arms.  It  is  lined  by 
ciliated  epithelium,  and  contains  a  fluid  with  amoeboid  cells. 
A  few  of  these  have  a  pigment  which  probably  aids  in 
respiration  ;  others  are  phagocytes,  which  get  rid  of  injurious 
particles  through  the  "  skin  gills  ; "  others  continue  the  work 
of  digestion. 

Water  Vascular  System. 

When  we  watch  a  starfish  crawling  up  the  side  of  a  rock 
we  see  that  scores  of  tube  feet  are  protruded  from  the  ventral 
groove  of  each  arm,  that  these  become  long  and  tense,  and 
that  their  sucker-like  terminal  discs  are  pressed  against  the 
hard  surface.  There  they  are  fixed,  and  towards  them  the 
starfish  is  gently  lifted.  The  protrusion  is  effected  by  the 
internal  injection  of  fluid  into  the  tube  feet,  the  fixing  is  due 
to  the  subsequent  withdrawal  of  the  water  producing  a 
vacuum  between  the  ends  of  the  tube  feet  and  the  rock. 

As  to  the  course  of  the  fluid,  it  is  convenient  to  begin  with  the  madre- 
poric  plate,  which  lies  between  the  bases  of  two  of  the  arms  (the  bivium}. 
This  plate  is  a  complex  calcareous  sieve,  with  numerous  perforating 
canals  and  external  pores.  It  may  be  compared  to  the  rose  of  a  water- 
ing pan,  but  the  holes  are  much  more  numerous,  and  lead  into  small 
canals  which  converge  into  a  main  ciliated  canal.  The  latter  runs  down 
through  the  body,  and  is  like  a  complex  calcareous  filter.  It  is  called 
the  stone  canal. 


WATER    VASCULAR   SYSTEM. 


233 


The  stone  canal  leads  into  a  water  ring  round  about  the  mouth.  From 
this  circumoral  ring  are  given  off  nine  glandular  bodies  (Tiedemann's 
bodies),  and  five  radial  tubes,  one  for  each  of  the  arms.  Considerations 
of  symmetry  suggest  that  there  should  be  ten  glandular  bodies,  but  the 
stone  canal  has  taken  the  place  of  one.  In  many  starfishes  there  are  five 
or  ten  little  reservoirs  (Polian  vesicles)  opening  into  the  circumoral  ring, 
but  in  Asterias  rtibens  these  are  hardly  distinguishable  from  the  first 
ampullae  of  the  radial  vessels. 

Along  each  arm,  then,  there  runs  a  radial  vessel.  It  lies  in  the 
ambulacral  groove  beneath  the  shelter  of  the  rafter-like  ossicles.  From 
it  branches  are  given  off  to  the  bases  of  the  tube  feet,  but  from  each  of 
these  bases  a  canal  ascends  between  each  pair  of  ambulacral  ossicles,  and 
expands  into  an  ampulla  or  reservoir  on  the  dorsal  or  more  internal  side. 
The  fluid  in  the  system  may  pass  from  the  radial  vessels  into  the  tube 
feet,  and  from  the  tube  feet  it  can  flow  back,  not  into  the  radial  vessel, 


FIG.  76. — Diagrammatic  cross  section  of  starfish  arm. 
(After  LUDWIG.) 

n.,  radial  nerve  ;  b.v.,  radial  blood  vessel  according  to  Ludwig,  sep- 
tum in  blood  vessel  according  to  others  •  iv.v.,  radial  water  vessel ; 
am.,  ampulla ;  tf.,  tube  foot ;  p.c.,  a  pyloric  caecum  cut  across  ;  s.p., 
a  calcareous  spine  ;  g. ,  a  skin  gill  ;  lac.,  spaces  in  the  skin  ;  go.^  ova 
in  ovary;  a.o.,  ambulacral  ossicle. 

but  into  the  ampullae.  There  are  muscles  on  the  walls  of  the  tube  feet, 
ampullae,  and  vessels.  At  the  end  of  each  arm,  there  is  a  long  unpaired 
tube  foot,  which  seems  to  act  as  a  tactile  tentacle,  and  has  also  olfactory 
significance. 

To  recapitulate,  the  madreporic  plate  leads  into  the  stone  canal,  this 
passes  into  the  ring  round  the  mouth  with  its  nine  vesicles,  from  the  ring 
radial  vessels  run  along  the  arms,  they  give  off  branches  to  the  tube  feet, 
and  the  base  of  each  tube  foot  communicates  with  an  ampulla. 


234  ECHINODERMA. 

Vascular  System. — We  have  not  yet  reached  certainty  in  regard  to 
this  system.  German  authorities,  e.g.*  Ludwig,  describe  (i)  a  radial 
blood  vessel  above  the  nerve  in  each  arm  ;  (2)  a  circumoral  vessel 
around  the  mouth  ;  (3)  a  heart  lying  beside  the  stone  canal  and  leading 
into  (4)  an  aboral  ring  which  gives  off  vessels  to  the  genital  organs. 

But  others  say  that  the  so-called  "  heart "  is  a  solid  glandular  organ, 
that  the  aboral  ring  is  merely  the  connecting  strand  or  rhachis  of  the 
genital  organs,  and  that  the  radial  and  circumoral  vessels  described  are 
really  thickened  septa  within  the  true  vessels. 

French  authorities  describe  (a)  a  radial  perihremal  space  or  blood 
vessel  divided  by  a  median  mesentery,  and  (b]  the  union  of  these  in  a 
circumoral  ring.  But  the  latter  encloses  (c)  another  annular  vessel  with 
which  a  sinus  (d)  surrounding  the  stone  canal  communicates.  Finally, 
an  aboral  pentagon  (e)  gives  off  five  pairs  of  genital  blood  vessels. 

Respiratory  System. 

From  the  dorsal  surface  and  sides  of  a  starfish  in  a  pool, 
numerous  transparent  processes  may  be  seen  hanging  out 
into  the  water.  They  are  the  simplest  possible  respiratory 
structures,  contractile  outgrowths  of  the  skin,  with  cavities 
continuous  with  the  ccelome,  and  are  called  "  skin  gills." 
It  is  likely  that  pigmented  cells  of  the  body  cavity  fluid  act 
like  rudimentary  red  blood  corpuscles  ;  the  water  vascular 
system  may  help  in  aeration  ;  and  the  whole  body  is  of 
course  continually  washed  with  water. 

Excretory  System. 

The  uskin  gills"  are  said  to  have  an  excretory  function  ;  for 
phagocytes,  bearing  waste,  seem  to  traverse  their  walls.  It 
may  also  be  that  excretion  is  somehow  concerned  in  forming 
the  carbonate  of  lime  skeleton,  but  facts  are  wanting. 

Reproductive  System. 

The  sexes  are  separate,  and  they  are  like  one  another, 
both  externally  and  internally.  The  organs  develop  periodi- 
cally, and  lie  in  pairs  in  each  arm.  Each  is  branched  like 
an  elongated  bunch  of  grapes,  and  is  surrounded  by  a  blood 
sinus.  Each  has  a  separate  duct,  which  opens  on  a  porous 
plate,  between  the  bases  of  the  arms  on  the  dorsal  surface. 
In  Asterina  gibbosa,  however,  the  eggs  are  extruded  ven- 
trally.  The  eggs  are  fertilised  in  the  water,  and  the  free 
swimming  larva,  which  will  be  described  along  with  those  of 
of  the  other  classes,  is  known  as  a  Bipinnaria  or  as  a 
Brachiolaria. 


OPHIUROIDEA    OR  BRITTLE  STARS.  235 

Other  Starfishes. 

Astropecten  and  most  forms  related  to  it  have  blind  food  canals  ; 
Brisinga  has  9-12  long  arms,  arising  abruptly  from  a  small  disc  as  in 
Brittle  stars,  and  has  no  ampullae,  eye  spots,  or  skin  gills  ;  Luidia  has 
three-bladed  pedicellariie ;  in  most  forms  the  genital  ducts  end  on  plates 
with  a  single  aperture,  and  so  on. 

The  commonest  European  forms  are  species  of  Asterias  or  Aster- 
acanthion,  Astropecten,  Cribrella,  Solaster,  Goniaster. 

The  largest  are  such  as  Asterias  gigantea  (from  the  Pacific  coast  of 
N.  America),  measuring  2  feet  in  diameter,  or  Pycnopodia  helianthoides, 
about  a  yard  in  diameter,  and  with  over  twenty  arms. 

There  are  many  deep  sea  forms,  such  as  the  ophiuroid-like  Brisinga, 
the  widely  distributed  Hymenaster,  and  the  blue  Porcellenaster  ccertileus, 
but  the  majority  occur  in  water  of  no  great  depth. 

Parental  care  is  incipient  among  Asteroids,  for  a  large  Asterias  has 
been  seen  sheltering  its  young  within  its  arms  :  there  is  a  definite  brood 
pouch  in  the  form  of  a  sort  of  tent  on  the  dorsal  surface  of  Pleraster. 

Many  Asteroids  break  very  readily,  or  throw  off  their  arms  when 
these  are  seized.  Professor  Forbes  describes  how  a  fine  specimen  of 
Luidia  thus  escaping  him  gave  a  "  wink  of  derision  "  as  it  passed  over 
the  side  of  the  boat.  The  lost  parts  are  slowly  regenerated,  and  strange 
forms  are  often  found  in  process  of  regrowth.  Thus  the  "  comet  form  " 
of  starfish  occurs  when  a  separated  arm  proceeds  to  grow  the  other  four. 
Asteroidea  first  occur  in  Silurian  strata. 


Class  OPHIUROIDEA.     Brittle  stars,  e.g.,  the  common 
Ophiopholis  bellis. 

The  body  of  a  brittle  star  differs  from  that  of  a  starfish  in 
the  abruptness  with  which  the  arms  spring  from  the  central 
disc  (cf.  Brisinga).  These  arms  are  muscular,  and  useful  in 
wriggling  and  clambering  ;  they  do  not  contain  outgrowths 
of  the  gut,  nor  reproductive  organs.  Moreover  there  is  no 
ambulacral  groove,  and  the  tube  feet  which  project  on  the 
sides  are  too  small  to  be  of  locomotor  service.  The  madre- 
poric  plate  is  situated  on  the  ventral  surface,  usually  on 
one  of  the  plates  around  the  mouth.  The  food  canal  ends 
blindly. 

The  reproductive  organs  lie  in  pairs  between  the  arms, 
and  open  into  pockets  or  bursse  formed  from  inturnings  of 
the  skin,  which  communicate  with  the  exterior  by  slits 
opening  at  the  bases  of  the  arms.  Water  currents  pass 
in  and  out  of  these  pockets,  which  probably  have  both 
respiratory  and  excretory  functions. 


236  ECHINODERMA. 

The  free  swimming  larva  is  a  Pluteus^  very  like  that  of 
Echinoids. 

Ophiuroids  are  first  found  in  Silurian  strata. 

1.  Euryalicla.     Skin  without  plates,   arms  simple  or  branched  and 

capable  of  being  rolled  up. 

A  s  trophy  ton.     Gorgonocephalus. 

2.  Ophiurida.     Skin  with  plates,  arms  simple. 

OphiopholtS)  Ophiocoma,  Ophiothrix,  are  common  genera. 
Amphiura  squamata  is  hermaphrodite. 


Class  ECHINOIDEA.     Sea  Urchins,  e.g.,  the  common  Echinus 
edulis,  Strongylocentrotus  lividus. 

Most  sea  urchins  live  off  rocky  coasts,  and  not  a  few 
shelter  themselves  sluggishly  in  holes.  They  move  by  means 
of  their  tube  feet  and  spines,  and  seem  to  feed  on  seaweeds, 
and  on  the  organic  matter  found  in  mud  and  other  deposits. 
After  the  perils  of  youth  are  past,  the  larger  forms  have  few 
formidable  enemies. 


"' 


,  £kin,  and  Skeleton. 

The  hard  and  pficluy  body  is  more  or  less  spherical. 
The  food  canal  begms~in  the  middle  of  the  lower^surface  ; 
it  ends  at  the  opposite  pole  in  the  middle  of  an  aj/ical  disc 
formed  of  a  central  plate  surrounded  by  five  "  ocular  "  and 
five  "  genital  "  plates.  The  ocular  or  radial  plates  bear 
eye  specks  ;  the  genital  or  basal  plates  bear  the  apertures  of 
x  the  genital  ducts,  but  one  of  the  five  is  modified  as  the 
madreporic  plate.  From  pole  to  pole  run  ten  meridians  of 
calcareous  plates  which  fit  one  another  firmly  ;  five  of  these 
(in  a  line  with  the  ocular  plates)  are  known  as  ambulacral 
areas,  for  through  their  plates  the  locomotor  tube  feet  are 
extruded  ;  the  five  others  (in  a  line  with  the  genital  plates) 
are  called  inter-ambulacral  areas,  and  bear  spines,  not  tube 
feet.  Altogether,  therefore,  there  are  ten  meridians,  and 
each  meridian  area  has  a  double  row  of  plates.  On  the  dry 
shell  from  which  the  spines  have  been  scraped,  the  ambu- 
lacral plates  are  seen  to  be  perforated  by  small  pores,  four 
pairs  or  so  to  each  plate.  Through  each  pair  of  pores  a 
tube  foot  is  connected  with  an  internal  ampulla.  In  the  ( 
starfish  the  ambulacral  areas  are  wholly  ventral,  and  the 


THE  NERVOUS  SYSTEM.  237 

apical  area  seen  on  the  dorsal  surface  of  the  young  forms  is 
not  demonstrable  in  the  adult. 

The  "  posterior "  ambulacra,  those  between  which  the 
modified  basal  or  madreporic  plate  lies,  are  often  distin- 
guished as  the  "  bivium,"  the  other  three  form  the  "  trivium," 
and  the  middle  one  of  the  three  is  "  anterior." 

On  the  shell  there  are  obviously  many  spines,  most 
abundant  on  the  inter-ambulacral  areas.  Their  bases  fit 
over  ball-like  knobs,  and  are  moved  upon  these  by  muscles. 
But  besides  these,  there  are  two  modified  forms  of  spines, — 
(a)  the  minute  pedicellariae,  with  three  snapping  blades  on 
a  soft  stalk,  and  sometimes  with  apical  glands  ;  and  (b}  small 
globular  sphaeridia,  which  show  some  structural  resemblances 
to  otocysts.  It  is  said  that  like  true  otocysts  they  are  con- 
cerned with  the  perception  of  direction  of  motion. 

In  front  of  the  mouth  project  the  tips  of  five  teeth,  which 
move  against  one  another,  grasping  and  grinding  small 
particles.  They  are  fixed  in  five  large  sockets,  and  along 
with  fifteen  other  pieces  form  "  Aristotle's  lantern,"  a  complex 
masticating  apparatus,  of  whose  history  we- know  little.  It 
surrounds  the  pharynx,  and  is  swayed  about  and  otherwise 
moved  by  muscles,  many  of  which  are  attached  to  five  beams 
which  project  inward  from  the  margin  of  the  shell  round 
about  the  mouth. 

As  in  other  Echinoderms,  the  skeleton  of  lime  is  meso- 
dermic.  The  shell  is  covered  externally  by  a  delicate 
ciliated  ectoderm,  beneath  which,  in  a  thin  layer  of  con- 
nective tissue,  there  is  a  network  of  nerve  fibres,  and  some 
ganglion  cells.  Internally,  there  is  another  thin  layer  of 
connective  tissue,  and  a  ciliated  epithelium  lining  the  body 
cavity.  The  skeleton  grows  by  the  formation  of  new  plates 
around  the  apical  disc,  and  also  by.  the  individual  increase 
of  each.  In  a  few  forms  the  shell  retains  some  plasticity. 

Nervous  System. 

The  nervous  system  consists  of  a  ring  around  the  mouth,  of 
radial  branches  run,  ing  up  each  ambulacral  area,  and  of  the 
superficial  network.  Tube  feet,  sphaeridia,  pedicellariae,  and 
spines  are  all  under  nervous  control,  and  each  radial  nerve 
ends  in  the  "  eye  speci's "  of  the  apical  "  ocular  plates." 
It  is  probable  that  all  the  tube  feet  are  sensory,  and 


238  ECHINODERMA. 

this  is  certainly  the  main  function  of  ten  which  lie  near 
the  mouth. 

Alimentary  Canal. 

The  alimentary  canal  passes  through  Aristotle's  lantern, 
and  the  intestinal  portion  lies  in  two  and  a  half  coils  around 
the  inside  of  the  shell  to  which  it  is  moored  by  mesenteries. 
It  contains  fine  gravel,  sand,  and  some  organic  debris.  It 
ends  near  the  centre  of  the  apical  disc,  whence  the  pedi- 
cellariae  have  been  seen  removing  the  faeces. 

Accompanying  the  first  coil  of  the  gut  is  a  canal  or 
"  siphon,"  which  opens  into  the  gut  at  both  ends.  Accord- 
ing to  Cuenot,  a  current  of  water  traverses  this  tube,  which 
thus,  by  reason  of  its  thin  walls,  carries  oxygen  to  the  cor- 


FIG.  77. — Ventral  half  of  Sea  Urchin.       (From  CARUS, 
after  TIEDEMANN.) 

</,  Aristotle's  lantern  in  centre  ;  oe,  oesophagus  ;  z,  intestine ; 
S,  intestinal  blood  vessel  ;  R,  radial  water  vessel  in  an  ambulacra! 
area  ;  A ,  an  inter-ambulacral  area  ;  m,  muscles  of  the  lantern. 

puscles  of  the  body  fluid.  The  spacious  body  cavity  is 
lined  by  ciliated  epithelium  and  contains  a  "  perivisceral " 
fluid,  whose  corpuscles  have  a  respiratory  pigment  (echino- 
chrome).  When  the  fluid  of  a  perfectly  fresh  sea  urchin  is 
emptied  out,  the  contained  corpuscles  unite  in  plasmodia, 
forming  composite  amoeboid  clots  (cf.  Proteomyxa,  &c.). 

Water  Vascular  System. 

The  madreporic  plate  communicates  with  a  membranous 
stone  canal,  which  runs  downwards  into  a  circular  vessel 


RESPIRATORY  AND  EXCRETORY  SYSTEMS.      239 

near  the  upper  end  of  the  lantern.  This  gives  off  five  inter- 
radial  transparent  vesicles,  and  five  radial  vessels  which  run 
down  the  sides  of  the  lantern  and  up  each  ambulacral  area. 
Each  radial  vessel  gives  off  numerous  lateral  branches, 
which  communicate  with  the  internal  ampullae  and  thence 
with  the  external  tube  feet.  When  the  tube  feet  are  made 
tense  with  fluid,  they  extend  beyond  the  limit  of  the  spines, 
and  are  attached  to  the  surface  of  the  rock  over  which  the 
sea  urchin  slowly  drags  itself.  The  sucker  at  the  tip  of  each 
tube  foot  bears  small  calcareous  plates  regularly  arranged, 
indeed  there  is  hardly  any  part  of  an  Echinoderm  in  which 
lime  may  not  be  deposited.  Before  bending  upwards 
from  the  base  of  the  lantern,  each  radial  vessel  gives  off  a 
branch  to  two  large  tentacle-like  tube  feet  without  attach- 
ing discs.  The  five  pairs  lie  near  the  mouth,  and  are 
sensitive. 

The  Blood  Vascular  System  is  not  readily  traced,  and  there  is  un- 
certainty as  to  many  points.  Along  the  stone  canal  lies  an  enigmatical 
structure,  to  which  such  names  as  "  plexiform  organ,"  "  ovoid  gland," 
"dorsal  organ,"  and  "heart"  are  given.  Its  structure  is  like  that  of 
the  smaller  glandular  enlargements  found  on  the  vascular  system. 
According  to  some,  it  gives  origin  to  some  of  the  amceboid  cells  of  the 
body  cavity  fluid.  It  is  connected  superiorly  with  the  five  genital 
organs,  inferiorly  with  a  circular  vessel  surrounding  the  pharynx  at  the 
top  of  the  lantern,  within  and  beneath  the  water  ring.  This  vascular 
ring  seems  to  be  connected,  by  branches  at  least,  with  the  five  pockets 
of  the  water  ring.  A  distinct  vessel  arises  from  the  ring  and  runs  along 
the  inner  or  ventral  surface  of  the  intestine,  while  another  on  the 
opposite  side  seems  to  originate  from  capillaries.  It  is  likely  enough 
that  there  may  be  radial  blood  vessels  or  spaces  in  the  ambulacral  areas. 
The  fluid  cannot  be  distinguished  from  that  of  the  body  cavity  ;  it 
contains  corpuscles,  some  of  which  have  pigment. 

Respiratory  and  Excretory  Systems. 

On  the  area  round  about  the  mouth  there  are  ten  hollow 
outgrowths,  which  resemble  the  skin  gills  of  starfishes.  As 
already  mentioned,  the  pigmented  cells  of  the  body  cavity 
fluid  seem  able  to  absorb  oxygen.  The  water  vascular 
system  plays  here  a  very  important  part  in  respiration. 
Waste  products  seem  simply  to  accumulate  in  the  tissues, 
but  Hartog  maintains  that  the  water  vascular  system  helps 
in  excretion. 


24o  ECHINODERMA. 

Reproductive  System. 

The  sexes  are  separate,  and  like  one  another.  Five 
branched  yellow-brown  ovaries  or  rose-white  testes  lie 
interradially  under  the  apex  of  the  shell,  and  open  by 
separate  ducts  on  the  five  genital  or  basal  plates.  In 
spring  the  apical  disc  may  be  seen  covered  with  orange 
ova  or  milky-white  spermatozoa. 

The  eggs  are  fertilised  externally  by  sperms  wafted  from 
adjacent  sea  urchins,  and  the  free  swimming  larva,  which  we 
shall  afterwards  describe,  is  called  a  Pluteus. 

Classification  of  Echinoidea. 

1.  Palseo-echinoidea.     Extinct  forms,  apparently  with  a  plastic  test, 

of  overlapping  and  variable  plates.      They  appear   in    Lower 
Silurian  rocks. 

2.  Desmosticha.     Regular  and  symmetrical  sea  urchins  like  Echinus. 

e.g.,  Cidaris,  without  external  gills. 

Diadema,  a  species  has  been  described  as  covered  with 

compound  eyes. 

Cyanosoma  urens,  the  spines  contain  a  poison  apparatus. 
Echinothuridse  have  flexible  tests. 

3.  Clypeastroidea.      Shield  shaped,  and  often  flat.      The  food  canal 

ends  outside  the  apical  disc  on  the  posterior  inter-radius. 
e.g.,  Clypeaster. 

4.  Petalosticha.     Heart  shaped.     The  mouth  is  ex-centric,  the  food 

canal  ends  away  from  the  apical  disc.  There  are  no  masticat- 
ing organs.  On  the  dorsal  surface  the  ambulacral  areas  dilate 
from  the  apex  outwards,  and  contract  again  towards  the  margin 
in  the  form  of  "  petals."  The  anterior  area  is  often  different 
from  the  other  four. 
e.g.,  Spatangus. 

Hernias ter  and  some  others  carry  their  young  among 
their  spines. 

Class  HOLOTHUROIDEA.      Sea  Cucumbers. 

The  Holothurians  do  not  at  first  sight  suggest  the  other 
Echinoderms,  for  they  are  like  plump  worms,  and  the 
calcareous  skeleton  is  not  prominent.  But  closer  examina- 
tion shows  the  characteristic  pentamerous  symmetry.  ?•"*' 
the  occurrence  of  calcareous  plates  in  the  skin.  Ti' 
seem  to  be  absent  in  the  unique  pelagic  Pelagothuria. 

Holothurians  occur  in  most  seas,  from  slight  to  ,ous 
great  depths.  Their  food  consists  of  small  animals,  anc^sej 


organic  par 
things  in  tl 
the  pharyn 
over  contn 
a  side  ruj; 
escape,  an 

The  we 
equidistant 
But  three  v 
mated   on   a 
convex  dorsal 
form. 

The  walls  of  L 
skeleton  is  repres< 
of  lime  scattered 
and  on  a  few  ot1 

The  nervous 
which  the  five  • 
unite,  and  fro^ 
organs  are  r 
have  "  ear 
on  the  dor 

From  tl 
ten,  or  m 
pole.      1 
contra  ^' 
pair 
foi 


flUlv 

systt 
Was 
but 
in  e 


)  with  fivi 
c)  with  i 
the  bod] 


— T. 


.nd 

iese 


v^ery 
lof- 


FEATHER  STARS.  243 

have  ampullae  and  tube  feet,  as  in  sea  urchins.  But  there 
are  many  divergences,  especially  in  the  reduction  of  the  tube 
feet  areas.  Instead  of  tube  feet,  or  along  with  them,  there 
are  often  conical  processes  or  papillae  without  terminal  discs. 
These  are  especially  common  on  the  dorsal  surface.  The 
blood  vascular  system  is  not  very  definite,  and  seems  to  con- 
sist mainly  of  spaces  in  the  connective  tissue,  e.g.,  around  the 
pharynx  and  along  the  intestine. 

The  sexes  are  usually  separate.  The  reproductive  organs 
do  not  exhibit  radial  symmetry,  and  are  branched  tubes 
which  open  within  or  just  outside  the  circle  of  tentacles. 
They  and  other  internal  organs  of  Holothurians  are  often 
very  brightly  coloured.  The  larva  is,  in  most  cases,  what  we 
shall  afterwards  describe  as  an  Auricularia.  Sometimes, 
however,  the  larval  stage  is  skipped,  as  in  Cucumaria  crocea 
and  Psolus  ephippiger  where  the  eggs  and  young  are  attached 
to  the  back  of  the  mother.  In  Cucumaria  lavigata  there 
is  an  invaginated  brood  pouch;  in  Synapta  vivipara  and 
others  the  body  cavity  serves  as  a  brood  pouch. 

The  calcareous  plates  of  Holothurians  are  found  as  far 
back  as  Carboniferous  strata. 

Classification. 

1.  Elasipoda  :  primitive  deep  sea  forms,  bilaterally  symmetrical,  with 

tube  feet  on  the  ventral  surface  only,  and  with  papillae  on 
the  back.     The  stone  canal  often  opens  externally  by  a 
pore.     There  are  no  respiratory  trees. 
e.g.)  Kolga,  Elpidia. 

2.  Pedata  :  with  well-developed  tube  feet  and  papillae. 

e.g.,  Holothuria,  Cucumaria,  Psolus. 

3.  Apoda  :  without  radial  canals,  tube  feet,  or  respiratory  trees. 

e.g.,  Synapta,  a  remarkable  animal,  especially  apt  to  break 
in  pieces  ;  pinnate  tentacles  ;    hermaphrodite  ;    with 
beautiful  calcareous  anchors  and  plates  in  the  skin. 
Semper  has  described  a  strange  animal,  Rhopalodina  lageniformis , 
from  the  Congo  coast.     It  is  like  a  globular  flask,  with  mouth  and  anus 
close  together  at  the  narrow  end,  with  ten  ambulacral  areas. 

Class  CRINOIDEA.     FEATHER  STARS. 
Commonest  Type,  Antedon  rosaceus. 

The  feather  stars  or  sea  lilies  differ  from  other  Echino- 
derms  in  being  fixed  permanently  or  temporarily  by  a  jointed 


244  ECHINODERMA. 

stalk.  The  modern  Comatulids,  e.g.,  the  rosy  feather  star 
(Comatula  or  Antedon  rosaceus]  leave  their  stalk  at  a  certain 
stage  in  life  ;  but  the  other  Crinoids,  e.g.,  Pentacrinus,  are 
permanently  stalked  like  almost  all  the  extinct  stone  lilies  or 
encrinites  once  so  abundant.  Most  of  them  live  in  deep 
water,  and  many  in  the  great  abysses.  An  anchorage  is 
found  on  rocks  and  stones,  or  in  the  soft  mud,  and  great 
numbers  grow  together — a  bed  of  sea  lilies.  The  free 
Comatulids  swim  gracefully  by  bending  and  straightening 
their  arms,  and  they  have  grappling  "  cirri "  on  the  aboral 
side,  where  the  relinquished  stalk  was  attached.  By  these 
cirri  they  moor  themselves  temporarily.  Small  organisms — 
Diatoms,  Protozoa,  minute  Crustaceans — are  wafted  down 
ciliated  grooves  on  the  arms  to  the  central  mouth,  which  is 
of  course  on  the  upturned  surface.  Some  members  of  the 
class,  e.g.,Comatula,  are  infested  by  minute  parasitic  "worms" 
(Myzostomidae)  allied  to  Chaetopods,  which  form  galls  on  the 
arms.  A  lost  arm  can  be  replaced,  and  even  the  visceral 
mass  may  be  regenerated  completely  within  a  few  weeks 
after  it  has  been  lost. 

The  animal  consists  of  (i)  a  cup  or  calyx,  (2)  an  oral  disc  forming  the 
lid  of  this  cup,  (3)  the  radiating  "  arms,"  and  (4)  the  stalk  supporting  the 
whole. 

The  calyx  consists  of  the  topmost  segment  of  the  stalk,  a  centro-dorsal 
plate,  and  several  rows  of  radial  plates,  which  lead  on  to  the  brachial 
plates  of  the  arms.  When  Comatulids  break  off  from  their  larval  stalk, 
they  carry  with  them  the  centro-dorsal  plate,  (C.D.  in  Fig.  79),  which 
becomes  the  central  part  of  their  calyx,  and  bears  the  cirri. 

The  oral  disc,  turned  upwards,  is  supported  by  plates.  Here  the  anus 
also  is  situated.  The  arms  usually  branch  in  dichotomous  fashion,  and 
thus  ten,  twenty,  or  more  may  arise  from  the  original  five.  But  the 
growing  point  continues  to  fork  dichotomously,  like  the  leaf  of  many 
ferns,  and  as  each  alternate  fork  remains  short,  a  double  series  of  lateral 
"  pinnules  "  results.  The  arms  are  supported  by  calcareous  plates.  The 
stalk  usually  consists  of  numerous  joints,  especially  in  extinct  forms,  in 
some  of  which  it  measured  over  fifty  feet  in  length.  Except  in  Holopus, 
and  in  the  stalked  stage  of  Antedon,  the  stalk  bears  lateral  cirri. 

The  nervous  system  is  remarkable  in  being  double.  On  the  upturned 
surface  of  each  arm,  beneath  the  food  wafting  ciliated  grooves,  there  is 
a  subepithelial  nervous  band,  probably  in  great  part  sensory  (Fig.  79,  h}. 
These  bands  are  united  in  a  ring  or  plexus  around  the  mouth.  So  far 
the  Crinoid  is  like  a  starfish.  But  on  the  dorsal  surface  the  main  mass 
lies — an  antambulacral  motor  and  sensory  nervous  system,  consisting  of  a 
central  capsule,  with  branches  to  the  cirri  and  to  the  arms  (d  and  a  in 
Fig.  79).  Cuenot  asserts  that  this  system,  though  to  a  very  slight  extent, 


FEATHER  STARS. 


245 


is  also  represented  in  starfishes.  Apart  from  the  superficial  epithelium 
there  are  no  sensory  structures. 

The  ciliated  food  canal  descends  from  the  mouth  into  the  cup,  and 
curves  up  again  to  the  anus,  which  is  usually  ex-centric  in  position.  The 
last  part  of  the  gut  is  expanded  to  form  an  anal  tube,  which  during  life 
is  in  constant  movement,  and  has  apparently  a  respiratory  function. 
From  the  cup,  where  the  body  cavity  is  in  great  part  filled  with  con- 
nective tissue  and  organs,  two  ccelomic  canals  extend  into  each  of  the 
arms.  They  communicate  at  the  apices  of  the  arms  and  pinnules,  and 
currents  pass  up  one  and  down  the  other. 

The  blood  vascular  system  consists  of  a  circumoral  ring,  which  is  con- 
nected with  a  radial  vessel  under  each  ambulacral  nerve,  and  with  a 
circumcesophageal  plexus.  There  is  also  a  "  plexiform  organ,"  k<  lying 
interradially  in  the  disc  anteriorly  to  the  mouth  "  (g  in  Fig.  79).  It  en- 
closes the  barren  central  part  of  the  reproductive  system  (the  axial  genital 


FIG.  79. — Diagrammatic  vertical  section  through  disc 
and  base  of  one  of  the  arms  of  Antedon  Rosaceus.  (After 
MILNES  MARSHALL.) 

The  section  is  interradial  on  the  left,  radial  on  the  right.  /?., 
ciliated  openings  in  body  wall;  h.,  subepithelial  ambulacral  nerve; 
/.,  water  vascular  canal ;  k.,  tentacle;  r.,  mouth;  s.,  intestine;^-., 
central  plexus,  with  "chambered  organ"  at  its  base;  RT..-R-$., 
radial  plates  ;  JSr.,  brachial  plates  ;  «.,  muscle  ;  a.,  axial  nerve  cord  ; 
d.,  central  capsule  ;  C.D.,  centro-dorsal  plate  ;  /.,  cirri ;  e.,  branches 
from  central  capsule  to  cirri. 

stolon),  and  has  connections  with  the  above  mentioned  plexus,  with  the 
vessels  to  the  organs,  and  with  a  strange  "chambered  organ  "  which  lies 
within  the  central  aboral  nervous  system. 

The  water  vascular  system  consists  as  usual  of  a  circumoral  ring  and 
radial  vessels.  These  lie  under  the  corresponding  blood  vascular  system. 
But  the  system  is  divergent  in  several  ways  ;  (a)  water  passes  into  it  by 
several  ciliated  and  branched  water  tubes  which  hang  from  the  ring,  and 
from  the  origins  of  the  radial  vessels,  into  the  body  cavity ;  (b]  water 


246  ECHINODERMA. 

passes  from  the  exterior  into  the  body  cavity  by  numerous  ( 1 500  on  the 
disc  of  Antedon  rosaced]  ciliated  water  pores  which  pierce  the  disc,  and 
sometimes  the  arms  also  ;  (c)  "  the  radial  water  vascular  vessels  give  off 
alternately  to  the  right  and  left,  in  groups  of  three  each,  delicate  tubular 
branches,  respiratory  in  function,  which  form  the  tentacles  homologous 
with  tube  feet." 

The  sexes  are  separate,  and  a  process  suggestive  of  sexual  union 
has  been  observed  in  Antedon.  The  reproductive  organs  extend  as 
tubular  strands  from  the  disc  along  the  arms,  but  are  rarely  functional 
except  in  the  pinnules,  from  each  of  which  the  elements  burst  out  by 
one  duct  in  females,  by  one  or  two  fine  canals  in  males. 

There  are  about  400  living  species  in  twelve  genera,  but  about  1 500 
species  in  200  genera  are  known  from  the  rocks.  The  class  is  obviously 
decadent.  It  is  represented  in  the  Cambrian,  and  attained  its  maximum 
development  in  Silurian,  Devonian,  and  Carboniferous  times. 

The  oval  ciliated  larva  of  Antedon^  the  only  one  known,  is  less  quaint 
than  that  of  other  Echinoderms. 

Classification  of  Crinoidea. 

1.  Pakeo-crinoidea  ( =  Tesselata).     Palseozoic  forms.     The  symmetry 

of  the  calyx  is  not  always  pentamerous. 

2.  Neo-crinoidea  ( =  Articulata  +  Holopus  and  Marsupites}. 

Mesozoic  and  recent.  The  calyx  always  has  pentamerous 
symmetry.  The  recent  forms  include  the  stalked  Pentacrinus, 
RhizocrimtS)  <5rV.,  and  the  free  Comatulids,  which  pass  through 
a  stalked  Pentacrinus  stage,  e.g.,  Antedon. 

Holopus  is  a  remarkable  deep  sea  form  with  direct  ancestors 
in  the  Upper  Silurian.  Marsupites  is  an  extinct  Crinoid  which 
had  no  stalk. 


Class  BLASTOIDEA.     Wholly  extinct. 

The  Blastoicls  are  first  found  in  the  Upper  Silurian,  later  than  Cystoids 
and  Crinoids ;  they  had  their  golden  age  in  the  Carboniferous  and 
Devonian  times,  but  then  disappeared.  Their  body  was  ovate,  with  five 
ambulacral  areas,  with  each  groove  of  which  jointed  pinnules  were 
associated. 

Class  CYSTOIDEA.     Wholly  extinct. 

The  Cystoids  are  first  found  in  the  Lower  Silurian  rocks,  had  their 
golden  age  in  Upper  Silurian  times,  and  died  out  in  the  Carboniferous 
ages.  Their  body  was  ovate  or  globular,  sessile  or  shortly  stalked, 
covered  with  polygonal  plates  often  irregularly  arranged.  Some  (accord- 
ing to  Bell,  the  more  primitive)  types  were  "never  fixed,  and  had  not 
fixed  ancestors."  They  seem  usually  to  have  borne  two  to  five  feeble, 
unbranched  arms. 

Both  Cystoids  and  Blastoids  seem  to  have  been  half  smothered  in  lime, 
and  perhaps  this  is  in  part  the  explanation  of  their  extinction. 


DEVELOPMENT  OF  ECHINODERMS.  247 

Development  of  Echinoderms. 

The  ovum  undergoes  total  segmentation,  and  a  hollow 
ball  of  cells  or  blastosphere  results.  Apart  from  two  alleged 
cases  of  delamination,  the  gastrula  is  always  formed  by  the 
invagination  pf  this  blastosphere.  Ectoderm  and  endoderm, 
or  epiblast  and  hypoblast,  are  thus  established. 

The  mesoblast  has  a  twofold  origin  :  (a)  from  "  mesen- 
chyme  "  cells,  which  immigrate  from  the  invaginated  hypo- 
blast  into  the  segmentation  cavity ;  (b)  by  the  outgrowing  of 
one  or  more  ccelome  pouches  from  the  gastrula  cavity  or 
archenteron.  It  is  thus  that  the  body  cavity  and  the  rudi- 
ments of  the  water  vascular  system  arise. 

According  to  Hertwig's  fundamental ,  thesis  this  double 


FIG.  80. — Stages  in  development  of  Echinoderms. 
(After  SELENKA.) 

i.  Section  of  blastula  of  Synapta  digitata  (Holothuroid)  with  a 
hint  of  gastrulation  ;  2.  Section  of  Gastrula  of  Toxopneustes  brevi- 
spinosus  (sea  urchin);  ec.,  ectoderm;  en.,  endoderm;  m.y  seg- 
mentation cavity  with  mesenchyme  cells  in  it ;  3.  Section  of  larva 
of  Asterina  gibbosa  (starfish,) ;  £/.,  blastopore  ;  G.,  mesenteron  ; 
v.p.,  vaso-peritoneal  vesicle  ;  r.  and  /.,  right  and  left  sides. 

origin  is  a  primitive  condition,  and  the  mesenchyme  here, 
as  always,  is  non-epithelial  and  gives  rise  to  the  connective 
tissues  and  to  the  vascular  system.  On  the  other  hand,  it 
has  been  asserted  that  in  Echinoderms  .the  mesenchyme  is 
not  purely  a  "  packing  tissue,"  but  may  acquire  a  distinctly 
epithelial  character.  Many  of  the  early  mesenchyme  cells 
are  calciferous,  combining  to  form  the  larva  skeleton. 


248  ECHINODERMA. 

The  larva  is,  first  of  all,  a  slightly  modified,  diffusely 
ciliated  gastrula.  It  becomes  more  modified,  but  preserves 
a  bilateral  symmetry.  In  Holothuroids,  Echinoids,  Asteroids, 
and  Ophiuroids,  the  larva  becomes  quaintly  modified  by  the 
outgrowth  of  external  processes,  and  the  formation  of  special 
ciliated  bands.  The  larva  of  Crinoids  (/.£.,  of  Antedon  only) 
is  not  so  divergent. 

The  larva  does  not  grow  directly  into  the  adult.  On  the 
contrary  the  adult  arises,  for  the  most  part,  from  new  growth 
within  the  larva.  The  structures  peculiar  to  the  larva  are 
absorbed,  or  in  part  thrown  off.  Only  in  a  very  few  cases 
is  the  development  direct. 

Following  the  excellent  account  of  Echinoderm  development,  in  the 
Vergleichende  Entwicklungsgeschichte  der  wirbellosen  Thieren  (Jena, 
1890),  by  Korschelt  and  Heider,  we  distinguish  four  stages  : — 

I.   The  formation  of  the  primary  germinal  layers*  of  the  mesenchyme, 
and  of  the  mouth  and  anus. 

Ectoderm  and  endoderm  are  established  by  the  invagination  of  the 
blastosphere.  The  result  is  a  ciliated  gastrula.  From  the  invaginat- 
ing  endoderm,  somewhat  amoeboid  cells  are  liberated  into  the  persisting 
segmentation  cavity,  and  form  the  mesenchyme  tissue  alluded  to  above. 
The  gastrula  cavity  or  archenteron  is  the  larval  mid  gut ;  the  blastopore, 
or  mouth  of  the  gastrula,  seems  usually  to  become  the  anus ;  but  an 
invagination  taking  place  at  the  other  end  forms  a  short  fore  gut  or 
stomatodseum. 

2.    The  formation  of  the  enteroccel  (body  cavity]  and  the  hydroccel 
(water  vascular  system). 

There  is  a  close  connection  between  the  origin  of  the  body  cavity  and 
that  of  the  water  vascular  system.  Both  are  the  results  of  an  outgrowth 
or  of  outgrowths  from  the  gastrula  cavity  or  archenteron,  into  the  sur- 
rounding space  between  endoderm  and  ectoderm.  As  they  have  a 
common  origin,  the  outgrowth  or  outgrowths  which  give  rise  to  enteroccel 
and  hydroccel  may  be  termed  vaso-peritoneal. 

There  is  not  perfect  agreement  as  to  this  united  origin,  but  the  follow- 
ing facts  are  generally  recognised. 

In  Holothuroids  there  is  a  single  outgrowth  which  gives  rise  to 

both  body  cavity  and  water  vascular  system. 

In  Echinoids,  Asteroids,  and  Ophiuroids,  there  are  two  out- 
growths, from  the  left  of  which  the  water  vascular  system 
arises. 

In  Crinoids  (Antedon},  there  are  three  outgrowths,  that  which 
gives  rise  to  the  water  vascular  system  being  independent  of 
the  pair  which  form  the  body  cavity. 

In  most  cases  a  dorsal  pore  bringing  the  hydroccel  into  com- 
munication with  the  exterior  has  been  detected. 


DEVELOPMENT  OF  ECHINODERMS.  249 


3.   The  differentiation  of  the  typical  larval  forms. 

The  celebrated  comparative  anatomist  and  physiologist,  Johannes 
Muller,  was  the  first  to  show  that  the  various  types  of  Echinoderm 
larvae  might  be  derived  from  one  fundamental  form. 

"This  fundamental  type  is  an  elongated,  oval  or  pear  shaped  larva, 
which  is  somewhat  flattened  on  its  ventral  side.  It  has  arisen  from  a 
gastrula,  whose  blastopore  has  become  the  anus,  while  the  archenteron 
is  bent  towards  the  ventral  surface,  where  it  communicates  by  the  larval 
mouth  with  the  exterior.  Besides  these  two  apertures,  the  larva  has  a 
third,  namely,  the  dorsal  pore  of  the  water  vascular  system.  The  cilia, 
with  which  the  larva  was  at  first  uniformly  covered,  partly  disappear, 
and  persist  only  in  restricted  regions  or  ciliated  bands."  (Korschelt  and 
Heider.) 

Crinoids. — The  simplest  Echinoderm  larva  is  that  of  Antedon^  a 
somewhat  modified  oval,  with  five  transverse  rings  of  cilia  (the  most 
anterior  is  less  distinct),  and  a  posterior  terminal  tuft. 

Holothuroids.  The  larva  of  Holothuroids  (an  Auricularid]  is  much 
quainter.  Its  diffuse  cilia  are  succeeded  by  a  wavy  longitudinal  band, 


4 
3 
FIG.  81. — Forms  of  Echinoderm  Larva.     (After  MULLER.) 

m.,  Mouth ;  #.,  anus.     The  dark  lines  indicate  the  ciliated  bands. 

1.  Supposed  primitive  type  from  which  the  various  forms  may  be 
derived. 

2.  Auricularia  of  Holothurian. 

3.  Bipinnaria  of  Asteroid. 

4.  Pluteus  of  Ophiuroid. 

which  in  the  Pupa  stage  breaks  into  transverse  rings,  usually  five  in 
number.  The  pre-oral  region  becomes  large. 

Asteroids.  Nearest  the  Auricularia  is  the  larva  of  starfishes,  which 
has  the  same  enlarged  pre-oral  region.  There  are  two  ciliated  bands, 
of  which  the  ad-oral  is  smaller,  the  ad-anal  much  larger.  They  are 
extended  peripherally  by  the  development  of  soft  arms,  and  such  a  larva 
is  known  as  a  Bipinnaria.  But  this  may  be  succeeded  by  a  Brachiolaria 
stage,  in  which  three  warty  arms  are  formed  at  the  anterior  dorsal  end, 
independently  of  the  ciliated  bands. 

Ophiuroids  and  Echinoids.  In  the  Pluteus  larvae  characteristic  of 
these  classes,  the  pre-oral  region  remains  small,  while  the  post-anal 
region  becomes  large.  There  is  one  undulating  ciliated  band,  the 
course  of  which  is  much  modified  by  the  growth  of  six  long  arms,  with 


250  EQHINODERMA. 

temporary  calcareous  supports.     This  quaint  form  is  often  compared  to 
a  six -legged  easel. 

4.    The  modification  of  the  larva  into  the  adult  Echinoderm. 

This  history  is  so  intricate  and  so  difficult  to  understand  without 
models,  that  it  may  be  better  simply  to  state  that  the  development  is 
indirect,  that  the  adult  is  a  new  formation  within  the  larva,  retaining 
the  water  vascular  system  and  mid  gut,  but  absorbing  or  rejecting  the 
provisional  larval  structures.  As  certain  parts  are  broken  down,  others 
are  built  up,  chiefly  through  the  agency  of  the  wandering  amoeboid  cells 
of  the  mesenchyme.  The  first  steps  in  the  upbuilding  of  the  adult,  and 
especially  of  its  skeleton,  are  to  some  extent  parallel  in  the  five  classes. 

One  of  the  most  important  changes  is  that  from  bilateral  to  radial 
symmetry.  In  connection  with  this,  it  has  been  conjectured  that  the 
primitive  ancestor  was  bilaterally  symmetrical,  and  that  the  radiate 
symmetry  was  acquired  by  early  sessile  or  sedentary  Echinoderms,  such 
as  the  Cystoids.  As  we  have  already  seen,  the  adults  in  the  different 
classes  tend  to  acquire  an  independent  and  secondary  bilateral  symmetry. 

It  is  very  difficult  to  compare  the  Echinoderm  larvse,  even  in  their 
simplest  form,  with  those  of  other  animals.  The  nearest  type  is  perhaps 
the  Tornaria  of  Balanoglossus,  but  it  again  is  very  unique.  One 
naturally  tries  to  compare  the  Echinoderm  larva  with  the  Trochosphere 
of  Annelids,  but  the  differences  are  very  marked. 

Pedigree  and  Relationships  of  Echinoderms. 

Concerning  the  exact  relationships  of  the  different  classes  of  Echino- 
derma,  there  is  still  considerable  doubt.  The  following  account  is 
based  upon  the  views  set  forth  by  Professor  Jeffrey  Bell,  but  the 
student  will  do  well  to  realise  that  in  this,  as  in  most  problems  of 
phylogeny,  there  is  little  certainty. 

The  Holothurians  have  no  aboral  system  of  plates,  and  the  radial 
symmetry  does  not  effect  the  reproductive  organs.  These  two  negative 
characters,  combined  with  some  positive  ones,  may  indicate  that  the 
Holothurians  are  primitive,  and,  as  is  certainly  suggested  by  their 
external  appearance,  have  affinities  with  the  supposed  "  worm-like " 
ancestors  of  Echinoderms. 

Again,  some  members  of  the  heterogenous  class  of  Cystoids  are 
extremely  primitive,  but  differ  from  the  Holothurians  in  the  possession 
of  an  aboral  system  of  plates,  alternately  radial  and  interradial.  From 
this  primitive  Cystoidean  stock,  two  branches  diverge.  The  one  leads  > 
to  the  sessile  Cystoids,  Blastoids,  and  Crinoids  (Pelmatozoa),  the  other 
to  the  free  Echinoidea,  Asteroidea,  and  Ophiuroidea.  Of  these  the 
existing  Asteroidea  and  Ophiuroidea  are  late  divergences  from  a  common 
stock. 


[TABLE. 


CONTRASTS  BETWEEN  ECHINODERMS. 


251 


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The  body  is  elongated 
and  worm-like,  with  a  tough 
muscular  skin,  in  which 
limy  plates  are  embedded. 

They  move  partly  by  mus- 
cular writhings,  partly  by 
means  of  the  tube  feet. 

There  is  a  circumoral 
nerve  ring  with  radial 
branches.  Sometimes  there 
are  "ear  sacs." 

The  mouth  surrounded 
by  tentacles  is  at  or  near 
one  pole,  the  anus  at  or 
near  the  other. 

The  circumoral  water 
ring  communicates  with 
the  tentacles  ;  the  madre- 
poric plate  usually  opens 
into  the  body  cavity  ;  the 
tube  feet  are  often  restricted 
and  often  mere  papillae 
without  terminal  discs. 

The  reproductive  organs 
are  branched  tubes  in  the 
body  cavity  ;  they  open 
near  the  base  of  the  wreath 
of  tentacles,  and  do  not 
exhibit  a  five  rayed  ar- 
rangement. 

Larva  —  an  A  uricularia. 

254  CRUSTACEA. 

and  there  in  England  and  Ireland,  and  is  common  on  the 
Continent.  It  is  absent  from  districts  where  the  water 
contains  little  lime.  The  food  is  very  varied — from  roots 
to  water  rats ;  cannibalism  also  occurs.  The  animals  swim 
backwards  by  powerful  tail  strokes,  or  creep  forwards  on 
their  "walking  legs."  Their  life  is  tolerably  secure,  but 
frequent  moultings  occur  which  are  expensive  and  hazardous. 
When  hatched  the  young  are  like  miniature  adults ;  for  a 
time  they  cling  beneath  the  tail  of  the  mother. 

External  Appearance. 

The  head  and  thorax  are  covered  by  a  continuous  (cephalo- 
thoracic)  shield ;  the  abdomen  shows  obviously  distinct 
segments  movable  upon  one  another.  As  indicated  by  the 
appendages,  there  are  three  groups  of  segments  or  metameres 
—five  in  the  head,  eight  in  the  thorax,  six  in  the  abdomen, 
as  well  as  an  unpaired  piece  or  telson  on  which  the  food 
canal  ends.  (According  to  some  authorities  there  are  twelve 
segments  in  the  cephalothorax,  and  seven  in  the  abdomen.) 
Each  of  the  nineteen  segments  bears  a  pair  of  append- 
ages. Among  other  external  characters  may  be  noticed  the 
stalked  movable  eyes,  the  two  pairs  of  feelers,  the  mouth 
with  six  pairs  of  appendages  crowded  round  it,  the  gills 
under  the  side  flaps  of  the  thorax,  and  the  varied  post-oral 
appendages. 

'(i)  The  external  shell  or  cuticle,  composed -of 
various  strata  of  chitin,  coloured  with  pig- 
ments, hardened  with  lime  salts  ; 

The  BODY  WALL    I  (2)  The    ectoderm,    epidermis,    or    hypodermis, 
consists  of: —        1  which  makes  and  remakes  the  cuticle  ; 

(3)  An  internal  connective  tissue  layer  or  dermis, 
with  pigment,  blood  vessels,  and  nerves. 
Internal  to  this  lie  the  muscles. 

Between  the  rings  and  at  the  joints  the  cuticle  contains 
no  lime,  and  is  therefore  pliable.  As  a  sacrificed  product 
of  epidermic  cells,  it  is  dead  and  cannot  expand.  Hence, 
as  long  as  the  animal  continues  to  grow  periodic  moulting 
is  necessary.  The  old  husk  becomes  thinner,  a  new  one  is 
formed  beneath  it,  a  split  occurs  across  the  back  just  behind 
the  shield,  the  animal  withdraws  its  cephalothorax  and  then 
its  abdomen,  and  an  empty  but  complete  shell  is  left  behind. 


APPENDAGES.  255 

The  moulting  is  preceded  by  an  accumulation  of  glycogen 
in  the  tissues,  and  this  is  probably  utilised  in  the  rapid 
growth  which  intervenes  between  the  casting  of  the  old  and 
the  hardening  of  the  new  shell. 

How  thorough  the  ecdysis  or  cuticle  casting  is,  will  be  appreciated 
when  we  notice  that  the  covering  of  the  eyes,  the  hairs  of  the  ears,  the 
lining  of  the  fore  gut  and  hind  gut,  the  gastric  mill,  and  the  tendinous 
inward  prolongations  of  the  cuticle  to  which  some  of  the  muscles  are 
attached  are  all  got  rid  of  and  renewed.  The  moults  occur  in  the 
warm  months,  eight  times  in  the  first  year,  five  times  in  the  second, 
thrice  in  the  third,  after  which  the  male  moults  twice,  the  female  once  a 
year,  till  the  uncertain  limit  of  growth  is  reached.  It  is  not  clearly 
known  in  what  form  the  animals  procure  the  carbonate  of  lime  which  is 
deposited  in  the  chitinous  cuticle,  but  experiments  made  by  Mr.  Irvine 
at  Granton  Marine  Station  proved  that  a  carbonate  of  lime  shell  could 
be  formed  by  crabs  even  when  the  slight  quantity  of  carbonate  of  lime 
in  sea  water  was  replaced  by  the  chloride.  Moulting  is  an  expensive 
and  exhausting  process,  and  great  mortality  is  associated  with  the 
process  itself  or  with  the  defenceless  state  which  follows.  The  process 
is  a  disadvantage  attendant  on  the  advantage  of  armature.  Inequalities 
in  the  legs  are  usually  due  to  losses  sustained  in  combat,  but  these  are 
gradually  repaired  by  new  growth. 

The  surface  of  the  body  bears  hairs  or  bristles  of  various 
kinds.  These  have  their  roots  in  the  epidermis,  and  are 
made  anew  at  each  moult.  There  are  simple  glands  beneath 
the  gill  flaps,  and  on  the  abdomen  of  the  female  there  are 
cement  glands,  the  viscid  secretion  of  which  serves  to  attach 
the  eggs. 

Appendages. 

The  limbs  of  a  Crustacean  usually  exhibit  considerable 
diversity ;  in  different  regions  of  the  body  they  are  adapted 
for  different  work ;  yet  all  have  the  same  typical  structure, 
and  begin  to  develop  in  the  same  way.  In  other  words, 
they  are  serially  homologous  organs,  illustrating  division  of 
labour.  Typically  each  consists  of  a  two-jointed  basal  piece 
(protopodite),  and  two  jointed  branches  rising  from  this — an 
internal  endopodite  and  an  external  eocopodite ;  but  in  many 
the  outer  branch  disappears.  The  protopodite  has  usually 
two  joints,  a  basal  or  proximal  coxopodite,  and  a  distal 
basipodite;  the  five  joints  which  the  endopodite  frequently 
exhibits  are  named  from  below  upwards — ischio-,  mero-, 
carpo-,  pro-,  dactylo-podites — details  of  some  use  in  the 
comparison  and  identification  of  species. 


256 


CRUSTACEA. 


THE   APPENDAGES   OF   THE   CRAYFISH. 


No. 

NAME. 

FUNCTION. 

STRUCTURE. 

i 

Antennules    (pre- 

Tactile,         olfactory, 

Typical. 

oral  ?) 

with     ear     sac     at 

base. 

2 

Antennae    (pre- 

Tactile,    opening     of 

Small  exopodite. 

oral?) 

kidney  at  base. 

3 

Mandibles. 

Masticatory. 

Four  joints,  of  which  three 

form  the  palp  (endopodite 

and  upper  joint  of  protopo- 

"rfx-N 

dite). 

ffi^ 

4 

ist  Maxillae. 

9 

Thin  single  jointed  protopo- 

dite, small  endopodite,  no 

exopodite. 

5 

2nd  Maxillae. 

Produces      respira- 

Thin   protopodite,    filamen- 

tory current. 

tous        endopodite,        the 

"baler"    is    formed    from 

the     epipodite,     probably 

along  with  the  exopodite. 

6 

ist  Maxillipedes 
(foot-jaws). 

9 

Thin  protopodite,  small  en- 
dopodite, large  exopodite. 

7 

2nd  Maxillipedes 

9 

Two-jointed  protopodite,  five- 
jointed     endopodite,    long 

exopodite. 

8 

3rd  Maxillipedes 

Masticatory. 

Two  -jointed       protopodite, 
large    five  -  jointed    endo- 

rt   . 

podite,  slender  exopodite. 

O  oo" 

H" 

9 

Forceps  (clawed). 

Fighting,  seizing. 

No  exopodite.     In  the  claw 
the  last  joint  bites  against  a 

10 

Walking  Legs 

prolongation  of  the  second 

(clawed). 

last. 

ii 

»              j> 

Genital     opening    in 

female. 

12 

» 

Without  claws. 

13 

j? 

Genital     opening    in 

male. 

.4 

Modified      swim- 
merets  in   male, 

(  Serve    in    the    male 
Xas   canals   for   the 

Protopodite  and  endopodite 
form  a  canal  ;  no  exopodite. 

in    female   rudi- 

seminal  fluid. 

mentary. 

a 

15 

Modified      swim- 

All  the  three  parts. 

£ 

merets  in  male, 

0(0" 

normal  in  female. 

,£> 

16 

Swimmerets. 

C  Move    slightly    like 

n 

^1 

T7 

?  } 

oars,     and     carry 

j, 

18 

» 

I      the    eggs     in     the 

» 

MUSCULAR  SYSTEM.  257 

We  can  fancy  how  the  Crustacean  form  of  limb  might  arise  from  the 
biramose  parapodium  of  a  Polychsete.  The  hard  chitinous  cuticle  of 
the  Arthropod  makes  joints  possible  and  necessary.  In  regard  to  the 
foregoing  list  it  should  be  noted  that  the  eye  stalks  are  no  longer 
included  in  the  series  since  their  development  is  not  like  that  of  the 
limbs,  and,  moreover,  that  though  the  two  pairs  of  antennae  lie  far  in 
front  of  the  mouth,  it  is  possible  that  they  were  originally  post-oral. 
With  many  of  the  thoracic  appendages,  gills,  plate-like  epipodites,  and 
setae  are  associated. 

It  is  interesting  to  connect  the  structure  of  the  appendages  with  their 
functions.  Thus  it  may  be  seen  that  the  great  paddles  are  fully  spread 
when  the  crayfish  drives  itself  backwards  with  a  stroke  of  its  tail,  while 
in  straightening  again  the  paddles  are  drawn  inwards,  and  the  outer 
joint  of  the  exopodite  bends  in  such  a  way  that  the  friction  is  reduced. 

It  is  likely  that  some  of  the  crowded  mouth  parts,  e.g.,  the  first 
maxilke,  are  almost  functionless.  The  hard  toothed  knob  which  forms 
the  greater  part  of  the  mandible  is  obviously  well  adapted  to  its 
crushing  work. 

In  connection  with  the  skeleton,  the  student  should  also 
notice  the  beak  (rostrum}  projecting  between  the  eyes  ;  the 
triangular  area  (epistoma)  in  front  of  the  mouth,  and  the 
slight  upper  and  lower  lips  ;  how  the  gills  are  protected  by 
lateral  flaps  of  the  body  wall ;  that  each  posterior  segment 
consists  of  a  dorsal  arch  (tergum\  side  flaps  (pleura),  a 
ventral  bar  (sternum],  while  the  little  piece  between  the 
pleura  and  the  socket  of  the  limb  is  dignified  by  the  name 
of  epimeron.  The  hindmost  piece  (telson)  on  which  the 
food  canal  ends  ventrally  is  regarded  by  some  as  a  distinct 
segment,  by  others  as  an  unpaired  appendage.  The  most 
difficult  fact  to  understand  clearly,  is  that  the  cuticle  of 
certain  mouth  parts  (e.g.,  the  mandibles),  and  of  the  ventral 
region  of  the  thorax,  is  folded  inwards,  forming  chitinous 
"  tendons  "  or  insertions  for  muscles,  protecting  the  ventral 
nerve  cord  and  venous  blood  sinus,  and  above  all,  con- 
stituting the  complex,  apparently,  but  not  really,  internal, 
"  endophragmal  "  skeleton  of  the  thorax. 

Muscular  System. 

The  muscles  are  white  bundles  of  fibres.  On  minute 
examination  these  show  clearly  that  transverse  striping 
which  is  always  well-marked  in  rapidly  contracting  ele- 
ments. They  are  inserted  on  the  inner  surface  of  the 
cuticle,  or  on  its  internal  foldings  (apodemata).  The  most 
important  sets  are — (i)  the  dorsal  extensors  or  straighteners 

17 


FIG.  82. — Appendages  of  Norway  Lobster. 

Ex,  Exopodite  ;  En,  endopodite  ;  protopodite  dark  throughput ;  Ep,  epipodite. 
i.  Antennule,  E,  position  of  ear  ;  2.  antenna,  K,  opening  of  kidney  ;  3.  mandible, 
P,  palp  ;  4.  first  maxilla  ;  5.  second  maxilla,  B,  baler ;  6.  first  maxillipede  ;  7.  second 
maxillipede  ;  8.  third  maxillipede — the  basal  joint  of  protopodite  is  called  coxopodite, 
the  next  basipodite  ;  the  five  joints  of  the  endopodite  are  called, — ischiopodite  (z) ; 
meropodite  (;;z)  ;  carpopodite  (c) ;  propodite  (/)  ;  dactyloppdite  (ct) ;  9.  forceps  ; 
(7)  coxopodite,  (6)  basipodite  ;  10-13.  walking  legs  ;  14.  modified  male  appendage  ; 
15-18.  small  swimmerets  ;  19.  large  paddles. 


NERVOUS  AND  SENSORY  SYSTEMS.  259 

of  the  tail ;  (2)  the  twisted  ventral  muscles,  most  of  which 
are  flexors  or  benders  of  the  tail,  which  have  harder  work, 
and  are  much  larger  than  their  opponents  ;  (3)  those  mov- 
ing the  appendages ;  (4)  the  bands  which  work  the  gastric 
mill. 

Nervous  System. 

The  supra-oesophageal  nerve  centres  or  ganglia,  forming 
the  brain,  have  been  shunted  far  forward  by  the  growth  of 
the  pre-oral  region.  We  thus  understand  how  the  nerve 
ring  round  the  gullet,  connecting  the  brain  with  the  ventral 
chain  of  twelve  paired  ganglia,  is  so  wide. 

The  dorsal  or  supra-oesophageal  ganglia  are  three  lobed, 
and  give  off  nerves  to  eyes,  antennules,  antennae,  and 
food  canal,  besides  the  commissures  to  the  sub-cesophageal 
centres. 

The  sub-cesophageal  ganglia,  the  first  and  largest  of  the 
ventral  dozen,  innervate  the  six  pairs  of  appendages  about 
the  mouth.  There  are  other  five  ganglia  in  the  thorax,  and 
six  more  in  the  abdomen. 

Though  the  ganglia  of  ^ach  pair  are  in  contact,  the  ventral 
chain  is  double,  and  at  one  place,  between  the  4th  and  5th 
ganglia,  an  artery  (sternal)  passes  between  the  two  halves 
of  the  cord.  From  each  pair  of  ganglia  nerves  are  given 
off  to  appendages  and  muscles,  and  apart  from  the  brain, 
these  minor  centres  are  able  to  control  the  individual  move- 
ments of  the  limbs.  In  the  thoracic  region  the  cord  is  well 
protected  by  the  cuticular  archway  already  referred  to. 

From  the  brain,  and  from  the  commissure  between  it  and 
the  sub-cesophageal  ganglia,  nerves  are  given  off  to  the  food 
canal,  forming  a  complex  visceral  or  stomato-gastric  system: 
Similarly  from  the  last  ganglia  of  the  ventral  chain,  nerves 
go  to  the  hind  gut.  If  the  brain  be  regarded  as  the  fusion 
of  two  pairs  of  ganglia,  as  the  development  suggests,  and 
the  sub-cesophageal  as  composed  of  six  fused  pairs,  then 
these,  along  with  the  eleven  other  pairs  of  the  ventral  chain, 
give  a  total  of  nineteen  nerve  centres, — a  pair  for  each  pair 
of  appendages. 

Sensory  System. 

A  skin  clothed  with  chitin  is  not  likely  to  be  in 
itself  very  sensitive,  but  some  of  the  setae  are.  These  are 


260 


CRUSTACEA. 


not   mere   outgrowths  of  the  cuticle,   but  are  continuous 
with  the  living  epidermis   beneath,  and  though  some  are 
only    fringes,    both    experiments    and 
histological     examination     show    that 
others  are  tactile. 

On  the  under  surface  of  the  outer 
fork  of  the  antennules,  there  are  special 
innervated  setae  which  have  been 
credited  with  a  smelling  function. 

Other  likewise  specialised  hairs  have 
sunk  into  a  sac  at  the  base  of  the 
antennules,  and  are  spoken  of  as 
auditory.  The  sac  opens  by  a  bristle- 
guarded  slit  on  the  inner  upper  corner 
of  the  expanded  basal  joint,  and  con- 
tains a  gelatinous  fluid  and  small 
"otoliths"  which  seem  to  be  foreign 
particles.  This  "ear"  is  somehow 
connected  with  directing  the  animal's 
movements.  In  some  other  Crus- 
taceans, the  auditory  hairs  are  lodged 
in  an  open  depression ;  this  has  be- 
come an  open  sac  in  the  Crayfish,  a 
closed  bag  in  the  Crab. 

Small  hairs  on  the  upper  lip  of  the 
mouth  have  been  said  to  have  a  tasting 
function,  but  imagination  is  apt  to 
help  conclusions  as  to  the  precise 
nature  of  the  sensitiveness  of  such 
simple  structures. 


p.r- 


R 

N 


FIG.    83. — A    single 
eye  element  or  omma- 


The  stalked  eyes,  which  used  to  be  tidium  of  the  Lobster- 
regarded  as  appendages,  arise  in  de-  (After_G'  H;  PARKER'} 
velopment  from  what  are  called  "  pro- 
cephalic  lobes "  on  the  head.  They 
are  compound  eyes,  that  is,  they  con- 
sist of  a  multitude  of  elements,  each 
of  which  is  structurally  complete  in 
itself.  On  the  outside  there  is  a 
cuticular  cornea,  divided  into  square  facets,  one  for  each 
of  the  optic  elements.  Then  follows  a  focussing  layer, 
corresponding  to  the  epidermis,  consisting  of  many  crystalline 


c,  Cornea  ;  c.h,  corneal 
hypodermis ;  cp,  cap  of 
crystalline  cone ;  co,  crys- 
talline cone  ;  d.r,  distal 
retinula  elements ;  p.r, 
proximal  retinula  ele- 
ments ;  R,  rhabdome ; 
N,  nerve  fibre. 


ALIMENTARY  SYSTEM.  261 

cones.  Each  crystalline  cone  is  composed  of  four  crystalline 
cells,  which  taper  internally.  Internal  to  each  crystalline 
cone  lie  a  number  of  retinula  cells.  The  innermost  of 
these  surround  four  little  red  rods,  united  closely  into  what 
is  called  a  rhabdome.  At  its  base,  a  nerve  fibre  enters 
from  the  adjacent  optic  ganglion  at  the  end  of  the  optic 
nerve.  Thus  each  element  consists  of  corneal  facet,  crystal- 
line cone,  and  retinula,  and  the  retinula  consists  of  internal 
rhabdome,  and  external  retinula  cells.  Between  the  in- 
dividual optic  elements,  lie  some  pigment  cells.  Opinions 
differ  as  to  the  visual  powers  of  Crustaceans,  but  their  eyes 
are  able  to  form  images  of  external  objects,  and  these 
images  are  erect,  not  inverted  as  in  the  eyes  of  Vertebrates. 

Alimentary  System. 

The  food  canal  consists  of  three  distinct  parts,  a  fore 
gut  or  stomatodaeum  developed  by  an  intucking  from  the 
anterior  end  of  the  embryo,  a  hind  gut  or  proctodasum 
similarly  invaginated  from  the  posterior  end,  and  a  mid  gut 
or  mesenteron  which  represents  the  original  cavity  of  the 
gastrula. 

The  mouth  has  been  shunted  backwards  from  the  anterior 
end  of  the  body,  so  that  the  antennules  and  antennae  lie  far 
in  front  of  it.  The  fore  gut,  which  is  lined  by  a  chitinous 
cuticle,  includes  a  short  gullet,  on  the  walls  of  which  there 
are  small  glands  hypothetically  called  "  salivary,"  and  a 
capacious  gizzard,  or  "  stomach,"  which  is  distinctly  divided 
into  two  regions.  In  the  anterior  (cardiac)  region  there  is 
a  complex  mill ;  in  the  posterior  (pyloric)  region  there  is  a 
sieve  of  numerous  hairs.  The  mill  is  very  complex,  but 
there  is  no  difficulty  in  dissecting  it  carefully,  nor  in  seeing 
at  once  that  there  are  supporting  "  ossicles  "  on  the  walls 
with  external  muscles  attached  to  them,  and  internally 
projecting  teeth  which  clash  together  and  grind  the  food. 
Three  of  the  teeth  are  conspicuous  ;  a  median  dorsal  tooth 
is  brought  into  contact  with  two  large  laterals.  On  each 
side  of  the  anterior  part  of  the  gizzard,  there  are  two  limy 
discs  or  gastroliths,  which  are  broken  up  before  moulting, 
and  though  quite  inadequate  to  supply  sufficient  carbonate 
of  lime  for  the  new  skeleton,  seem  to  have  some  relation  to 
this  process.  The  occurrence  of  chitinous  cuticle,  hairs, 


262 


CRUSTACEA. 


teeth,  and  gastroliths  in  the  "  stomach,"  is  intelligible  when 
the  origin  of  the  fore  gut  is  remembered,  and  so  is  the  dis- 
mantled state  of  this  region  when  moulting  occurs. 

The  mid  gut  is  very  short,  but  it  is  the  digestive  and 
absorptive  region.  From  it,  there  grows  out  on  each  side  a 
large  digestive  gland  with  two  ducts.  This  gland  is  more 
than  a  "liver,"  more  even  than  a  " hepatopancreas."  It 
absorbs  peptones  and  sugar,  and  makes  glycogen  like  the 
Vertebrate  liver,  its  digestive  juices  are  comparable  to  those 
of  the  pancreas  and  the  stomach  of  higher  animals.  The 
hind  gut  is  long  and  straight.  It  is  lined  by  a  chitinous 


FIG.  84. — Longitudinal  Section  of  Lobster,  showing  some 
of  the  organs. 

H,  Heart ;  A.O,  ophthalmic  artery  ;  a.a,  antennary  artery  ;  a.h, 
hepatic  artery  ;  St,  sternal  artery ;  S.A,  superior  abdominal  artery  ; 
M.G,  mid  gut :  D.G,  digestive  gland  ;  H.G,  hind  gut ;  Ex,  exten- 
sor muscles  of  the  tail ;  Fl,  flexor  muscles  of  the  tail ;  I. A,  inferior 
abdominal  artery  ;  G.  gizzard  ;  C,  cerebral  ganglia. 

cuticle,  as  its  origin  suggests.      There  are  a  few  minute 
glands  on  its  walls. 

Body  Cam'fy. 

The  space  between  the  gut  and  the  body  wall  is  for  the 
most  part  filled  up  by  the  muscles  and  the  organs,  but 
there  are  interspaces  left  which  contain  a  fluid  with  amoeboid 
cells.  These  interspaces  seem  to  represent  enlarged  blood 
sinuses  (a  haemoccele)  rather  than  a  true  body  cavity  or 


VASCULAR  AND  RESPIRATORY  SYSTEMS.        263 

ccelome.      One  of  the   spaces  forms  the  pericardium,   or 
chamber  in  which  the  heart  lies. 

Vascular  System. 

Within  this  non-muscular  pericardium,  and  moored  to  it 
by  thin  muscular  strands,  lies  the  six-sided  heart,  which 
receives  pure  blood  from  the  gills  (via  the  pericardium) 
and  drives  it  to  the  body. 

The  arterial  system  is  well  developed.  Anteriorly,  the 
heart  gives  off  a  median  artery  to  the  eyes  and  antennules, 
a  pair  of  arteries  to  the  antennae,  and  a  pair  to  the  digestive 
gland.  Posteriorly,  there  issues  a  single  vessel,  which  at 
once  divides  into  a  superior  abdominal,  running  along  the 
dorsal  surface,  anoT  a  sternal  which  goes  vertically  through 
the  body.  This  sternal  passes  between  the  connectives 
joining  the  4th  and  5th  ventral  ganglia,  and  then  divides 
into  an  anterior  and  posterior  abdominal  branch.  All  these 
arteries  are  continued  into  capillaries. 

From  the  tissues  the  venous  blood  is  gathered  up  in 
channels,  which  are  not  sufficiently  defined  to  be  called  veins. 
It  is  collected  in  a  ventral  venous  sinus,  and  passes  into  the 
gills.  Thence  purified  by  exposure  on  the  water-washed  sur- 
faces, it  returns  by  six  vessels  on  each  side  to  the  pericardium. 
From  this  it  enters  the  heart  by  six  large  and  several  smaller 
apertures,  which  admit  of  entrance  but  not  of  exit. 

The  blood  contains  amoeboid  cells,  and  the  fluid  or  plasma 
includes  a  respiratory  pigment,  haemocyanin  (bluish  when 
oxidised,  colourless  when  deoxidised),  and  a  lipochrome 
pigment,  called  tetronerythrin.  Both  of  these  are  common 
in  other  Crustaceans. 

Respiratory  System. 

Twenty  gills — vascular  outgrowths  of  the  body  wall — lie 
on  each  side  of  the  thorax,  sheltered  by  the  flaps  of  the 
shield.  A  current  of  water  from  behind  forwards  is  kept 
up  by  the  activity  of  the  baling  portion,  or  scaphognathite, 
of  the  second  maxilla.  Venous  blood  enters  the  gills  from 
the  ventral  sinus,  and  purified  blood  leaves  them  by  the  six 
channels  leading  to  the  pericardium. 

Observed  superficially,  the  gills  look  somewhat  like 
feathers  with  plump  barbs,  but  their  structure  is  much  more 


264  CRUSTACEA. 

complex.  The  most  important  fact  is  that  they  present  a 
large  surface  to  the  purifying  water,  while  both  the  stem  and 
the  filaments  which  spring  from  it  contain  an  outer  canal 
continuous  with  the  venous  sinus,  and  an  inner  canal  com- 
municating with  the  channels  which  lead  back  to  the 
pericardium  and  heart. 

Three  sets  of  gills  are  distinguishable.  To  the  basal  joints  of  the 
six  appendages  from  the  second  maxillipede  to  the  fourth  large  limb 
inclusive,  the  podobranchs  are  attached.  They  come  off  with  the 
appendages  when  these  are  pulled  carefully  away,  and  each  of  them 
bears  in  addition  to  the  feathery  portion  a  simple  lamina  or  epipodite. 
The  membranes  between  the  basal  joints  of  the  appendages  and  the 
body,  from  the  second  maxillipede  to  the  fourth  large  limb  inclusive, 
bear  a  second  set,  the  arthrobranchs ,  which  have  no  epipodites.  In 
connection  with  the  second  maxillipede  there  is  a  single  arthrobranch, 
in  connection  with  each  of  the  five  following  appendages  there  are  two, 
so  that  there  are  eleven  arthrobranchs  altogether.  There  remain  three 
pleurobranckSy  one  on  the  epimeron  of  the  fifth  large  limb,  and  two 
others  quite  rudimentary  on  the  two  preceding  segments.  The  bases 
of  the  podobranchs  bear  long  setae. 

In  Nephrops  and  the  common  lobster  the  number  and  arrangement  of 
the  gills  is  slightly  different. 

Excretory  System. 

A  kidney  or  "  green  gland  "  lies  behind  the  base  of  each 
antenna,  and  its  opening  is  marked  by  a  conspicuous  knob 
on  the  basal  joint  of  that  appendage.  Each  kidney  consists 
of  a  dorsal  sac  communicating  with  the  exterior,  and  of  a 
ventral  coiled  tube  which  forms  the  proper  renal  organ. 
The  latter  is  supplied  with  blood  from  the  antennary  and 
abdominal  arteries,  and  forms  as  waste  products  uric  acid 
and  greenish  guanin.  Each  kidney  may  be  regarded  as 
homologous  with  a  nephridium. 

In  Palcemon,  the  kidney  is  connected  by  a  glandular  duct 
with  a  delicate  dorsal  "nephro-peritoneal  sac,"  possibly  coelo- 
mic.  more  probably  an  enlargement  of  the  nephridial  system. 

The  crayfish  has  also,  near  the  gills,  small  branchial  glands  which 
excrete  carcinuric  acid  from  the  blood,  and  also  help  in  phagocytosis, 
that  important  process  in  which  wandering  amoeboid  cells  resist  infection 
and  help  to  repair  injuries  (cf.  possible  function  of  thymus  in  Fishes). 

Reproductive  Organs. 

The  male  crayfish  is  distinguished  from  the  female  by  his 
slightly  slimmer  build,  and  by  the  copulatory  modification 


REPRODUCTIVE   ORGANS.  265 

of  the  first  two  pairs  of  abdominal  appendages.  In  both 
sexes  the  gonads  are  three  lobed,  and  communicate  with  the 
exterior  by  paired  ducts. 

The  testes  consist  of  two  anterior  lobes  lying  beneath  and 
in  front  of  the  heart,  and  of  a  median  lobe  extending  back- 
wards. Each  lobe  consists  of  many  tubules  within  which 
the  spermatozoa  develop.  From  the  junction  of  each  of  the 
anterior  lobes  with  the  median  lobe,  a  genital  duct  or  vas 
deferens  is  given  off.  This  has  a  long  coiled  course,  is  in 
part  glandular,  and  ends  in  a  short  muscular  portion  opening 
on  the  last  thoracic  limb.  The  spermatozoa  are  at  first  disc- 
like  cells,  they  give  off  on  all  sides  long  pointed  processes 
like  those  of  a  Heliozoon,  and  remain  very  sluggish.  The 


FIG.  85. — Female  reproductive  organs  of  Crayfish. 
(After  SUCKOW.) 

ov,  Ovaries  ;  ov' ,  fused  posterior  part ;  od,  oviduct :  vu,  female 
aperture  on  the  second  walking  leg. 

seminal  fluid  is  milky  in  appearance,  and  becomes  thicker 
in  its  passage  through  the  genital  ducts.  It  is  possible  that 
the  genital  ducts  represent  modified  nephridia,  and  that  the 
cavities  of  the  gonads  are  coelomic. 

The  ovaries  are  like  the  testes,  but  more  compact.  The 
eggs  are  liberated  into  the  cavity  of  the  organ,  and  pass  out 
by  short  thick  oviducts  opening  on  the  second  pair  of 
walking  legs.  As  they  are  laid  they  seem  to  be  coated  with 
the  secretion  of  the  cement  glands  of  the  abdomen,  and  the 


266 


CRUSTACEA. 


mother  keeps  her  tail  bent  till  the  eggs  are  glued  to  the 
small  swimmerets. 

Before  this,  however,  sexual  union  has  occurred.  The 
male  seizes  the  female  with  his  great  claws,  throws  her  on 
her  back,  and  deposits  the  seminal  fluid  on  the  ventral 
surface  of  the  abdomen.  The  fluid  flows  down  the  canal 
formed  by  his  first  abdominal  appendages,  and  these  seem 
to  be  kept  clear  by  the  movements  of  the  next  pair,  which 


FIG.  86. — Section  through  the  egg  of  Astacus  after  the  com- 
pletion of  segmentation.     (After  REICHENBACH.) 

st.  Stalk  of  the  egg  ;  ch,  chorion  envelope  ;  bl,  peripheral  blastoderm, 
within  which  are  the  yolk  pyramids  (dark). 

are  also  modified.  On  the  abdomen  of  the  female  the 
agglutinated  spermatozoa  doubtless  remain  until  the  eggs 
are  laid,  when  fertilisation  in  the  strict  sense  is  achieved. 

The  Development  has  been  very  fully  worked  out,  and  is  of  interest 
in  being  direct,  without  the  metamorphosis  so   common   among   the 


DEVELOPMENT. 


267 


Arthropoda.  The  spherical  ovum  is  surrounded  by  a  cuticular  vitelline 
membrane,  and  contains  a  considerable  quantity  of  yolk.  After  ferti- 
lisation the  segmentation  nucleus  divides  in  the  usual  way  into  2,  4, 
8,  and  so  on,  but  this  nuclear  division  is  not  followed  by  division  of  the 
plasma.  Eventually  the  nuclei,  each  surrounded  by  a  small  amount  of 
protoplasm,  approach  the  surface  of  the  egg  and  arrange  themselves 
regularly  round  it.  The  peripheral  protoplasm  then  segments  round 
these  nuclei,  and  thus  we  have  a  central  core  of  unsegmented  yolk 
enveloped  by  a  peripheral  ring  of  rapidly  dividing  cells.  In  the  central 
yolk  free  nuclei  may  be  frequently  found,  these  are  the  so-called  yolk 


pd 


S.S 


FIG.  87. — Longitudinal  section  of  later  embryo  of 
Astacus.     (After  REICHENBACH.) 

ec,  Kctoderm  ;  7/2,  mesoderm  cells  ;  cg>  cerebral  ganglia  ;  st>  stoma- 
todaeum  ;  A,  anus :  T,  telson ;  g,  ventral  ganglia  ;  ss,  sternal  sinus  ; 
4dt  proctodffium  ;  /t,  heart ;  7/z^",  mid  gut ;  yolk  pyramids  (dark). 

nuclei.      Such  a  type  of  segmentation  is  called  peripheral  or  centro- 
lecithal,  and  is  very  characteristic  of  Arthropod  eggs. 

Over  a  particular  region  of  the  segmented  egg,  known  as  the  "  ventral 
plate,"  the  cells  begin  to  thicken  ;  at  this  region  an  invagination  occurs, 
which  represents  the  gastrula.  At  the  anterior  lip  of  the  blastopore  the 
mesoderm  appears,  being  many  celled  from  the  first.  Soon  the  blasto- 
pore closes ;  the  cavity  of  the  gastrula  thus  becomes  a  closed  sac — the 
future  mid  gut.  The  cells  of  this  archenteron  take  up  the  core  of  yolk 


FIG.  88. — Embryo  of  Crayfish,  flattened  out,  with  removal  of  yolk, 
magnified  about  40  times.     (After  REICHENBACH.) 

Note  rudiments  of  eyes  and  appendages,  and  in  the  middle  line  the  nervous  system. 


SYSTEMATIC  SURVEY  OF  CRUSTACEA. 


269 


into  themselves  in  a  way  which  early  suggests  their  future  digestive 
function.  On  the  surface  of  the  egg  there  have  already  appeared 
ectodermic  thickenings — the  so-called  eye  folds, — rudiments  of  the 
appendages,  and  of  the  thoracic  and  abdominal  regions. 

In  the  later  stages  invaginations  of  the  ectoderm  form  the  fore  and 
hind  gut,  which  grow  inward  from  opposite  ends  to  meet  the  endoder- 
mic  mid  gut,  also  the  ear  sac  and  the  green  glands.  The  gills  are 
formed  in  great  part  from  ectodermic  outgrowths  or  evaginations. 
From  the  mid  gut  the  digestive  gland  is  budded  out.  The  heart,  the 
blood  vessels,  blood,  and  muscles  are  due  to  the  mesoderm. 

As  usual,  the  nervous  system  arises  from  an  ectodermic  thickening. 
The  eye  arises  partly  from  the  optic  ganglia  of  the  "brain,"  partly 
from  the  "eye  folds,"  and  partly  from  the  epidermis. 

When  the  young  crayfish  are  hatched  from  the  egg  shells,  they  still 
cling  to  these,  and  thereby  to  the  swimmerets  of  the  mother.  In  most 
respects  they  are  miniature  adults,  but  the  cephalothorax  is  convex  and 
relatively  large,  the  rostrum  is  bent  down  between  the  eyes,  the  tips  of 
the  claws  are  incurved  and  serve  for  firm  attachment,  and  there  are  other 
slight  differences.  The  noteworthy  fact  is  that  the  development  is  com- 
pleted wilhin  the  egg  case,  and  that  it  is  continuous  without  metamor- 
phosis. (The  shortened  life  history  of  the  crayfish  is  interesting  in 
relation  to  its  fresh  water  habitat,  where  the  risks  of  being  swept  away 
by  currents  are  obviously  great ;  but  it  must  also  be  remembered  that 
the  tendency  to  abbreviate  development  is  a  general  one.N  There  is 
some  maternal  care  in  the  crayfish,  for  the  young  are  said  sometimes  to 
return  to  the  mother  after  a  short  exploration  on  their  own  account. 

SYSTEMATIC  SURVEY  OF  THE  CLASS  CRUSTACEA. 


(i)  Entomostraca,  lower  forms. 

They  are  usually  small  and 
simple. 

The  number  of  segments  and  ap- 
pendages is  very  variable. 

The  larva  is  generally  hatched  as  a 

simple  unsegmented  Nauplius. 
There  is  no  gastric  mill. 


(2)  Malacostraca,  higher  forms. 
They  are  usually  larger  and  more 

complex. 
The  head  consists  of  5,  the  thorax 

of  8,   the  abdomen  of  6  (7  in 

Leptostraca)  segments. 
The  larva  is  usually  higher  than  a 

Nauplius. 
There  is  a  gastric  mill. 


{A pus,     Bronchi- 
pus,  and  Artemia 
(brine-shrimps), 
Daph  nia^Moina, 
Polyphemus. 

2.  Ostracoda,  Cypris,  Cypridina. 

3.  Copepoda,     Cyclops,    Arguhts, 

many  parasites. 

4.  Cirripedia,  acorn  shells  and  bar- 

nacles,    e.g.,     Balamts     and 
Lepas. 


Leptostraca,  e.g. 
Arthrostraca, 

Thoracostraca, 


Nebalia. 
Amphipods 

(sand     hoppers, 

&c.). 
Isopods       (wood 

lice,  &c.). 
Cuma. 
Squilla. 
Mysis. 
Shrimp,     lobster, 

crayfish,  crab. 


270  CRUSTACEA. 

FIRST  SUB-CLASS.     ENTOMOSTRACA. 
These  are  the  more  primitive  Crustaceans,  often  small 
and  simple,  with  a  variable  number  of  segments  and  append- 
ages.    The  newly  hatched  larva  is  usually  a  Nauplius.     The 
adult  may  retain  the  unpaired  simple  frontal  eye,  which  is 
always  found  in  the  Nauplius,  and  has  no  gastric  mill. 
Order  i.  Phyllopoda. 
Order  2.  Ostracoda. 
Order  3.  Copepoda. 
Order  4.  Cirripedia. 

Order  I.  Phyllopoda.     In  these  at  least  four  pairs  of  swimming  feet 
bear   respiratory  plates.     The  body  is  generally  well  segmented, 
and  is  protected  by  a  shield-like  or  bivalve  shell.     The  mandibles 
are  without  palps,  and  the  maxillse  are  rudimentary. 
(a)  Branchiopoda.      The  body  has  numerous  segments  and  (10-20 
or  more)  appendages  with  respiratory  plates.     The  shell  is 
rarely  absent,  usually  shield-like  or  bivalved.     The  heart  is  a 
long  dorsal  vessel  with  numerous  openings.     The  eggs  are  able 
to  survive  prolonged  desiccation  in  the  mud. 

Branchipus,  a  beautifully  coloured  fresh  water  form,  with 

hardly  any  shell. 

Artemia.  Brine  shrimps.  Periodically  parthenogenetic. 
By  gradually  changing  the  salinity  of  the  water, 
Schmankewitsch  was  able,  in  the  course  of  several 
generations,  to  modify  A.  salina  into  A.  milhlenhausii^ 
and  vice  versa.  Artemia  fertilis  is  one  of  the  four 
animals  known  to  occur  in  the  dense  waters  of  Salt  Lake. 
Limnadia^  with  bivalve  shell.  Periodically  parthenogenetic. 
A  mollusc-like  bivalve  shell  is  still  more  marked  in 
Estheria. 

Apus,  a  fresh  water  form  with  a  large  dorsal  shield. 
Periodically  parthenogenetic.  One  species  herma- 
phrodite. 

Of  these  Apus  is  certainly  the  most  interesting.  It  is  over  an  inch  in 
length,  and  therefore  a  giant  among  Entomostraca.  It  has  an 
almost  world  wide  distribution.  "It  possesses  peculiarities  of 
organisation  which  mark  it  out  as  an  archaic  form,  probably 
standing  nearer  to  the  extinct  ancestors  of  the  Crustacea  than 
almost  any  other  living  member  of  the  group."  The  appendages 
are  very  numerous  and  mostly  leaf-like.  They  may  be  regarded 
as  representing  a  primitive  type  of  Crustacean  limb.  Professor 
Ray  Lankester  enumerates  them  as  follows  : — 

fi.  Antenna. 
Pre-oral.          -!  2.  Second   antenna.     (This   is   sometimes    absent,   and 

apparently  always  in  certain  species. ) 
^3.  Mandible. 
Oral.  -[  4.  Maxilla. 

( 5.  Maxillipede. 


CLASSIFICATION  OF  CRUSTACEA.  271 

(6.  First  thoracic  foot  (leg-like). 

Thoracic  |  7-16.  Other  ten  thoracic  feet  (swimmers). 

(Pre-genital).i  The   i6th  in  the  female  carries  an  egg  sac  or  brood 
t     chamber.    There  are  eleven  thoracic  rings  on  the  body. 
Abdominal     f  17-68.  Fifty-two  abdominal  feet,  to  which  there  corres- 
( Post-genital).  \  pond  only  seventeen  rings  on  the  body. 

The  large  dorsal  shield  is  not  attached  to  the  segments  behind  the  one 
bearing  the  maxillipedes.  Many  of  the  thin  limbs  doubtless 
function  as  gills.  The  genital  apertures  are  on  the  i6th  append- 
ages. The  anus  is  on  the  last  segment  of  the  body. 
There  is  a  pair  of  ventral  ganglia  to  each  pair  of  limbs  ;  the  ventral 
nerve  cords  are  widely  apart ;  and  the  cephalic  ganglion  is 
remarkably  isolated.  Professor  Ray  Lankester  called  this 
cephalic  ganglion  an  "archi-cerebrum,"  to  emphasise  its  pre- 
oral  position  and  its  distinctness  from  the  posterior  ganglia. 
Subsequent  research  has  shown,  however,  that  in  Apus,  as  in 
other  Crustaceans,  the  cephalic  ganglion  is  a  "  syn-cerebrum, 
i.e.,  it  is  composed  of  pre-oral  ganglia  fused  with  post-oral  ganglia 
which  have  been  shunted  forwards. 

(b)  Cladocera.  Small  laterally  compressed  "water  fleas,"  with 
few  and  somewhat  indistinct  segments.  The  shell  is  usually 
bivalved.  The  head  often  projects  freely.  The  second  pair 
of  antennae  are  large,  two-branched,  swimming  appendages, 
and  there  are  4-6  pairs  of  other  swimming  organs.  The 
heart  is  a  little  sac  with  one  pair  of  openings.  An  excretory 
organ  (the  shell  or  maxillary  gland)  opens  in  the  region  of 
the  second  maxillae.  It  is  the  Entomostracan  equivalent  of 
the  antennary  green  gland  of  Malacostraca.  The  males  are 
usually  smaller  and  much  rarer  than  the  females.  The  latter 
have  a  brood  chamber  between  the  shell  and  the  back. 
Within  this  many  broods  are  hatched  throughout  the 
summer.  Periodic  parthenogenesis  (of  the  "  summer  ova") 
is  very  common.  "Winter  eggs,"  which  require  fertilisa- 
tion, are  set  adrift  in  a  part  of  the  shell  modified  to  form  a 
protective  cradle  or  ephippium. 

Daphnia,  Moina,  Sida,  Polyphemus,  Leptodora,  and  many 
other  "  water  fleas  "  are  extraordinarily  abundant  in  fresh 
water,  and  form  part  of  the  food  of  many  fishes.  A  few 
occur  in  brackish  and  salt  water. 

Order  2.  Ostracoda.     Small  Crustaceans,  usually  laterally  compressed, 
with  an  indistinctly  segmented  or  unsegmented  body,  rudimentary 
abdomen,    and    bivalve    shell.      There   are   only   seven   pairs    of 
appendages. 
_,  .   Cypris  (fresh  water),  Cypridina  (marine). 

Order  3.  Copepoda.  Elongated  Crustaceans,  usually  with  distinct  seg- 
ments. There  is  no  dorsal  shell.  There  are  five  pairs  of  biramose 
thoracic  appendages,  but  the  last  may  be  rudimentary  or  absent. 
The  abdomen  is  without  limbs,  and  of  its  five  segments  the  first  two 
are  sometimes  united.  The  females  carry  the  eggs  in  external 
ovisacs.  Many  are  ecto-parasitic,  especially  on  fishes  ("fish  lice  ") 


272  CRUSTACEA. 

and  are  often  very  degenerate.     The  free  living  Copepods  form  an 
important  part  of  the  food  supply  of  fishes. 

->_  Cyclops,  free  and  exceedingly  prolific  in  fresh  water.     Cetochilus 
free  and  abundant  in  the  sea. 

Sapphirina,  a  broad  flat  marine  form  about  quarter  of  an  inch 
long,  occasionally  parasitic.  The  male  surpasses  all  animals 
in  the  brilliancy  of  its  "phosphorescent"  colour. 

Chondr acanthus.  As  in  many  other  cases,  the  parasitic  females 
carry  the  pigmy  males  attached  to  their  body. 

Caligus,  a  very  common  genus  of  "  fish  lice." 

Lerncea,  Penella,  etc.  The  adult  females  are  parasitic,  and 
almost  worm-like.  The  males  and  the  young  are  free.  That 
the  males  are  often  free  and  not  degenerate,  while  their 
mates  are  parasitic  and  retrogressive,  may  be  understood  by 
considering  (i)  the  greater  vigour  and  activity  associated 
with  maleness  ;  (2)  the  fact  that  parasitism  affords  safety  and 
abundance  of  nutrition  to  the  females  during  the  reproduc- 
tive period. 

Arguhis,  a  divergent  form  temporarily  parasitic  on  carp,  &c.  It 
has  a  shield-like  cephalothorax  and  a  small  cleft  abdomen. 
A  protrusible  spine  projects  in  front  of  the  blood  sucking 
mouth;  the  mandibles  and  first  maxillae  are  adapted  for 
piercing  ;  the  second  maxilke  or  maxillipedes  for  adhesion. 
There  are  four  pairs  of  two-branched  swimming  appendages. 
There  are  two  large  compound  eyes.  The  female  has  no 
ovisacs ;  the  eggs  are  laid  on  foreign  objects. 

Order  4.    Cirripedia.     Barnacles   and   acorn   shells,    and   some   allied 
degenerate  parasites. 

Marine  Crustaceans,  which  in  adult  life  are  fixed  head  down- 
wards. The  body  is  indistinctly  segmented,  and  is  enveloped 
in  a  fold  of  skin,  usually  with  calcareous  plates.  The  anterior 
antennae  are  involved  in  the  attachment,  the  posterior  pair 
are  rudimentary.  The  oral  appendages  are  small,  and  in 
part  atrophied.  In  most  there  are  six  (or  less  frequently 
four)  pairs  of  two-branched  thoracic  feet,  which  sweep  food 
particles  into  the  depressed  mouth.  The  abdomen  is  rudi- 
mentary. There  is  no  heart.  The  sexes  are  usually  com- 
bined, out  dimorphic  unisexual  forms  also  occur.  The  herma- 
phrodite individuals  occasionally  carry  pigmy  or  "  comple- 
mental "  males.  The  spermatozoa  are  mobile,  which  is 
unusual  among  Crustacea. 

Lepas,  the  ship  barnacle,  is  as  an  adult  attached  to  floating  logs  and 
ship  bottoms.  The  anterior  end  by  which  the  animal  fixes  itself  is 
drawn  out  into  a  long  flexible  stalk,  containing  a  cement  gland,  the 
ovaries,  &c.,  and  involving  in  its  formation  the  first  pair  of  antennae  and 
the  front  lobe  of  the  head.  The  second  antennae  are  lost  in  larval  life. 
The  mouth  region  bears  a  pair  of  small  mandibles  and  two  pairs  of  small 
maxillae,  the  last  pair  united  into  a  lower  lip.  The  thorax  has  six  pairs 
of  two-branched  appendages,  and  from  the  end  of  the  rudimentary 


CLASSIFICATION  OF  CRUSTACEA.  273 

abdomen  a  long  penis  projects.  At  the  base  of  this  lies  the  anus. 
Around  the  body  there  is  a  fold  of  skin,  and  from  this  arise  fiye  calcare- 
ous plates,  an  unpaired  dorsal  carina,  two  scuta  right  and  left  anteriorly, 
two  terga  at  the  free  posterior  end.  The  nervous  system  consists  of  a 
brain,  an  cesophageal  ring,  and  a  ventral  chain  of  five  or  more  ganglia. 
There  is  a  fused  pair  of  rudimentary  eyes.  No  special  circulatory  or 
respiratory  organs  are  known.  Two  excretory  (?)  tubes  lead  from 
(ccelomic)  cavities  to  the  base  of  the  second  maxillee,  and  are  probably 
comparable  with  shell  glands  and  with  nephridia.  There  is  a  complete 
food  canal  and  a  large  digestive  gland.  Beside  the  latter  lie  the  branched 
testes,  whose  vasa  deferentia  unite  in  an  ejaculatory  duct  in  the  penis. 
From  the  much  branched  ovaries  in  the  stalk,  the  oviducts  pass  to  the 
first  thoracic  legs,  where  they  pass  into  a  cement  making  sac,  opening  to 
the  exterior.  The  eggs  are  found  in  flat  cakes  between  the  external  fold 
of  skin  and  the  body. 

The  life  history  is  most  interesting.     Nauplius  larvae  escape  from  the 


FIG.  89. — Acorn  shell  (Balanus  tintinnabulum). 
(After  DARWIN.) 

^,  Tergum  ;  s,  scutum  ;  d,  opening  of  oviduct ;  _/J  mantle  cavity ;  x, 
depressor  muscle  of  tergum;  g,  depressor  muscle  of  scutum;  h,  ovi- 
duct ;  r,  outer  shell  in  section  ;  a,  adductor  muscle  of  scuta. 

egg  cases,  and  after  moulting  several  times  become  like  little  Cyprid 
water  fleas.  The  first  pair  of  appendages  become  suctorial,  and  after  a 
period  of  free  swimming,  the  young  barnacle  settles  down  on  some 
floating  object,  mooring  itself  by  means  of  the  antennary  suckers,  and 
becoming  firmly  glued  by  the  secretion  of  the  cement  glands.  During 
the  settling  and  the  associated  metamorphosis,  the  young  barnacle  fasts, 
living  on  a  store  of  fat  previously  accumulated.  Many  important 
changes  occur,  the  valved  shell  is  developed,  and  the  adult  form  is 
gradually  assumed.  While  the  early  naturalists,  such  as  Gerard  (1597), 
regarded  the  barnacle  as  somehow  connected  with  the  barnacle  goose, 
18 


274 


CRUSTACEA. 


and  zoologists,  before  J.  Vaughan  Thompson's  researches  (1829),  were 
satisfied  with  calling  Cirripedes  divergent  Molluscs,  we  now  know  clearly 
that  they  are  somewhat  degene- 
rate Crustaceans.  We  do  not 
know,  however,  by  what  con- 
stitutional vice,  by  what  fatigue 
after  the  exertions  of  adoles- 
cence, they  are  forced  to  settle 
down  to  sedentary  life. 

The  food  consists  of  small 
animals,  which  are  swept  to 
the  mouth  by  the  waving  of  the 
curled  legs.  Growth  is  some- 
what rapid,  but  the  usual  skin 
casting  is  much  restricted  ex- 
cept in  one  genus.  Neither  the 
valves,  nor  the  uniting  mem- 
branes, nor  the  envelope  of  the 
stalk,  are  moulted,  though  dis- 
integrated portions  may  be  re- 
moved in  flakes  and  renewed 
by  fresh  formations.  In  the 
allied  genus  Scalpellum,  some 
are  like  Lepas^  hermaphrodites, 
without  complementary  males 
(Sc.  balanoides]  ;  others  are 
hermaphrodite,  with  comple- 
mentary males  (Sc.  villosum\ 
&c.  ;  and  others  are  unisexual, 
but  the  males  are  minute  and 
parasitic  (Sc.  regium). 

Balanus,  the  acorn  shell,  en- 
crusts the  rocks  in  great  num- 
bers between  high  and  low 
water  marks.  It  may  be  de- 
scribed, in  Huxley's  graphic 
words,  as  a  crustacean  fixed  by 
its  head,  and  kicking  the  food 
into  its  mouth  with  its  legs. 
The  body  is  surrounded,  as  in 
Lepas,  by  a  fold  of  skin,  which 
forms  a  rampart  of  six  or  more 
calcareous  plates,  and  a  four- 
fold lid,  consisting  of  two  scuta  ,-, 

and  two  terga.     When  covered    FlG'  9°-  -Development  of  Sacculma. 
by   the   tide,    the   animal 


B 


(After  DELAGE.) 
(Not  drawn  to  scale.) 


tide,  the  animal  pro- 
trudes and  retracts  between  the 
valves  of  the  shell  six  pairs  of 
curl-like  thoracic  legs.  The 
structure  of  the  acorn  shell  is 
in  the  main  like  that  of  the  barnacle,  but  there  is  no  stalk. 


A.  Free  swimming  Nauplius,  with  three 
pairs  of  appendages ;  B.  Pupa  stage ;  C. 
Adult  protruding  from  the  tail  of  a  crab. 


CLASSIFICATION  OF  CRUSTACEA.  275 

The  life  history  also  is  similar.  A  Nauplius  is  hatched.  It  has  the 
usual  three  pairs  of  legs,  an  impaired  eye,  and  a  delicate  dorsal  shield. 
It  moults  several  times,  grows  larger,  and  acquires  a  firmer  shield,  a 
longer  spined  tail,  and  stronger  legs.  Then  it  passes  into  a  Cypris  stage, 
with  two  side  eyes,  six  pairs  of  swimming  legs,  a  bivalve  shell,  and  other 
organs.  As  it  exerts  itself  much  but  does  not  feed,  it  is  not  unnatural 
that  it  should  sink  down  as  if  in  fatigue.  It  fixes  itself  by  its  head  and 
antennae,  and  is  glued  by  the  secretion  of  the  cement  gland.  Some  of 
the  structures,  e.g.,  the  bivalve  shell,  are  lost ;  new  structures  appear, 
e.g.,  the  characteristic  Cirriped  legs  and  the  shell.  Throughout  this 
period,  which  Darwin  called  the  "pupa  stage,"  there  is  external 
quiescence,  and  the  young  creature  continues  to  fast.  The  skin  of  the 
pupa  moults  off ;  the  adult  structures  and  habits  are  gradually  assumed. 
At  frequent  periods  of  continued  growth,  the  lining  of  the  shell  and 
the  cuticle  of  the  legs  are  shed.  In  spring  these  glassy  cast  coats  are 
exceedingly  common  in  the  sea.  Acorn  shells  feed  on  small  marine 
animals.  They  fix  themselves  not  to  rocks  only,  but  also  to  shells, 
floating  objects,  and  even  to  whales  and  other  animals. 

Alcippe  and  Cryptophialus  (with  only  three  or  four  pairs  of  feet)  live 
in  the  shells  of  other  Cirripedes  or  of  Molluscs  ;  Proteolepas  is  parasitic 
in  the  mantle  of  other  Cirripedes,  and  like  a  grub. 

On  the  ventral  surface  of  the  abdomen  of  crabs,  Sacculina,  the  most 
degenerate  of  all  parasites,  is  often  found.  Its  complete  history  has 
been  beautifully  worked  out  by  Professor  Delage.  It  is  in  shape  an 
ovoid  sac,  and  is  attached  about  the  middle  of  a  segment.  On  the 
lower  surface  of  the  sac  there  is  a  cloacal  aperture,  opening  into  a  large 
brood  chamber,  usually  distended  with  eggs  contained  in  chitinous 
tubes.  The  brood  chamber  surrounds  the  central  "visceral  mass," 
consisting  of  a  nerve  ganglion,  a  cement  gland  which  secretes  the  egg 
cases,  and  the  hermaphrodite  reproductive  organs ;  of  digestive  or 
vascular  systems  there  is  no  trace.  The  parasite  is  attached  by  a 
peduncle,  dividing  up,  within  the  body  of  the  crab,  into  numerous 
"roots,"  which  have  been  compared  to  the  placenta  of  a  mammalian 
fcetus.  The  "roots"  ramify  within  the  body  of  the  crab,  and  by  them 
the  Sacculina  obtains  nutrition  and  gets  rid  of  its  waste  products  ;  it  is 
therefore  practically,  even  at  this  stage,  an  endoparasite.  The  larvae 
leave  the  brood  chamber  as  Nauplii,  they  moult  rapidly  and  become 
Cyprid  larvae.  These  fix  themselves  by  their  antennae  to  young  crabs, 
at  the  uncalcified  membrane  surrounding  the  base  of  the  large  bristles 
of  the  back  or  appendages.  The  thorax  and  abdomen  are  cast  off 
entirely ;  the  structures  within  the  head  region  contract ;  eyes,  tendons, 
pigment,  the  remaining  yolk,  and  the  carapace  are  all  lost ;  and  a  little 
sac  remains,  which  passes  into  the  interior  of  the  crab.  Eventually  it 
reaches  the  abdomen,  and,  as  it  approaches  maturity,  the  integuments 
of  the  crab  are  dissolved  beneath  it,  and  the  sac-like  body  protrudes  ; 
essentially,  however,  Sacculina  is  always  endoparasitic.  It  appears  to 
live  for  three  years,  during  which  time  the  growth  of  its  host  is  arrested, 
and  no  moult  occurs. 


276  CRUSTACEA. 

SECOND  SUB-CLASS.     MALACOSTRACA. 

These  are  higher  Crustaceans  in  which  the  body  consists 
of  three  regions  with  a  constant  number  of  segments,  five  to 
the  head,  eight  to  the  thorax,  and  six  to  the  abdomen 
(except  in  forms  like  Nebalia,  which  have  seven).  The 
terminal  piece  or  telson  of  the  abdomen  is  regarded  by 
some  zoologists  as  a  distinct  segment.  Apart  from  this 
telson,  and  also  the  segment  next  to  it  in  Nebalia,  all  the 
segments  bear  paired  appendages.  More  or  less  of  the 
thorax  is  fused  to  the  head  region,  and  the  anterior  thoracic 
limbs  are  usually  auxiliary  to  mastication.  Two  compound 
lateral  eyes  and  a  gastric  mill  are  always  present.  There  is 
an  antennary  excretory  gland,  probably  comparable  with 
the  Entomostracan  maxillary  gland.  (The  female  genital 
apertures  are  on  the  third  last  pair  of  thoracic  legs,  the  male 
apertures  on  the  last  pair.  Very  few  are  hatched  in  the 
Nauplius  stage,  many,  however,  at  the  Zoaea  level,  while 
others  have  no  metamorphosis  at  all. 

Legion  i.     Leptostraca.     Nebalia. 

Legion  2.     Arthrostraca,  with   three   orders,  Anisopoda, 

Isopoda,  Amphipoda. 
Legion  3.     Thoracostraca,    with   four   orders,    Cumacea, 

Stomatopoda,  Schizopoda,  Decapoda. 

Legion  I.     Leptostraca. 

Marine  Crustaceans  of  great  systematic  interest,  because  they  retain  in 
many  ways  the  simplicity  of  ancestral  forms,  and  link  Malacostraca  to 
Phyllopods.  The  most  important  genus  is  Nebalia. 

A  bivalve  shell  covers  the  whole  of  the  lank  body,  except  the  last 
four  abdominal  segments  ;  the  head  is  free  from  the  thorax  ;  the  eight 
segments  of  the  thorax  are  free  from  one  another,  and  the  plate-like 
appendages  resemble  those  of  Phyllopods ;  the  abdomen  has  seven 
segments  and  a  telson  with  two  forks  ;  the  elongated  heart  extends  into 
the  abdomen,  and  has  seven  pairs  of  lateral  apertures  or  ostia.  Nebalia 
and  its  congeners  are  probably  related  to  certain  ancient  fossil  forms 
from  Palaeozoic  strata — Hymenocaris,  Ceratiocaris,  &c. 

Legion  2.     Arthrostraca.     (Edriophthalmata,  sessile  eyed.) 

There  is  no  shell  fold  or  shield,  except  in  the  order  Anisopoda.  The 
first  thoracic  segment  (rarely  with  the  addition  of  the  second)  is  fused  to 
the  head,  the  corresponding  appendages  serve  as  maxillipedes,  the  other 
thoracic  segments  (seven  or  six)  are  free.  The  eyes  are  sessile.  The 
heart  is  elongated. 


CLASSIFICATION  OF  CRUSTACEA.  277 

Order  I.  Anisopoda.  The  fusion  of  the  first  two  thoracic  segments 
to  the  head,  the  presence  of  a  cephalothoracic  shield,  and  other 
divergent  features  distinguish  Tanais,  Apsetides,  6°<:.,  from  the 
Isopoda. 

Order  2.  Isopoda.  The  body  is  flattened  from  above  downwards. 
The  first  thoracic  segment  is  fused  to  the  head,  while  the  other 
six  or  seven  are  free,  and  there  is  no  cephalothoracic  shield.  The 
abdomen  is  usually  short,  and  its  appendages,  usually  overlapped 
by  the  first  pair,  are  plate-like,  and  function  in  part  as  respiratory 
organs. 

^=*The  "wood  lice"  (Oniscus,  Porcellio)  are  familiar  animals 
which  lurk  in  damp  places  under  stones  and  bark,  and 
devour  vegetable  refuse.  Some  related  forms  (e.g., 
Armadillo),  which  roll  themselves  up,  are  called  "  pill 
bugs."  In  the  terrestrial  forms  there  is  obviously  a 
departure  from  the  ordinarily  aquatic  habit  of  Crustaceans, 
and  the  exopodites  of  some  of  the  abdominal  appendages 
have  tubular  air  passages. 
Asellus  is  a  very  common  form,  living  in  both  fresh  and  salt 

water. 

Idotea  is  not  uncommon  among  the  shore  rocks. 
The  "gribble"  ( Linmoria  lignorum)  is  a  destructive  marine 

Isopod  which  eats  into  wood. 

Among  the  marine  Cymothoidse  which  are  often  parasitic  on 
fishes,  some,  e.g.,  Cymothoe,  are  remarkable  in  their  sexual 
condition,  for  they  are  hermaphrodites  in  which  the  male 
organs  mature  and  become  functional  when  the  oviducts 
are  still  closed,  while  at  a  later  period  in  life  the  male 
organs  are  lost  and  the  animals  become  functionally 
female. 

The  Bopyridre  infest  the  gill  chambers  of  other  Crustaceans, 
e.g.,  prawns.  The  pigmy  males  are  usually  carried  about 
by  their  mates. 

Among  the  parasitic  Cryptoniscidce,  we  again  find  herma- 
phrodites with  associated  pigmy  males.  In  not  a  few 
cases  they  seriously  affect  the  reproductive  organs  of  their 
male  hosts. 

Many  of  these  Isopods,  like  not  a  few  other  Crustaceans,  are 
extremely  interesting  to  those  who  care  to  think  about  the 
problem  of  sex.  Thus,  to  cite  one  other  instance,  the  males  and 
females  in  the  genus  Gnathia  are  so  unlike,  that  they  have  been 
mistakenly  referred  to  different  sub-families. 

Order  3.  Amphipoda.  The  body  is  laterally  compressed.  In  most 
it  is  only  the  first  thoracic  segment  which  is  fused  to  the  head,  in 
the  "  no-body-crabs  "  (Caprellida),  and  "whale  lice"  (Cyamida}, 
two  segments  are  involved.  The  thoracic  limbs  bear  respiratory 
appendages.  Of  the  six  pairs  of  legs  w7hich  the  abdomen  usually 
bears,  the  anterior  three  are  usually  more  strongly  developed  as 
swimmers,  while  the  posterior  three — directed  backwards — are 
used  in  jumping. 


278  CRUSTACEA. 

Ganunarus  pulex   is  very  common  in  fresh  water.       Other 

species  occur  on  the  sea-shore.      There  also  the  "  Beach 

fleas  "  (Talitrus  and  Orchestia]  are  exceedingly  abundant. 

On  solid  ground  they  move  on   their  sides  in  a  strange 

fashion,  but  they  swim  very  swiftly. 
Hyperia,  Phronima,  and  many  marine  Amphipods,  have  a  habit 

of  living  as  commensals  with  other  animals. 
Caprella,  a  common  marine  gymnast  on   Hydroids,  &c.,  has 

the  trunk  of  the  body  reduced  to  the  quaintest  possible 

minimum. 

Legion  3.     Thoracostraca.     (Podophthalmata,  with  stalked  eyes.) 

Several  or  all  of  the  thoracic  segments  are  fused  to  the  head,  and 
there  is  a  cephalothoracic  shield  overlapping  the  gills.  The  two  eyes 
are  stalked  except  in  Cumacea. 

Order  I.  Cumacea.  The  cephalothoracic  shield  is  small,  and  four  or 
five  thoracic  segments  are  left  uncovered  and  free.  The  eyes  are 
sessile  and  adjacent  or  fused.  There  are  two  pairs  of  maxillipedes. 
The  females  have  no  abdominal  appendages  except  on  the  last 
segment.  The  genera  are  marine,  e.g.,  Cuma  or  Diastylis. 

Order  2.  Stomatopoda.  The  shield  is  still  small  and  does  not  cover 
the  three  posterior  thoracic  segments.  The  body  is  somewhat 
flattened,  the  abdomen  is  very  strong.  Five  anterior  thoracic 
appendages  are  directed  towards  the  mouth,  and  serve  to  catch 
food,  and  to  clamber.  The  five  anterior  abdominal  legs  carry 
feathery  gills,  the  sixth  pair  forming  swimming  paddles.  The 
elongated  heart  extends  into  the  abdomen,  which  also  contains  the 
reproductive  organs.  The  genera  are  marine,  e.g.,  Squilla. 
Order  3.  Schizopoda.  A  delicate  shield  covers  the  whole  of  the 
thorax,  but  there  is  still  some  freedom  as  to  one  or  more  of  the 
posterior  thoracic  segments.  The  eight  thoracic  appendages  are 
uniformly  biramose,  but  the  first  two  may  serve  as  maxillipedes. 
The  abdominal  appendages  of  the  male  are  strongly  developed, 
those  of  the  female  are  weak  except  the  last,  which  in  both  sexes 
form  paddles.  They  are  marine  forms,  e.g.,  Mysis  (without  gills 
on  the  thoracic  legs),  Lophogaster  and  Euphausia  (with  gills  on 
the  thoracic  legs).  The  last  named  starts  in  life  as  a  Nauplius. 
As  an  adult  it  has  luminous  organs  on  the  eye-stalks,  thoracic 
legs,  and  abdominal  segments. 

Order  4.   Decapoda.     The  shield  is  large  and  firm,  and  is  fixed  to  the 

dorsal  surface  of  all  the  thoracic  segments.       Of  the  thoracic 

appendages,  the  first  three  pairs  are  maxillipedes,  the  five  other 

pairs  are  jointed  walking  legs  (whence  the  term  Decapod). 

Sub-order   I.       Macrura.      Abdomen  long.      Homarus  (lobster)  ; 

Nephrops   (Norway   lobster,   sea   crayfish)  ;    Astactis  (fresh 

water  crayfish) ;  Palinurus  (rock  lobster),  whose  larva  was 

long  known  as  the  glass   crab    (Phyllosoma)  ;    Pencsus,   a 

shrimp  which  passes  through   Nauplius,   Zoeea,   and  Mysis 

stages  ;    Lucifer  and  Sergestes  are  also  hatched  at  a  stage 

antecedent  to  the  Zooea  ;    Crangon   vulgaris   (the   British 


GENERAL  NOTES   ON  CRUSTACEANS.  279 

shrimp) ;  PahEtnon,  Pandalus,  Hippolyte  (prawns) ;   Galathea 
(with  the  abdomen  bent  inwards)  ;    Pagurtis,  Etipagurus 
(hermit  crabs)  ;  Birgus  latro  (the  terrestrial  robber  or  palm 
crab),  in  which  the  upper  part  of  the  gill  cavity  is  shut  off  to 
form  a  "  lung,"  the  walls  having  numerous  vascular  plaits. 
Opinion  seems  to  incline  against  recognising  a  separate  sub- 
order (Anomura)  for  the  soft-tailed  hermit  crabs. 
Sub-order  2.     Brachyura.     Abdomen   short,  and  bent  under  the 
thorax.     It  is  narrow  in  the  male,  and  does  not  usually  bear 
more  than  two  pairs  of  appendages  ;    it  is  broader  in  the 
female,   and  bears  four  paired  appendages.       The  ventral 
ganglia  have  fused  into  an  oval  mass.     Cancer  (edible  crab)  ; 
Carcinus  m&nas  (shore  crab) ;   Portunus  (swimming  crab) ; 
Maia  (spider  crab)  ;    Lithodes  (stone   crab) ;    Porcellana  ; 
Dromia  (often  covered  by  a  sponge)  ;   Pinnotheres  (living 
inside   bivalves)  ;     Gelasimus  (fiddler   crab,    a   very  adept 
burrower)  ;   Telphusa  (a  fresh  water  crab) ;  Gecarcinus  (land 
crabs,  only  visiting  the  sea  at  the  breeding  season). 
*   History. — Fossil  Crustaceans  are  found  in  Cambrian  strata,  but  the 
highest  forms  (Decapoda)  were  not  firmly  established  till  the  Tertiary 
period.     Some  of  the  genera,  e.g.,  the  Branchiopod  Estheria,  living 
from  Devonian  ages  till  now,  are  remarkably  persistent  and  successful. 
How  the  class  arose,  we  do  not  know  :  it  is  probable  that  types  like 
Nebalia  give  us  trustworthy  hints  as  to  the  ancestors  of  the  higher 
Crustaceans  ;  it  is  likely  that  the  Phyllopods,  e.g.,  Apus,  bear  a  similar 
relation  to  the  whole  series  ;  the  Copepods  also  retain  some  primitive 
characteristics ;  but  it  is  difficult  apart  from  mere  guessing  to  say  any- 
thing definite  as  to  the  more  remote  ancestry. 

We  naturally  think  of  a  segmented  worm  type  as  a  plaus- 
ible starting-point  for  Crustaceans,  and  it  is  not  difficult 
to  understand  how  a  development  of  cuticular  chitin  would 
tend  to  produce  a  flexibly  jointed  limb  out  of  an  unjointed 
parapodium,  how  the  mouth  might  be  shunted  a  little  back- 
wards, and  two  appendages  and  ganglia  a  little  forwards, 
and  how  division  of  labour  would  result  in  the  differentia- 
tion of  distinct  regions. 

GENERAL  NOTES  ON  CRUSTACEANS. 

Of  a  class  that  includes  animals  so  diverse  as  crabs, 
lobsters,  shrimps,  "beach  fleas,"  "wood  lice,"  barnacles, 
acorn  shells,  and  "  water  fleas,"  it  is  difficult  to  state  general 
characteristics,  other  than  those  facts  of  structure  which  we 
have  already  summarised. 

Admitting  the  parasitism  of  many  Crustaceans,  and  the 
sedentary  life  of  barnacles  and  acorn  shells,  we  must  still 


280  CRUSTACEA. 

allow  that  great  activity  characterises  the  class.  With  this 
may  be  connected  the  brilliant  colouring,  the  power  of 
colour  change,  and  the  phosphorescence  of  many  forms. 

Except  some  primitive  and  degenerate  forms,  all  are  seg- 
mented. The  typical  appendage  consists  of  a  basal  piece 
with  two  jointed  branches.  The  cuticle  is  always  chitinous, 
and  often  very  much  calcified.  The  abundance  of  chitin 
may,  to  some  extent,  explain  the  absence  of  cilia  in  Crus- 
taceans and  other  Arthropods.  The  rigidity  of  the  cuticle 
partially  explains  the  necessity  of  frequent  moults.  As  the 
muscles  contract  very  rapidly,  they  illustrate  the  striated 
condition  with  great  clearness.  In  crabs  and  some  others 
the  ventral  ganglia  are  concentrated.  Sensory  organs  are 
generally  well  developed;  both  "eyes"  and  "ears"  may 
occur  away  from  the  head.  Much  of  the  alimentary  canal, 
which  is  almost  always  simple,  consists  of  fore  gut  and  hind 
gut.  These  are  anterior  and  posterior  invaginations  of  skin 
which  meet  the  mid  gut  or  archenteron — the  original  gastrula 
cavity — and  are  especially  large  in  the  higher  Crustaceans  or 
Malacostraca.  The  frequent  presence  of  a  gastric  mill  is 
quite  intelligible,  for  it  occurs  in  the  fore  gut.  The  body 
cavity  is  never  very  large,  being  mainly  filled  up  with 
muscles  and  organs,  and  of  a  true  coelome  there  is  little 
trace.  In  the  blood  hsemocyanin  is  the  commonest  respira- 
tory pigment.  In  the  body  or  skin  lipochrome  pigments, 
such  as  those  which  change  from  bluish  green  to  red  as  the 
lobster  is  boiled,  frequently  occur.  Of  modes  of  respiration, 
there  are  many  grades, — by  the  general  surface,  by  currents 
of  water  in  and  out  of  the  posterior  part  of  the  food  canal, 
by  thin  plates  on  the  legs,  by  well-formed  gills.  We  miss 
the  numerous  excretory  nephridia  of  Annelids ;  the  green- 
glands  of  lobsters,  &c.,  probably  represent  a  pair ;  the  shell- 
glands  of  Phyllopods  and  Copepods  and  some  other  struc- 
tures seem  to  be  in  part  at  least  excretory.  It  is  possible 
that  shell  making  is  an  organised  method  of  getting  rid  of 
some  waste  products.  There  are  many  peculiarities  con- 
nected with  reproduction ;  thus  parthenogenesis  for  pro- 
longed periods  is  common  among  "  water  fleas " ;  herma- 
phroditism  occurs  in  barnacles,  acorn  shells,  &c.  ;  the 
hermaphrodites  are  sometimes  accompanied  by  pigmy 
"  complemental "  males ;  the  two  sexes  are  often  very 


GENERAL   NOTES   ON  CRUSTACEANS. 


281 


diverse.  The  spermatozoa  are  usually  exceptional  in  being 
very  slightly  motile.  Some  appendages  are  often  modified 
for  copulation  or  for  carrying  the  eggs. 

Development. — The  ova  of  most  Crustacea  show  con- 
siderable similarity  to  those  of  Astacus,  and  the  segmen- 
tation is  typically  of  the  kind  already  described.  But 
while  this  is  the  most  typical  case  for  Crustacean,  and, 

indeed,  for  Arthropod 
development,  it  is  pos- 
sible, within  the  limits  of 
the  class  Crustacea,  to 
trace  out  a  complete 
series,  in  which  the  first 
term  is  a  segmentation 
of  the  complete  and 
equal  type,  like  that  of 
a  worm,  and  the  last  the 
purely  peripheral.  In  the 
same  way,  though  gastru- 
lation  is  usually  much 
disguised,  we  find  all 
cases  from  an  invagina- 
tion  of  the  simplest  em- 
bolic  type  (Lucifer),  and 
through  the  condition 
described  for  Astacus^  to 
the  formation  of  endo- 
derm  by  the  ingrowth 
of  a  solid  plug  of  cells 
(Arthrostraca,  &c.). 

FIG.  91—Zcxea  of  common  Shore  Crab  Compared  with  As- 
(Carcimts  mcenas}.  (After  FAXON.)  tacus,  however,  the  most 
The  appendages  are  numbered.  important  point  we  have 

to  notice  is  the  frequent 

occurrence  of  a  very  striking  metamorphosis  in  the  life 
history.  In  other  words,  the  larva  hatched  from  the  egg 
is  rarely  like  the  parent,  and  only  acquires  the  adult  char- 
acters after  a  series  of  profound  changes.  In  some  cases 
(Nebalia,  Mysis)  a  metamorphosis  takes  place  within  the 
egg-cases,  and  in  the  few  forms  in  which  development  seems 
to  be  direct,  slight  traces  of  metamorphosis  are  found. 


282  CRUSTACEA. 

Almost  all  the  lower  Crustaceans  and  the  higher  forms 
Euphausia  and  Penceus  are  hatched  in  a  Nauplius  stage. 
In  the  remaining  cases  the  Nauplius  stage  is  indicated 
within  the  egg  by  the  moulting  of  a  larval  cuticle  (so  in 
Astacus}.  The  Nauplius  is  characterised  by  a  typically 
rounded  body,  and  by  the  presence  of  three  pairs  of  append- 
ages, which  are  the  only  obvious  indications  of  segmenta- 
tion. The  first  pair  of  appendages  are  unbranched  and 
bear  larval  sense  organs,  the  next  two  are  biramose  swim- 
ming organs.  There  is  an  unpaired  median  eye,  but  no 
heart  and  frequently  no  hind  gut.  The  three  pairs  of 
appendages  become  the  first  and  second  pairs  of  antennae 
and  the  mandibles  of  the  adult.  The  head  region  of  the 
Nauplius  becomes  the  head  region  of  the  adult,  the  posterior 
region  also  persists,  the  new  growth  of  segments  and  append- 
ages takes  place  (with  numerous  moultings)  in  the  region 
between  these. 

The  second  important  form  of  larva  is  the  Zoaea,  which 
has  all  the  appendages  on  to  the  last  maxillipedes  inclusive, 
an  unsegmented  abdomen,  and  two  lateral  compound  eyes 
in  addition  to  the  unpaired  one  of  the  Nauplius  stage. 
Most  Decapoda  are  hatched  in  the  Zoaea  stage. 

-    (a)  The  crayfish  (Astacus]  is  hatched  almost  as  a  miniature  adult. 
The  development  is  therefore  very  direct  in  this  case. 

(b)  The  lobster  (Homarus}  is  hatched  in  a  Mysis  stage,  in  which  the 

thoracic  limbs  are  two-branched  and  used  for  swimming.  After 
some  moults  it  acquires  adult  characters. 

(c)  Crabs  are  hatched  in  the  Zocea  form,  and  pass  with  moults  through 

a  Megalopa  stage,  in  which  they  resemble  certain  Hermit  Crabs. 
The  abdomen  is  subsequently  tucked  in  under  the  thorax. 

(d)  Penceus  (a  kind  of  shrimp)  is  hatched  as  a  Nauplius,  becomes  a 

Zocea,  then  a  Mysis,  then  an  adult.  Its  relative  Lucifer  starts 
as  a  Meta-Nauplius  with  rudiments  of  three  more  appendages 
than  the  Nauplius.  Another  related  form,  Sergestes,  is  hatched 
as  a  Protozocea>  with  a  cephalothoracic  shield  and  an  unseg- 
mented abdomen.  Thus  there  are  two  grades  between  Nauplius 
and  Zocea. 

Three  facts  must  be  borne  in  mind  in  thinking  over  the  life  histories 
of  crayfish,  lobster,  crab,  and  Penceus: — (l)  there  is  a  general  tendency 
to  abbreviate  development,  and  this  is  of  more  importance  when  meta- 
morphosis is  expensive  and  full  of  risks  ;  (2)  there  is  no  doubt  that  larvae 
exhibit  characters  which  are  related  to  their  own  life  rather  than  to  that 
of  the  adult ;  (3)  it  is  a  general  truth,  that  in  its  individual  development 
the  organism  has  to  recapitulate  to  some  extent  the  evolution  of  the 
race,  that  ontogeny  recapitulates  phylogeny.  But  while  there  can  be  no 


BIONOMICS.  283 

doubt  that  the  metamorphoses  of  these  Crustaceans  is  to  some  extent 
interpretable  as  a  recapitulation  of  the  racial  history,  for  there  were 
unsegmented  animals  before  segmented  forms  arose,  and  the  Zocza  stage 
is  antecedent  to  the  Mysis,  &c.,  yet  it  does  not  follow  that  ancestral 
Crustaceans  were  like  Nauplii.  On  the  contrary,  the  Nauplius  must  be 
regarded  as  a  larval  reversion  to  a  type  much  simpler  than  the  ancestral 
Crustacean.  Moreover,  this  idea  of  recapitulation  offers  a  philosophical 
rather  than  a  material  explanation  of  the  facts. 

Bionomics. 

Most  Crustaceans  are  carnivorous  and  predatory ;  others 
feed  on  dead  creatures  and  organic  debris  in  the  water ;  a 
minority  depend  upon  plants. 

Parasitism  occurs  in  over  700  species,  in  various  degrees, 
and  of  course  with  varied  results.  Most  of  the  parasites 
keep  to  the  outside  of  the  host  (e.g..  Fish  lice),  and  suck 
nourishment  by  their  mouths ;  the  Rhizocephala  (e.g., 
Sacculimi),  send  ramifying  absorptive  roots  through  the  body 
of  the  host.  Sometimes  the  parasitism  is  temporary  (Ar- 
gulus] ;  sometimes  only  the  females  are  parasitic  (e.g.,  in 
Lernced).  The  parasites  tend  to  lose  appendages,  segmen- 
tation, sense  organs,  &c.,  but  the  reproductive  organs  become 
more  fertile.  The  hosts,  e.g.,  crabs  infested  by  Rhizo- 
cephala, are  sometimes  materially  affected,  and  even  ren- 
dered incapable  of  reproducing. 

Some  Crustaceans  live  not  as  parasites  but  as  commensals 
with  other  animals,  doing  them  no  harm,  though  sharing 
their  food.  Thus  there  is  a  constant  partnership  between 
some  hermit  crabs  and  sea  anemones.  The  hermit  crab  is 
concealed  and  protected  by  the  sea  anemone ;  the  latter  is 
carried  about  by  the  Crustacean  and  gets  fragments  of  food. 

Masking  is  also  common,  especially  among  crabs.  Some 
will  cut  the  tunic  off  a  sea  squirt  and  throw  it  over  their  own 
shoulders.  Many  attain  a  mask  more  passively,  for  they  are 
covered  with  hydroids  and  sponges,  which  settle  on  the 
shell.  There  is  no  doubt,  however,  that  some  actively 
mask  themselves,  for  besides  those  known  to  use  the 
Tunicate  cloak,  others  have  been  seen  planting  seaweeds 
on  their  backs.  The  protective  advantage  of  masking  both 
in  offence  and  defence  is  very  obvious. 

The  intelligence  of  crabs  and  some  of  the  higher  Crus- 
taceans is  well  developed.  Maternal  care  is  frequent. 


284  CRUSTACEA. 

Fighting  is  very  common,  but  the  loss  of  limbs  is  readily 
repaired. 

Deep  sea  Crustaceans  are  very  abundant,  and  often 
remarkable  "  for  their  colossal  size,  their  bizarre  forms,  and 
brilliant  red  colourings ; "  some  are  blind,  others  are 
brilliantly  phosphorescent.  Yet  more  abundant  are  the 
pelagic  Crustaceans  (especially  Entomostraca  and  Schizo- 
pods) ;  they  are  often  transparent  except  the  eyes,  often 
brightly  coloured  or  phosphorescent.  Many  Crustaceans 
live  on  the  shore,  and  play  a  notable  part  in  the  struggle 
for  existence  which  is  so  keen  in  that  densely  crowded 
region.  The  lower  Crustaceans  are  abundantly  represented 
in  fresh  water,  in  pools,  streams,  and  lakes.  A  few,  such  as 
wood  lice  and  land  crabs,  are  terrestrial,  and  some  blind 
forms  occur  in  caves. 


CHAPTER    XIV. 

'PERIPATUS,    MYRIOPODS,    AND    INSECTS. 

Series  ARTHROPODA.     Sub-division  TRACHEATA 
ANTENNATA. 

.    Classes  PROTOTRACHEATA. — Peripatus.     MYRIOPODA. — Centipedes 
and  Millipedes.     INSECTA. — Insects. 

THESE  three  classes  form  a  series  of  which  winged  insects 
are  the  climax.  The  type  Peripatus  is  archaic,  and  links 
the  series  to  the  Annelids ;  the  Myriopods  lead  on  to  the 
primitive  wingless  insects. 

We  may  speak  of  the  series  as  Tracheata  Antennata,  for 
all  breathe  by  tracheae — tubes  which  carry  air  to  the  recesses 
of  the  body — and  all  have  antennae. 

First   Class   of  Tracheata   Antennata — PROTOTRACHEATA, 
including  one  genus,  Peripatus. 

GENERAL  CHARACTERS. — The  body  is  worm-like  inform, 
soft  skinned^  and  without  external  segmentation. 

There  is  a  pair  of  prominent  pre-oral  antennce. 

The  true  appendages  are  — a  pair  of  jaws  in  the  mouth,  a 
pair  of  slime  secreting  oral  papilla,  numerous  pairs  of  short, 
imperfectly  jointed  legs,  each  with  two  claws,  and  a  pair  of 
anal  p  apt  lice.  The  legs  contain  peculiar  (coxal)  glands. 

Respiration  is  effected  by  numerous  trachece,  ivhose  openings 
are  somewhat  scattered  on  the  surface  of  the  body.  The  heart 
is  simply  an  elongated  dorsal  vessel  with  valvular  openings. 
There  is  a  series  of  excretory  tubes  or  nephridia.  The  halves 
of  the  ventral  nerve  cord  are  ividely  separate. 

The  single  genus   Peripatus   is   represented  by    numerous 


286         PERIPATUS,    MYRIOPODS,   AND  INSECTS. 


(tivelve)  species,  widely  distributed;  in  its  possession  of  trachea 
and  nephridia  it  is  an  interesting  connecting  link  ;  in  many 
ways  it  seems  to  be  an  old  fashioned  survivor  of  an  archaic 
type. 

The  species  of  Peripatus  are  beautiful  animals.  Professor 
Sedgwick  says — "The  exquisite  sensitiveness  and  continu- 
ally changing  form  of  the  antennae,  the  well- 
rounded  plump  body,  the  eyes  set  like  small 
diamonds  on  the  side  of  the  head,  the  deli- 
cate feet,  and,  above  all,  the  rich  colouring 
and  velvety  texture  of  the  skin,  all  combine 
to  give  these  animals  an  aspect  of  quite 
exceptional  beauty."  As  to  their  habits,  Mr. 
Hatchett  Jackson  says — "  They  live  under 
stones,  in  rotting  wood,  &c.,  in  moist  places, 
are  nocturnal  in  habit,  and  feed  on  insects, 
&c.,  which  they  ensnare  by  the  ejectioji  of 
slime  from  the  oral  papillae."  To  their  shy 
habits,  their  persistence  is  possibly  in  part 
due.  They  are  able  to  move  quickly,  some- 
what after  the  fashion  of  Millipedes,  especi- 
ally like  Scolopendrella.  Young  forms  roll  up 
when  touched,  and  have  been  seen  to  climb 
up  vertical  glass  plates. 


FIG.  92. — 
External  form 
of  Peripatus. 

(After  BAL- 
FOUR.) 


Note   antennae 
and  simple  feet. 


The  species  acknowledged  by  Sedgwick  are  : — Four 
from  South  Africa — P.  capensis^  P.  balfouri,  and  P. 
brevis  from  Table  Mountain,  and  P.  moseleyi  from 
near  Williamstown ;  two  from  Australasia — P.  nova 
zealandice  from  New  Zealand,  and  P.  leuckarti  from 
Queensland  ;  seven  from  neotropical  regions — P.  edwardsii  from  Cara- 
cas, P.  imthurmi  or  demeraranus  from  Demerara,  P.  trinidadensis  and 
P.  torqiiatus  from  Trinidad,  P.  iuliformis  from  St.  Vincent,  P.  chilensis 
from  Chili,  P.  quitensis  from  Ecuador,  besides  which  there  are  some 
doubtful  forms.  The  list  shows  how  widely  this  remarkable  genus  is 
distributed. 

As  the  different  species  have  similar  habits,  and  live  in  very  similar 
conditions,  the  differences  between  them  perhaps  illustrate  purely  con- 
stitutional variations. 

A  more  Detailed  Account  of  Peripatus. 

Form. — The  body  suggests  an  Annelid  or  a  caterpillar,  but,  apart  from 
the  appendages,  there  is  no  external  segmentation.  Over  the  soft  skin 
are  numerous  minute  warts  with  small  bristles.  The  mouth  is  ventral 
and  anterior  ;  the  anus  terminal  and  posterior. 


DETAILED  ACCOUNT  OF  PERIPATUS.  287 

Appendages. — The  two  large,  ringed  antennae  do  not  seem  to  be 
homologous  with  limbs.  The  first  pair  of  appendages — double  sickle- 
like  jaws — lie  in  the  mouth  cavity.  A  little  further  back  are  two  oral 
papillae  from  which  slime  is  exuded.  Then  there  are  the  14-42  stump- 
like  legs,  each  with  two  terminal  chitinous  claws.  In  the  young  P. 
capensis  the  leg  is  said  to  be  five -jointed,  but  in  the  adults  there  is  no 
trace  of  this.  In  respect  to  its  legs,  therefore,  Peripatus  is  hardly  an 
Arthropod. 

Skin.  —  The  chitinous  cuticle,  ordinarily  thick  in  Arthropods,  is 
delicate.  The  ectoderm  [hypodermis,  or  epidermis]  is  a  single  layer  of 
cells. 

The  Muscular  System  is  very  well  developed.  ( I )  Externally  there 
is  a  layer  of  circular  muscles  ;  (2)  within  this  lies  a  double  layer  of 
diagonal  fibres  ;  (3)  internally  there  are  strong  longitudinal  bundles. 
Finally,  in  connection  with  this  internal  layer,  there  are  fibres  which 
divide  the  body  cavity  into  a  median  and  two  lateral  compartments. 
The  median  includes  heart,  gut,  slime  glands,  reproductive  organs  ; 
the  laterals  include  the  nerve  cords,  the  salivary  glands  ;  the  legs  con- 
tain nephridia  and  coxal  or  crural  glands.  Striped,  rapidly  contracting 
muscles  are  characteristic  of  Arthropods,  but  m  Peripatus  the  muscles 
are  unstriped,  excepting  those  which  work  the  jaws  and  are  perhaps  the 
most  active. 

The  Nervous  System  consists  of  a  dorsal  brain  and  two  widely  separate 
lateral  ventralnerve  cords.  These  are  connected  transversely  by  numer- 
ous commissures,  are  slightly  swollen  opposite  each  pair  of  legs  to  which 
they  give  off  nerves,  and  are  united  posteriorly  over  the  anus.  There 
are  only  hints  of  ganglia,  but  there  is  a  continuous  layer  of  ganglionic 
cells.  The  brain  is  very  homogeneous,  simpler  than  that  of  most  Insects. 
From  the  brain  nerves  pass  to  the  antennae,  &c. ,  and  two  viscerals  or 
sympathetics,  soon  uniting,  innervate  the  anterior  part  of  the  gut. 
Sense  organs  are  represented  by  two  simple  eyes  on  the  top  of  the  head. 
These  are  most  like  the  eyes  of  some  marine  Annelids.  Behind  each 
there  lies  a  special  optic  lobe  connected  with  the  brain,  but  the  eye  itself 
arises  as  a  dimple  in  the  skin. 

Alimentary  Canal. — Round  about  the  mouth,  papillae  seem  to  have 
fused  to  form  a  "  mouth  cavity,"  which  includes  the  mandibles,  a  median 
pad  or  tongue,  and  the  opening  of  the  mouth  proper.  The  mouth  leads 
into  a  muscular  pharynx,  into  which  opens  the  common  duct  of  two 
large  salivary  glands,  which  extend  far  back  along  the  body.  Mouth, 
pharynx,  and  short  oesophagus  are  lined  by  a  chitinous  cuticle,  like  that 
of  the  exterior.  The  long  digestive  region  or  mid  gut  extends  from  the 
second  leg  nearly  to  the  end  of  the  body.  Its  walls  are  plaited.  Finally, 
there  is  a  short  rectum,  lined  by  a  chitinous  cuticle. 

Circulatory  System. — The  dorsal  blood  vessel  forms  a  long  contractile 
heart.  It  lies  within  a  pericardial  space,  and  receives  blood  by  seg- 
mentally  arranged  apertures  with  valves.  The  circulation  is  mostly  in 
ill-defined  spaces  in  the  apparent  body  cavity  or  "  haemoccele." 

The  Respiratory  System  consists  of  very  long  and  very  fine  unbranched 
tracheae,  which  are  widely  distributed  in  the  body  ;  a  number  open 
together  to  the  exterior  in  flask-like  depressions.  These  openings  or 
stigmata  are  diffuse  and  irregular  in  Peripatus  edwardsii,  but  in  P<  capensis 


288         PERIPATUS,    MYRIOPODS,   AND  INSECTS. 


there  is  a  dorsal  and  ventral  row  on  each  side.  In  P.  novce  zealandice 
the  tracheae  are  said  to  be  branched. 

The  Excretory  System. — A  pair  of  nephridia  lie  in  each  segment. 
Each  consists  of  an  internal  terminal  funnel,  a  looped  canal,  and  a  wide 
vesicle  which  opens  near  the  base  of  each  leg.  They  are  not  very  differ- 
ent from  those  of  many  Annelids,  but  their  occurrence  in  a  Tracheate 
is  remarkable.  The  salivary  glands  and  the  genital  ducts  are  probably 
modified  nephridia.  It  may  be  noted,  too,  that  the  same  is  perhaps 
true  of  the  "  coxal  glands  "  of  Limulus  and  of  the  antennary  glands  of 
Crustaceans. 

Crural  or  Coxal  Glands  lie  in  the  legs  and  open  to  the  exterior.  Their 
meaning  is  uncertain,  their  occurrence  is  variable.  Thus  in  P.  edwardsii 
they  occur  in  the  males  only,  in  P.  capensis  they  are  present  in  both 
sexes.  In  the  male  of  P.  capensis  the  last  pair  are  very  long  (a.g., 
Fig.  93).  The  large  mucus  glands,  which  pour  forth  slime  from  the  oral 
papillae,  are  regarded  as  modified  coxal  glands. 

Reproductive  System. — (a)  Female  (of  P.  edwardsii]. — From  the  two 
bvaries,  which  are  surrounded  by  one  connective  tissue  sheath,  the  eggs 


F.I 


F.2 


FIG.  93. — Dissection  of  Peripatus  capensis. 
(After  BALFOUR.) 

at.,  Antennae;  or,p.,  oral  papillae;  e.g.,  cerebral  ganglia  ;  sl.d., 
duct  of  slime  gland  (sl.g.) ;  s.o.8,  segmental  organ  or  nephridium 
eighth  ;  v.c.,  ventral  nerve  connected  by  transverse  commissures  (co.) 
with  its  fellow;  v^.g.,  last  crural  gland;  s.o.i?,  seventeenth  neph- 
ridium; £-.0.,  genital  aperture;  A,  anus;  p.d.c.,  posterior  com- 
missure ;  F.if,  seventeenth  appendage :  a.g.,  last  crural  gland; 
F.I,  F.2,  first  and  second  legs  ;  oe.co.,  oesophageal  nerve  commis- 
sure. 

pass  by  two  long  ducts  leading  to  a  common  terminal  vagina  opening 
between  the  second  last  legs.  These  ducts  are  for  the  most  part  uteri, 
but  on  what  may  be  called  the  oviduct  portions  adjoining  the  ovaries, 
there  are  two  pairs  of  pouches — (a)  a  pair  of  receptacula  seminis  (for 
storing  the  spermatozoa  received  during  copulation),  and  a  pair  of 
receptacula  ovorum  for  storing  fertilised  eggs. 

The  eggs  are  hatched  in  the  uteri,  and  all  stages  are  there  to  be  found 
in  regular  order.     The  young  embryos  seem  to  be  connected  to  the  wall 


DETAILED  ACCOUNT  OF  PERIPATUS. 


of  the  uterus  by  what  has  been  called  a  "  placenta,"  so  suggestive  is  it  of 
mammalian  gestation.  The  older  embryos  lose  this  "  placenta,"  but 
each  lies  constricted  off  from  its  neighbours.  When  born  the  young 
resemble  the  parents  except  in  size  and  colour.  In  P.  novce  zealandice, 
the  ova  pass  from  the  ovary  into  the  uterus  in  December,  and  the  young 
are  born  in  July — a  long  period  of  gestation. 

(d)  Male  (of  P.  edwardsii].  The  male  elements  are  produced  in  small 
testes,  pass  thence  into  two  seminal  vesicles,  and  onwards  by  two  vasa 
deferentia  into  a  long  single  ejaculatory  duct,  which  opens  in  front  of  the 
anus.  In  the  ejaculatory  duct  the  spermatozoa  are  made  into  a  long 

packet  or  spermatophore,  which 
is  attached  to  the  female  during 
copulation. 

[While  it  is  characteristic  of 
Arthropods,  in  which  the  de- 
velopment of  chitin  is  so  pre- 
dominant, that  ciliated  epi- 
thelium is  absent,  it  seems  that 
in  Peripatus,  which  is  much  less 
chitinous  than  the  others,  ciliated 
cells  occur  in  some  parts  of  the 
reproductive  ducts,  and  perhaps 
also  at  the  internal  funnels  of 
the  nephridia.  This  is  indeed 
what  one  would  expect.] 

Development  of  Peripatus. — 
There  is  a  strange  variety  of 
development  in  different  species 
of  this  genus.  Thus  there  is 
much  yolk  in  the  ovum  of  P. 
novce  zealandice,  extremely  little 
in  that  of  P.  capensis.  In  the 
former  species  the  yolk  has  a 
manifold  origin  ;  it  is  said  to 
arise  in  the  protoplasm  or  the 
ovum  itself  from  the  breaking 
up  of  the  germinal  vesicle,  from 
surrounding  follicle  cells,  and 
from  yolk  present  within  the 
ovary.  In  P.  capensis  and  P. 
a,  Anus  ;  bl,  blastopore  ;  m,  mouth  ;  balfouri  spermatozoa  reach  the 
&.i?e™  SeSmentS;  ""  2°ne  °f  °vary,  anS  there  probably  the 

ova  are  fertilised,  but  in  P.  novce 

zealandice  the  spermatozoa  are  confined  to  the  receptaculum  seminis, 
near  which  fertilisation  seems  to  occur.  In  the  maturation  of  the  ova 
of  P.  capensis  and  P.  balfouri  two  polar  bodies  are  extruded  as  usual, 
but  none  have  been  observed  in  the  case  of  P.  novce  zealandice. 

In  P.  capensis  the  "  segmentation  "  is  remarkable,  for  true  cleavage  of 

cells  does  not  occur.     The  fully  "segmented"  ovum  does  not  exhibit 

the  usual  cell  limits.     It  is  a  protoplasmic  mass — or  syncytium — with 

many  nuclei.     Even  when  the  body  is  formed,  the  continuity  of  cells 

19 


7 


FlG.  94. — Embryos  of  Peripatus 
capensis,  showing  closure  of  blas- 
topore  and  curvature  of  embryo. 
(After  KORSCHELT  and  HEIDER.) 


290        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

persists,  nor  does  the  adult  lack  traces  of  it.  To  Professor  Sedgwick, 
this  singular  fact  suggested  the  theory  that  the  Metazoa  may  have  begun 
as  multinucleate  Infusorian-like  animals. 

The  gut  appears  as  a  large  vacuole  within  the  rnultinucleated  mass, 
and  a  gastrula  stage  is  thus  established. 

In  the  ova  of  P.  nova  zealandicz,  which  have  much  yolk,  a  superficial 
multiplication  of  nuclei  forms  a  sort  of  blastoderm,  which  spreads  over 
almost  the  entire  ovum.  The  segmentation  in  this  case  has  been  called 
centrolecithal  (the  type  characteristic  of  Arthropods),  but  it  is  again  true 
that  for  a  long  time  the  cells  do  not  exist  as  well  defined  units.  It  has 
been  said,  indeed,  that  "  the  embryo  is  formed  by  a  process  of  crystallis- 
ing out  in  situ  from  a  mass  of  yolk,  among  which  is  a  protoplasmic 
reticulum  containing  nuclei." 

From  these  examples  the  student  will  perceive  how  difficult  it  is  to 
give  a  succinct  account  of  the  development  of  Peripatus. 


Development  of  Organs. 

The  hypodermis  is  ectodermic,  the  cuticle  an  external  product 
thereof. 

The  muscles  are  as  usual  derived  from  the  mesoderm,  which  arises 
from  two  ventral  mesodermic  strands.  These  are  subsequently  divided 
into  hollow  segments.  The  true  body  cavity  or  coelome  is  represented 
by  the  original  cavities  of  the  mesoderm  segments.  In  the  adult  this 
series  of  truly  ccelomic  cavities  is  hardly  represented  except  by  the  inner- 
most portions  of  the  nephridia.  The  apparent  body  cavity  is  a  secondary 
cavity,  consisting,  for  the  most  part,  of  blood  carrying  or  vascular 
spaces,  subsequently  established  in  the  mesoderm.  It  is  divided  into 
five  regions,  the  central  space,  the  two  lateral  cavities,  and  the  cavities 
of  the  legs. 

The  appendages  are  outgrowths  of  the  body  wall.  They,  and  all  the 
segmentally  arranged  parts,  develop  progressively  from  in  front  back- 
wards. 

The  nervous  system  is  derived  from  ectodermic  thickenings  which 
sink  inwards.  It  develops  from  in  front  of  the  mouth  backwards. 

The  food  canal  consists  of  the  long  endodermic  mid  gut  or  mesenteron 
(the  gastrula  cavity),  of  an  anterior  ectodermic  invagination  form- 
ing pharynx  and  gullet  (fore  gut  or  stomatodseum),  and  of  a  short 
posterior  ectodermic  invagination  forming  the  rectum  (hind  gut  or 
proctodseum). 

The  nephridia  have  a  twofold  origin.  The  internal  funnel  is  derived 
directly  from  part  of  a  mesodermic  segment  or  vesicle.  The  rest  of  the 
nephridium  is  invaginated  from  the  ectoderm. 

The  reproductive  organs  arise  on  the  epithelium  of  a  persistent  portion 
of  the  true  coelome  or  primitive  body  cavity. 

Zoological  Position  of  Peripatus. — Professor  Lang,  in  his  work  on 
Comparative  Anatomy,  summarises  the  synthetic  characters  of  Peripatus 
as  follows :  — 


MYRIOPODA. 


291 


ANNELID  CHARACTERISTICS.        TRACHEATE  CHARACTERISTICS. 


The  presence  of  tracheae. 

The  nature  of  the  heart  and  the 

lacunar  circulation. 
The   modification   of  appendages 

as  mouth  organs. 
The  form  of  the  salivary  glands. 
The    smallness    of    the    genuine 

body  cavity  or  ccelome. 


Segmentally  arranged  nephridia  as 
in  Chaetopods. 

Segmentally  arranged  coxal  glands, 
like  similar  glands  in  some 
Chaetopods. 

The  muscular  ensheathing  of  the 
body. 

Less  important  are  the  stump- 
like  legs  and  the  simple 
eyes. 

The  ladder  like  character  of  the  ventral  nervous  system  (cf.  primitive 
Molluscs,  Phyllopod  Crustaceans,  and  Nemerteans)  is  probably  primitive. 
That  salivary  glands  and  genital  ducts  are  homologous  with  nephridia  is 
a  fact  of  much  morphological  interest.  It  is  possible  that  the  slime 
glands  are  modifications  of  coxal  or  crural  glands,  and  that  the  latter  are 
homologous  with  the  parapodial  glands  of  some  Annelids.  It  is  not 
certain  that  the  antennae,  jaws,  and  oral  papillae  of  Peripatus  precisely 
correspond  to  the  antennae,  mandibles,  and  first  maxillae  of  Insects. 

Our  general  conclusion  is  that  Peripatus  is  an  archaic  type,  a  survivor 
of  forms  which  were  ancestral  to.  Tracheata  and  closely  related  to 
Annelids. 


Second  Class  of  Tracheata  Antennata.     MYRIOPODA. 
Centipedes  and  Millipedes. 

These  animals  retain  a  worm-like  shape ;  the  numerous 
rings  of  the  body  and  the  appendages  they  bear  are  very 
uniform ;  there  is  little  division  of  labour.  It  would  be  rash 
to  assert  that  any  of  the  modern  Myriopods  are  stages  in  the 
pedigree  of  Insects,  but  it  is  likely  that  the  two  classes  are 
branches  from  one  base.  Simple  wingless  insects,  known 
as  Collembola  and  Thysanura,  are  closely  approached  by 
such  Myriopods  as  Scolopendrella. 

Both  centipedes  and  millipedes  live  on  land,  but  two  or 
three  of  the  latter  (e.g.,  a  species  of  Geophilus)  occur  on  the 
seashore.  Most  are  very  shy  animals,  lurking  in  dark  places 
and  avoiding  the  light. 

The  head  bears  a  pair  of  antennae,  and  two  pairs  of 
appendages — mandibles  and  maxillae.  The  limbs  are  six- 
or  seven-jointed,  clawed,  and  very  uniform.  They  have 
many  more  legs  than  insects,  but  they  make  less  of  them. 
The  nervous  system,  heart,  excretory  tubules,  &c.,  are  like 
those  of  Insects. 


292         PERIPATUS,   MYRIOPODS,   AND  INSECTS. 


The  development  in  many  ways  suggests  and  leads  up  to 
that  of  Insects. 

MYRIOPODA. 


CENTIPEDES. 
CHILOPODA. 


MILLIPEDES. 
DIPLOPODA  (or  CHILOGNATHA). 


Carnivorous. 
Poisonous. 
Body  usually  flat. 

A  pair  of  appendages  to  each 
segment. 


Many-jointed  antennae. 

Toothed  cutting  mandibles. 

Each  maxilla  consists  of  an  ex- 
ternal palp,  and  a  bilobed  median 
portion. 

The  next  appendage  is  leg-like. 
Then  follows  a  large  basilar  plate, 
beside  which  are  the  two  poison 
claws. 

A  single  posterior  genital  aper- 
ture. 

Examples  Scolopendra. 
Lithobius. 


Vegetarian. 
Harmless, 
Body  cylindrical. 

By  the  imperfect  separation  of  the 
segments  all  but  the  most  anterior 
seem  to  have  two  pairs  of  append- 
ages each,  and  also  two  paired 
ganglia,  and  two  pairs  of  stigmata. 

Seven-jointed  antennae. 
Broad  masticating  mandibles. 
Maxillae  are  represented   by  a 
four-lobed  plate. 

No  basilar  plate. 


Genital  apertures  open  on  the 
second  or  third  pair  of  limbs. 

Examples— -Julus. 

Geophilus. 


In  reference  to  habitat,  it  is  interesting  to  note  that  at  least  two 
myriopods — Geophilus  submarinus  and  Linotcenia  maritima,  occur  on 
British  coasts. 

As  distinct  from  the  two  chief  sub-classes,  it  is  perhaps  necessary  to 
recognise  other  two — Pauropoda,  e.g.,  Pauropus,  and  Symphyla,  e.g., 
Scolopendrella.  The  last-named  approaches  closely  to  the  most  primi- 
tive insects  (Collembola  and  Thysanura). 

Third  Class  of  Tracheata  Antennata.     INSECTA. 

Insects  occupy  a  position  among  the  backboneless  animals 
like  that  of  birds  among  the  Vertebrates.  The  typical 
members  of  both  classes  have  wings  and  the  power  of  true 
flight,  richly  aerated  bodies,  and  highly  developed  nervous 
and  sensory  organs.  Both  are  very  active  and  brightly 


THE   COCKROACH.  293 

coloured.  They  show  parallel  differences  between  the  sexes, 
and  great  wealth  of  species  within  a  narrow  range.  One 
expects  to  find  that  insects,  like  birds,  have  a  high  body 
temperature. 

GENERAL  CHARACTERS. — Like  other  Arthropods •,  Insects 
have  segmented  bodies,  jointed  legs,  chitinous  armature,  and  a 
ventral  chain  of  ganglia  linked  to  a  dorsal  brain.  Compared 
with  Peripatus  and  Myriopods,  adult  insects  show  concentra- 
tion of  the  body  segments,  decrease  in  the  number  and  increase 
in  the  quality  of  the  appendages,  and  wings  withal. 

Insects  are  terrestrial  and  aerial,  and  rarely  aquatic 
animals  ;  usually  winged  as  adults,  breathing  by  means  of 
trachea,  and  often  with  a  metamorphosis  in  the  course  of  their 
growth. 

The  body  is  divided  into  three  distinct  regions, — head, 
thorax,  and  abdomen.  The  head  bears  three  pairs  of  mouth 
appendages  (=  legs),  and  a  pair  of  pre-oral  out-growths — 
the  antenna  ;  the  thorax  bears  a  pair  of  legs  on  each  of  its 
three  segments,  and,  typically,  a  pair  of  wings  on  each  of 
the  posterior  two ;  the  abdomen  has  no  appendages,  unless 
rudimentary  modifications  of  these  be  represented  by  stings, 
ovipositors,  &c. 

First  Type  of  Insects,  Periplaneta  (or  Blatta)* 
The  COCKROACH. 

The  cockroaches  found  in  Britain  are  immigrants,  either 
from  the  East  (P.  orientalis],  or  from  America  (P.  americand) ; 
the  two  species  closely  resemble  one  another.  They  are 
omnivorous  in  their  diet,  and  active  in  their  habits,  but 
they  hide  during  the  day  and  feed  at  night.  They  are 
ancient  insects,  for  related  forms  occurred  in  Silurian  ages  ; 
they  are  average  types,  for  they  are  neither  very  simple  nor 
very  highly  specialised.  Their  position  is  among  the 
Orthoptera,  in  the  same  order  as  locusts  and  grasshoppers. 
The  young  are  hatched  as  miniature  adults,  except  that 
wings  are  absent ;  in  other  words,  there  is  no  metamorphosis 
in  development. 

The  skin  consists  of  an  external  chitinous  cuticle  and  a 
subjacent  cellular  layer — the  epidermis  or  hypodermis — 
from  which  the  cuticle  is  formed.  The  newly  hatched 
cockroaches  are  white,  the  adults  are  dark  brown. 


294         PERIPATUS,   MYRIOPODS,   AND  INSECTS. 


External  Characters. 


THE  HEAD. 

APPENDAGES  OF  THE  HEAD. 

OTHER  STRUCTURES  ON  THE 

HEAD. 

It      is      vertically 

i.  A  pair  of  stout  toothed  mand- 

The antennae    (probably   not 

elongated   and 
separated  from 

ibles  working  sideways. 
2.  The  first  maxillae,  each  con- 

homologous with  append- 
ages), long,  slender,  many 

the  thorax  by  a 
neck. 

sisting  (a)  of  a  basal  piece  or 
protopodite  with  two  joints  — 

jointed,  tactile. 
The    large    black   compound 

a  basal  cardo,  a  distal  stipes  ; 

eyes. 

(b)    of    a    double    endopodite 

The  "upper  lip"  or  labrum, 

borne  by  the  basal  piece, 

in  front  of  the  mouth. 

and  consisting  of  an  inner 

The  white  oval  patches  near 

lacinia  and  a  softer  outer 

the  bases  of  the  antennae, 

Jalea  ; 
an  exopodite  or  maxillary 
palp  also  borne  by  the  basal 

possibly  sensory. 

piece,  and  consisting  of  five 

joints. 

3.  The  second  pair  of  maxillae, 

fused  together  as  the  "  labi- 

um,''  consisting  (a)  of  a  fused 

basal    piece    or    protopodite 

with  two  joints—  a  basal  sub- 

mentum,     a     smaller     distal 

mentum  ;    on  each  side  this 
protopodite  bears 

(b)  a  double  endopodite  (ligula) 

consisting     of     an      inner 

lacinia,  and  an  outer  para- 

glossa  ; 

(c)  an  exopodite  or  labial  palp, 

consisting  of  three  joints. 

THE  THORAX. 

THE  APPENDAGES  OF  THE 

OTHER  STRUCTURES  ON  THE 

It  consists  of  three 

THORAX. 

THORAX. 

segments  :  — 

(a)  prothorax, 
(b)  mesothorax, 

(a)  First  pair  of  legs. 
(b)  Second  pair  of  legs. 

(£)  A  pair  of  elytra  or  wing- 

covers    (modified    wings) 

rudimentary  in  female   of 
P.  orientalis. 

(c)  metathorax. 
(Each    segment    is 
bounded   by  a 

(c)  Third  pair  of  legs.    Each  leg 
consists  of  many  joints  —  a 
basal  "  coxa  "  with  a  small 

(c)    A    pair    of   membranous 
wings,  sometimes  used  in 
flight,  folded  when  not  in 

dorsal  tergum, 

"  trochanter  "  at  its  distal 

use,   absent   in    female  of 

and    ventral 

end,  a  "  femur,"  a  "  tibia," 

P.  orientalis. 

sternum.) 

a  six-jointed  tarsus  or  foot 

Between  the  segments  of  the 

ending  in  a  pair  of  claws. 

thorax    are    two    pairs  of 

respiratory    apertures     or 

stigmata. 

THE  ABDOMEN. 

APPENDAGES  (?)  OF  THE 

OTHER  STRUCTURES  ON  THE 

ABDOMEN. 

ABDOMEN. 

It  consists  of  10  (or 

Two   cigar-shaped    tactile    anal 

A  pair  of  stigmata  occur  be- 

1 1)  distinct  seg- 

cerci,  attached    under   the 

tween    the    edges    of   the 

ments,        with 

edges  of  the  last  tergum, 

terga  and    sterna    in    the 

terga  and  ster- 

are possibly  relics  of   the 

first  eight  abdominal  seg- 

na   as    in    the 

last  abdominal  appendages. 

ments.      There  are  there- 

thorax. 

The  ninth  sternum  of  the  male 

fore  twenty  stigmata  in  all. 

bears  a  pair  of  styles,  pos- 

The anus  is  terminal,  beneath 

sibly  relics  of  appendages. 
Both  sexes  have  complex   hard 

the   tenth    tergum   of  the 
abdomen  ;  a  pair  of  "podi- 

structures     (gonapophyses) 

cal  plates  "  lie  beside  it. 

beside  the  genital  apertures. 

The   genital   aperture   is   ter- 

They are  possibly  relics  of 

minal,  ventral  to  the  anus. 

appendages. 

The    opening    of    the    sper- 

matheca  —  the       female's 

receptacle      for     sperma- 

tozoa —  lies  on   the  ninth 

sternum  of  the  abdomen 

NERVOUS  SYSTEM. 


295 


Moulting,  which  involves  a  casting  of  the  cuticle,  of  the 
internal  lining  of  the  tracheae,  &c.,  occurs  some  seven  times 
before  the  cockroach  attains  in  its  fifth  year  to  maturity. 

The  muscles,  which  move  the  appendages,  and  produce 
abdominal  movements  essential  to  respiration,  are  markedly 
cross  striped. 

Nervous  System. — A  pair  of  supra-oesophageal  or  cerebral 
ganglia  lie  united  in  the  head.  As  a  brain,  they  receive 


•mx.p 


•~S.ni 


FIG.  95. — Mouth  appendages  of  Cockroach. 
DUFOUR.) 


(After 


I.  Mn,  Mandibles  ;  II.  First  Maxillae  ;  c,  cardo ;  st,  stipes ;  L, 
lacinia ;  G,  galea;  mx.p,  maxillary  palp;  III.  Second  Maxillae  or 
Labium  ;  s.m,  submentum  ;  m,  mentum  ;  L,  laciniae  ;  pg,  para- 
glossa  ;  l.p,  labial  palp. 

impressions  by  antennary  and  optic  nerves.  By  means  of  a 
paired  commissure  surrounding  the  gullet,  they  are  connected 
with  a  double  ventral  chain  of  ten  ganglia.  Of  these,  the 
first  or  sub-cesophageal  pair  are  large,  and  give  off  nerves  to 
the  mouth  parts,  &c. ;  from  each  of  the  ganglia  of  the 
thorax  and  the  abdomen  nerves  are  given  off  to  adjacent 
parts.  There  are  three  pairs  of  ganglia  in  the  thorax,  and  six 


296        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

in  the  abdomen,  of  which  the  last  is  the  largest.  From  the 
cesophageal  commissures  two  visceral  nerves  are  given  off, 
which  form  in  a  somewhat  complex  manner  the  innervation, 
of  gullet,  crop,  and  gizzard.  Besides  the  large  compound 
eyes,  there  are  other  sensory  structures — some  of  the  hairs 
on  the  skin,  the  maxillae  (to  some  extent  organs  of  taste), 
the  antennae  (tactile  and  olfactory),  the  anal  cerci  (tactile), 
and  possibly  the  oval  white  patches  on  the  head. 

Alimentary  System. — (i)  The  fore  gut  (stomatodaeum)  is 
lined  by  a  chitinous  cuticle  continuous  with  that  of  the  outer 
surface  of  the  body.  It  includes  (a)  the  buccal  or  mouth 
cavity,  in  which  there  is  a  tongue-like  ridge,  and  into  which 
there  opens  the  duct  of  the  salivary  glands  ;  (b)  the  narrow 
gullet  or  oesophagus  ;  (c)  the  swollen  crop  ;  (d}  the  gizzard 


FIG.  96. — Transverse  section  of  Insect.     (After  PACKARD.) 

k,  Heart ;  g,  gut ;    «,  nerve  cord  ;   st,  stigma  ;    tr,  trachea  ;   tv, 
wing  ;  f)  femur  of  leg. 

with  muscular  walls,  six  hard  cuticular  teeth,  and  some 
bristly  pads. 

There  is  a  pair  of  diffuse  salivary  glands  on  each  side  of 
the  crop,  and  between  each  pair  of  glands  a  salivary  recep- 
tacle. The  ducts  of  the  two  salivary  glands  on  each  side 
unite,  the  two  ducts  thus  formed  combine  in  a  median  duct, 
and  this  unites  with  another  median  duct  formed  from  the 
union  of  the  ducts  of  the  receptacles.  The  common  duct 
opens  into  the  mouth. 

(2)  The  mid  gut  (mesenteron)  is  lined  by  endoderm.  It 
is  short  and  narrow,  and  with  its  anterior  end  seven  or  eight 


REPRODUCTIVE  SYSTEM. 


297 


club-shaped  digestive  outgrowths  are  connected.  These 
seem  to  have  a  pancreatic  function. 

(3)  The  hind  gut  (proctodaeum)  is  lined  by  a  chitinous 
cuticle.  It  is  convoluted  and  divided  into  narrow  ileum, 
wider  colon,  and  dilated  rectum  with  six  internal  ridges. 
From  the  beginning  of  the  ileum,  the  excretory  Malpighian 
tubules  are  given  off. 

Respiratory  System. — The  tracheal  tubes,  which  have  ten 
pairs  of  lateral  apertures  or  stigmata,  ramify  throughout  the 
body. 

Circulatory  System. — The  chambered  heart  lies  along  the 
mid  dorsal  line  of  abdomen  and  thorax.  It  receives  blood 
by  lateral  valvular  apertures  from  the  surrounding  pericardial 
space,  and  drives  it  forwards  by  a  slender  aorta.  The  blood 
circulates,  however,  within  ill-defined  spaces  in  the  body. 

The  Excretory  System  consists  of  sixty  or  so  fine  (Mal- 
pighian) tubules,  which  rise  in  six  bundles  from  the  beginning 
of  the  ileum,  and  twine  through  the  "  fatty  body "  and  in 
the  abdominal  cavity. 

Reproductive  System 


OF  THE  MALE. 


OF  THE  FEMALE. 


The  testes  are  paired  organs,  sur- 
rounded by  the  fatty  body  below 
the  5th  and  6th  abdominal 
terga.  They  atrophy  in  the 
adult. 

From  the  testes,  two  narrow  ducts 
or  vasa  deferentia  lead  to  two 
seminal  vesicles. 

These  seminal  vesicles  (the  ' '  mush- 
room-shaped gland  ")  open  into 
the  top  of  the  ejaculatory  duct. 

This  duct  opens  on  the  loth  sternum. 
Beside  the  aperture  there  are 
copulatory  structures  (gona- 
pophyses).  With  the  ejaculatory 
duct  a  gland  is  associated. 


The  ovaries  are  paired  organs,  in 
the  posterior  abdominal  region, 
each  consisting  of  eight  ovarian 
tubes.  These  are  bead-like 
strings  of  ova  at  various  stages 
of  ripeness. 

From  the  ovarian  tubes  of  each  side, 
eight  eggs  pass  at  a  time  into 
a  short  wide  oviduct. 


The  two  oviducts  unite  and  open  in 
a  median  aperture  between  the 
8th  and  Qth  abdominal  sterna. 
Beside  the  aperture  are  hard 
structures  (gonapophyses)  which 
help  in  the  egg  laying.  Here 
also  a  pair  of  "  collet erial " 
glands  pour  out  their  cementing 
secretion  by  two  apertures. 
The  spermatheca  is  a  paired 
sac  with  a  single  aperture  on 
the  Qth  abdominal  sternum. 


298        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

Sixteen  ova,  one  from  each  ovarian  tube,  are  usually 
enclosed  within  each  egg  capsule.  The  latter  is  formed 
from  the  secretion  of  the  colleterial  glands.  Each  egg  is 
enclosed  in  an  oval  shell,  on  which  there  are  several  little 
holes  (micropyles),  through  one  of  which  a  spermatozoon 
enters.  Spermatozoa,  from  the  store  within  the  spermatheca, 
are  included  in  the  egg  capsule.  The  development  is 
similar  to  that  of  other  insects,  and  it  has  already  been 
mentioned  that  there  is  ho  metamorphosis. 

At  an  early  stage  in  development,  some  cells  associated  with  the 
mesoderm  are  set  apart  as  reproductive  cells,  and  originally  these  have 
a  segmental  arrangement  as  in  Annelids  ;  at  a  later  stage  other  meso- 
derm cells  join  these,  some  forming  ova,  others  epithelial  cells  around  the 
latter.  The  distinction  between  truly  reproductive  cells  and  associated 
epithelial  cells,  which  is  said  to  be  late  of  appearing  in  some  of  the 
higher  insects,  is  established  at  a  very  early  stage  in  the  cockroach. 


Second  Type  of  Insects.     The  BRITISH  HIVE  BEE 
(Apis  mellifica.) 

This  is  a  much  more  highly  specialised  type  than  the 
cockroach.  It  belongs  to  the  order  Hymenoptera. 

The  Hive  Bee  (Apis  mellifica)  is  a  native  of  this  country, 
and  is  the  species  most  commonly  found  domesticated.  It 
is  the  only  British  representative  of  the  genus  Apis,  and 
exhibits,  in  its  most  fully  developed  form,  the  social  life 
which  is  foreshadowed  among  the  Humble  Bees.  As  a 
consequence  of  this  social  life,  there  is  much  division  of 
labour,  which  expresses  itself  alike  in  habit  and  in  structure. 
The  males  (drones)  take  no  part  in  the  work  of  the  colony, 
and  have  solely  a  reproductive  function ;  the  females  are 
divided  into  two  groups — the  queen  bees  and  the  workers. 
In  the  workers,  which  do,  in  fact,  perform  all  the  work  of 
the  hive,  the  vegetative  organs  attain  their  highest  degree 
of  development,  but  the  reproductive  organs  are  normally 
abortive  and  functionless.  In  the  queens,  of  which  there  is 
but  one  adult  to  each  hive,  the  enormous  development  of 
the  reproductive  organs  seems  to  act  as  a  check  upon  the 
vegetative  organs,  which  are  of  less  advanced  type  than 
those  of  the  workers.  The  workers  are  further  divisible  into 
nurses,  which  are  young  and  do  not  leave  the  hive,  being 


THE  BRITISH  HIVE  BEE.  299 

occupied  with  the  care  of  the  larvae,  and  the  foraging  bees, 
which  are  older  workers,  and  gather  the  food  to  supply  the 
whole  colony. 

In  considering  the  relation  between  the  life  of  the  Hive 
Bee  and  that  of  many  allied  forms  (Bombus,  &c.),  it  is 
important  to  notice  that  the  habit  of  laying  up  stores  of 
food  material  for  the  winter,  enables  the  colony,  and  not 
merely  an  individual,  to  survive,  and  must  thus  have  greatly 
assisted  in  the  evolution  of  sociality. 

The  body  shows  the  usual  division  into  head,  thorax, 
and  abdomen,  and  varies  considerably  in  the  three  different 
types,  being  smallest  in  the  workers.  It  is  entirely  covered 
with  hairs,  some  of  which  are  sensitive,  while  others  are 
used  in  pollen  gathering,  &c. 

The  head  bears  antennae,  which  are  composed  of  a  long 
basal  and  numerous  smaller  joints.  They  are  marvellously 
sensitive,  serving  to  communicate  impressions,  and  also  con- 
taining organs  of  special  sense.  A  pair  of  compound  eyes, 
largest  in  the  drones,  and  three  median  ocelli  are  also 
present  in  the  head  region.  Of  the  true  appendages  of  the 
head,  the  mandibles  are  in  the  workers  very  powerful  and 
used  for  many  purposes  connected  with  comb  building.  In 
the  first  maxillae,  the  maxillary  palps  are  aborted,  but  internal 
lacinia,  external  galea,  and  basal  stipes  and  cardo  are  present 
as  usual.  The  second  pair  of  maxillae  are  much  modified 
to  form  the  labium  or  so-called  lower  lip.  The  united 
basal  joints  form  the  mentum  and  sub-mentum.  From  the 
mentum  at  either  side  springs  the  long  labial  palp,  which 
represents  the  outer  fork  of  the  typical  appendage.  The 
inner  fork  is  divided  into  two  parts  at  each  side,  of  these 
the  inner  (laciniae)  are  united  and  much  elongated,  the  two 
outer  or  paraglossae  are  free  and  closely  apposed  to  the 
laciniae;  the  whole  structure  is  known  as  the  ligula.  When  the 
bee  is  engaged  in  sucking  honey  from  a  flower,  the  maxillae 
and  labial  palps  are  closely  apposed  to  the  ligula,  and  thus 
an  air-tight  tube  is  formed.  When  not  in  use,  the  whole 
structure  is  folded  back  upon  itself. 

In  the  queen  and  in  the  drone  the  mouth  parts  are 
shorter,  and  are  not  used  in  honey  gathering. 

The  thoracic  appendages  consist  as  usual  of  three  pairs 
of  legs,  which  have  the  usual  parts.  On  the  first  leg,  at 


300        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 


the  junction  of  the  tibia  and  the  first  tarsal  joint,  there  is  a 
complicated  mechanism  which  is  employed  in  cleaning  the 
antennae ;  this  is  present  in  all  three  forms,  and  varies  with 
the  size  of  the  antennae.  In  the  workers  the  third  leg  is 
remarkably  modified  for  pollen  gathering  purposes.  The 
first  tarsal  joint  bears  regular  rows  of  stiff  straight  hairs  on 
which  the  pollen  grains  are  collected  ;  they  are  borne  to  the 
hive  in  the  pollen  basket,  placed  at  the  back  of  the  tibia, 
and  furnished  with  numer- 
ous hairs.  In  queen  and 
drone,  these  special  ar- 
rangements of  hairs  are 
absent. 

The  second  and  third 
thoracic  segments  bear 
each  a  pair  of  wings. 
These  are  largest  in  the 
drones  and  relatively 
smallest  in  the  queen, 
who  flies  but  seldom.  At 
the  base  of  each  wing 
there  is  a  respiratory 
spiracle. 

In  the  adult  queen  and 
worker,  the  abdomen  is 
divided  into  six  segments  ; 
in  the  drone,  into  seven. 
There  are  no  abdominal 
appendages.  On  the  ven- 
tral surface  in  the  worker, 
but  not  in  the  queen  or 
drone,  there  are  four  pairs 
of  wax  pockets  or  glands, 
which  secrete  the  wax 
which,  after  mastication 
with  saliva,  is  employed  in 
building  the  combs.  The  abdomen  also  bears  in  queen  and 
worker  five  pairs  of  spiracles,  but  in  the  drone,  on  account  of 
the  additional  segment,  there  are  six  pairs.  The  total  number 
of  spiracles  is  thus  fourteen  for  queen  and  worker,  and  sixteen 
for  the  drone.  The  posterior  region  of  the  abdomen  bears 


FIG.  97. — Head  and  mouth  parts 
of  Bee.     (After  CHESHIRE.) 

a,  Antenna ;  in,  mandible ;  g;  gum  flap 
or  epipharynx ;  mx.p,  maxillary  palp ; 
pg  and  mx,  galea  and  lacinia ;  /./,  labial 
palp  ;  /,  ligula  ;  b,  bouton  at  end. 


NERVOUS  AND  ALIMENTARY  SYSTEMS.          301 

the  complicated  sting.  In  the  worker,  this  consists  of  a  hard 
incomplete  sheath,  which  envelops  two  barbed  darts.  The 
poison  flows  down  a  channel  lying  between  the  darts  and 
the  sheath.  Ramifying  through  the  abdomen  are  found  the 
two  slender  coiled  tubes  which  constitute  the  poison  gland. 
At  the  posterior  end  of  the  body  these  unite  and  open  into 
a  large  poison  sac.  When  a  bee  uses  its  sting,  the  chitinous 
sheath  first  pierces  the  skin,  and  then  the  wound  is  deepened 
by  the  barbed  and  pointed  darts,  while  at  the  same  time 
poison  is  steadily  pumped  down  the  channel  mentioned 
above,  and  pours  out  by  minute  openings  at  the  bases  of 
the  darts.  The  poison  contains  formic  acid,  and  is  fatal  to 
the  bee  if  directly  introduced  into  its  blood.  Associated 
with  the  sting  there  are  a  pair  of  delicate  tactile  palps.  In 
the  queen,  the  sting  is  curved  and  more  powerful,  but  it  is 
apparently  only  used  in  combat  with  a  rival.  In  the  worker, 
the  sting,  and  with  it  a  portion  of  the  gut,  is  usually  lost 
after  use,  and,  in  consequence,  death  ensues  ;  the  queen,  on 
the  other  hand,  can  withdraw  her  sting  from  the  wound  with 
considerable  ease.  There  is  no  trace  of  sting  in  the  drone, 
as  is  natural  when  we  consider  that  it  is  merely  a  modifica- 
tion of  an  ovipositor. 

Nervous  System. 

In  the  adult  this  exhibits  considerable  fusion  of  parts. 
The  supra-cesophageal  ganglia  are  very  large,  and  send  large 
lateral  extensions  to  the  compound  eyes.  This  "  brain  " 
is  best  developed  in  the  active  workers.  The  sub-cesopha- 
geal  mass  is  formed  by  the  fusion  of  three  pairs  of  ganglia. 
In  the  thorax  there  are  two  pairs  of  ganglia,  of  which  the 
second  supplies  the  wings  and  the  two  last  pairs  of  legs. 
In  the  worker  there  are  five  pairs  of  abdominal  ganglia,  but 
in  the  queen  and  drone  only  four.  The  sense  organs  are 
the  simple  and  compound  eyes,  and  the  antennae,  which  are 
furnished  with  numerous  sensitive  structures. 

Alimentary  System. 

The  oesophagus  is  a  narrow  tube  which  runs  down  the 
thoracic  region.  In  the  abdominal  region  it  expands  into 
the  crop  or  honey  sac.  The  crop  opens  by  a  complicated 


302        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

orifice,  with  a  remarkable  stopper  arrangement,  into  the 
digestive  region  or  chyle  stomach,  which  is  separated  by  a 
pylorus  from  the  coiled  small  intestine.  The  inner  wall  of 
the  small  intestine  bears  numerous  rows  of  chitinous  teeth 
set  in  longitudinal  ridges,  and  is  perforated  by  the  apertures 
of  the  excretory  tubules.  At  the  junction  of  the  small  with 
the  large  intestine,  there  are  six  brownish  plates,  perhaps 
functioning  as  valves. 

In  connection  with  the  anterior  region  of  the  gut,  there  is  a  very 
complicated  series  of  glands.     First,  we  have  in  the  workers  only,  on 


B 


FIG.  98. — Nervous  system  of  Bee.     (After  CHESHIRE.) 

A,  Of  larva ;  B,  of  adult ;  «,  antenna ;  mx,  maxilla  ;  »*,  mandible  ; 
w,  origin  of  wing  ;  1-5,  abdominal  ganglia. 

either  side  of  the  head,  a  long  coiled  gland  which  is  intracellular  in 
type.  ^  It  is  largest  in  the  so-called  "  nurses  "  which  feed  the  young,  and 
diminishes  in  size  kter.  According  to  Mr.  Cheshire,  this  gland  secretes 
a  nitrogenous  fluid  which  is  furnished  to  all  the  larvae  in  their  early 
stages,  but  is  supplied  to  the  future  queen  during  the  whole  of  the 


OTHER  SYSTEMS. 


303 


feeding  period,  and  also  during  the  period  of  egg  laying  ;  this  secretion 
was  formerly  termed  **  royal  jelly."  In  addition  to  this  pair  of  glands, 
there  are  in  the  worker  three  other  gland  systems.  Of  these,  the  second 

and  third  pairs  have  a  common 
central  outlet  on  the  mentum,  and 
secrete  the  saliva  which  is  plen- 
tifully mixed  with  the  nectar  dur- 
ing suction.  The  fourth  pair  is 
small,  and  the  ducts  open  just 
within  the  mandible.  The  last 
three  pairs  of  glands  are  found 
also  in  drone  and  queen. 

The  method  of  feeding  in 
the  bee  differs  considerably 
in  the  three  types.  In  the 
worker,  the  honey  sucked  up 
from  flowers  is  mixed  with 
saliva,  passes  down  the  gul- 
let into  the  crop,  thence  by 
the  opening  of  the  "stomach 
mouth  "  it  may  reach  the 
true  stomach  and  so  be  di- 
gested,  or  may  be  carried  in 
the  crop  to  the  hive  and 
there  emptied  into  the  cells 
by  regurgitation.  The  pol- 
len, which  is  frequently  mixed 
with  the  honey,  is  separated 
from  the  latter  by  means  of 
the  stomach  mouth,  and  is 
digested.  Before  impregna- 
tion, the  queen,  like  the 


FIG.  99.  —  Food  canal  of  Bee. 
part  after  CHESHIRE.) 


(In 


mx>  Maxilla  ;  a,  antenna  ;  e,  eye  ;  s.g; 
salivary  glands  ;  a?,  oesophagus  ;  h.s, 
honey  sac;  s,  stopper;  c.s,  chylific 
stomach  ;  m.t,  malpighian  tubules  :  j.z", 
small  intestine  ;  /./,  large  intestine  ;  st, 
sting. 


worker,  feeds  on  pollen  and 
honey;  after  it,  she  is  always 
fed  by  the  attendant  workers. 
The  drones,  like  the  young 

workers,  avail  themselves  of  the  general  food  supply  of  the 

colony,  and  do  not  themselves  collect  honey. 

Other  Systems. 

The  respiratory  system  is  represented  by  the  ramifying 
tracheal  tubes.  They  open  to  the  exterior  by  the  lateral 
spiracles,  which  can  be  completely  closed.  In  connection 


304        PERIPATUS,    MYRIOPODS,   AND  INSECTS. 

with  the  tracheae  there  are  large  air  sacs  which  aid  greatly  in 
flight. 

The  circulatory  system  is  in  essentials  the  same  as  that 
of  the  cockroach.  The  blood  contains  a  few  nucleated 
amoeboid  corpuscles. 

The  excretory  system  consists  of  numerous  fine  Mal- 
pighian  tubules  which  open  into  the  small  intestine. 

Reproductive  System. 

In  the  drone  the  reproductive  organs  consist  of  a  pair  of 
testes,  each  furnished  with  a  narrow  vas  deferens,  expanding 
at  its  distal  end  into  a  seminal  vesicle.  The  seminal  vesicles 
open  into  the  ejaculatory  duct,  and  at  their  junction  a  large 
paired  mucus  gland  opens.  When  maturity  is  reached  the 
testes  diminish  in  size,  while  the  spermatozoa  accumulate  in 
the  terminal  expanded  part  of  the  ejaculatory  duct,  and  there 
become  aggregated  into  a  compact  spermatophore.  With 
the  terminal  portion  of  the  male  duct  copulatory  organs 
are  associated. 

Mating  takes  place  only  once  in  the  life  of  the  queen,  and 
is  followed  by  the  death  of  the  drone. 

In  the  queen  the  large  ovaries  occupy  considerable  space  in  the  abdo- 
minal region.  As  usual,  each  consists  of  numerous  (100-150)  ovarian 
tubes  containing  ova  in  various  stages  of  development.  The  ovarian 
tubes  open  into  the  right  and  left  oviducts,  which  again  unite  to  form  the 
common  oviduct.  With  the  anterior  portion  of  the  common  duct  the 
globular  spermatheca  is  associated.  In  connection  with  it  there  is  a 
gland  corresponding  to  the  mucus  gland  of  the  male.  The  oviduct 
terminates  in  a  copulatory  pouch. 

Previous  to  laying,  the  eggs  are  fertilised  by  sperms  set  free  from  the 
spermatheca.  In  the  case  of  drone  eggs  this  liberation  of  spermatozoa 
does  not  take  place,  and  the  eggs  in  consequence  are  parthenogenetic. 
Queens  which  have  never  mated,  or  which  have  exhausted  their  stock  of 
male  elements,  habitually  lay  drone  eggs,  but  those  which  are  laying 
abundant  fertilised  eggs  at  times  also  lay  unfertilised  eggs.  This  with- 
holding of  spermatozoa  is  said  to  be  "voluntary,"  and  related  to  the 
needs  of  the  colony,  but  the  physiological  reason  is  unknown. 

The  workers  possess  female  organs  similar  in  type  to  those  of  the 
queen,  but  of  an  extremely  rudimentary  nature. 

The  eggs  are  laid  singly  in  the  cells  of  the  comb,  at  the  rate  of  about 
two  per  minute,  for  weeks  together.  They  are  of  the  usual  insect  type. 
According  to  the  size  of  the  cell  in  which  it  is  deposited,  and  the  food 
with  which  it  is  furnished,  the  fertilised  ovum  develops  into  a  worker 
or  into  a  queen.  The  development  takes  place  within  the  cell,  and 
includes  a  complete  metamorphosis. 


CLASSIFICATION  OF  INSECTS. 


305 


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Hymenoptera.  Ants,  bees,  wasps,  gall  flies,  saw  : 
Menogn.  or  Metagn.  ,  or  a  sort  of  compromise  1 
with  four  transparent  wings.  Larvas  are  footle* 

Lepidoptera.  Butterflies  and  moths. 
Metagn.  Two  pairs  of  uniform,  scaly  wings. 

Diptera.  Two  winged  flies.  House  fly,  gad  fly, 
Metagn.,  but  sometimes  with  power  of  biting, 
folded  wings,  and  posterior  "balancers"  or  ' 
footless  maggot,  without  a  distinct  head. 

Siphonaptera  or  Aphaniptera.  Fleas. 
Metagn.  ,  but  also  with  power  of  piercing.  Nc 
Ectoparasitic.  Larva  —  a  footless  maggot. 

Coleoptera.  Beetles. 
Menogn.  ,  rarely  Metagn.  Fore  wings  modifie 
folded  when  not  in  use.  Larvas  very  diverse,  £ 
bee  parasites  Strepsiptera  are  probably  allied. 

Trichoptera.  Caddis  flies. 
Menogn.  Hind  wings  usually  larger  than  fore 
The  body  is  hairy,  rarely  scaly.  The  larvae 
usually  live  in  water  within  special  cases,  and  c 

Panorpata.  Scorpion  flies. 
Menogn.  Two  pairs  of  narrow  membranoi 
Larva  —  like  a  caterpillar. 

Neuroptera.  Ant  lions  and  lace  winged  flies. 
Menogn.  Two  pairs  of  glassy  wings  with  man 
live  in  water,  and  have  tracheal  gills. 

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306         PERIPATUS,   MYRIOPODS,    AND  INSECTS. 


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Rhynchota  or  Hemiptera,  e.g.,  Phylloxera,  Aphide 
bugs,  water  scorpions,  lice.  (The  male  cocci 
metamorphosis.  ) 
The  mouth  parts  are  adapted  for  sucking  and  for 
of  wings  or  none.  The  parasitic  forms  have  no 
several  respects  degenerate. 
Thysanoptera,  e.g.,  Thrips. 
Ametab.  Suctorial  mouth  organs.  Wings  very  r 
absent.  Only  three  or  four  pairs  of  stigmata.  O 
Corrodentia,  e.g.,  Bird  lice,  termites. 
Ametab.  Mouth  parts  adapted  for  biting.  Win 
lice  have  no  compound  eyes. 
Orthoptera,  e.g.,  Cockroach,  locust,  cricket,  mole 
"  walking  leaf." 
Ametab.  Mouth  parts  adapted  for  biting.  Anteri 
firmer  than  those  behind,  or  modified  into  wing  cc 
times  absent. 

Plecoptera,  e.g.,  Perla. 
Hemimetab.  Mouth  parts  adapted  for  biting.  Tw< 
The  larvae  live  in  water,  and  breathe  by  tracheal 

the  adult. 
Odonata,  Dragon  flies. 
Hemimetab.  Mouth  parts  adapted  for  biting. 
•  wings.  The  larvae  live  in  water,  and  breathe  by 

Ephemerida,  May  flies. 
Hemimetab.  Mouth  parts  of  adult  somewhat  c 
Fore  wings  large,  hind  wings  small  or  absent.  L 
by  tracheal  gills,  and  have  biting  mouth  organs. 
Dermaptera,  Earwigs. 
Ametab.  Mouth  parts  adapted  for  biting.  Ant< 
large,  but  folded  both  longitudinally  and  crosswii 

Collembola,  Springtails,  e.g.,  Podura,  Smynthurus. 
Thysanura,  e.g.  Campodea,  Lepisma. 

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FORM— APPENDAGES.  307 


GENERAL   NOTES    ON    INSECTS. 

The  main  characteristics  of  insects  have  already  been 
described  in  the  two  types  chosen,  but  we  here  revise  them 
in  general  terms. 

Form. 

The  body  of  an  adult  insect  may  be  divided  into  three 
distinct  regions  : — 

1.  The  undivided  head,  which  consists  of  at  least  three  fused  seg- 

ments, as  it  bears  three  pairs  of  appendages. 

2.  The  median  thorax,  divided  into  pro-,  meso-,  and  meta-thoracic 

segments,  each  with  a  pair  of  legs,  the  last  two  often  with 
wings. 

3.  The  abdomen  with  about  eleven  rings,  usually  without  trace  of 

limbs. 

But  this  is  only  the  crude  anatomy  of  form.  One  must 
think  of  the  long  dragon  fly  with  outspread  wings,  and  of  the 
compact  cockchafer,  of  the  thin-waisted  wasps  and  long- 
bodied  butterflies,  of  house  fly  and  cricket,  of  large  moths 
and  beetles,  and  the  almost  invisible  insect  parasites. 

Appendages. 

Insects  "feel  their  way,"  test  food,  and  apparently  com- 
municate impressions  to  one  another  by  means  of  a  pair  of 
jointed  feelers  or  antennae,  situated  in  front  of  the  head. 
Unlike  the  organs  of  a  similar  name  in  Crustaceans,  the 
antennae  are  not  usually  ranked  among  the  appendages 
strictly  so-called.  They  seem  to  be  pre-oral  outgrowths. 

It  was  a  step  of  some  importance  in  morphology  when  Savigny 
showed  that  the  three  pairs  of  appendages  about  the  mouth  were 
homologous  with  the  other  appendages,  i.e.,  were  masticatory  legs. 

(i.)  Furthest  forward  lie  two  mandibles,  the  biting  and  cutting  jaws. 
These  are  single  jointed,  and  thus  differ  from  the  organs  of  the  same 
name  in  the  crayfish,  which  bear  a  three-jointed  palp  in  addition  to  the 
hard  basal  part.  In  those  insects  which  suck  and  do  not  bite,  e.g.,  adult 
butterflies,  the  mandibles  are  reduced. 

(2.)  Next  in  order  is  \h.e  first  pair  of  maxillce.  Each  maxilla  consists 
of  a  basal  piece  (protopodite),  an  inner  fork  (endopodite),  and  an  outer 
fork  (exopodite).  I  use  these  names  from  Crustacean  terminology, 


308        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

after  the  example  of  Marshall  and  Hurst.  The  entomologists  divide 
the  protopodite  into  a  lower  joint  the  cardo,  and  an  upper  the  stipes,  the 
endopodite  into  an  internal  lacinia,  and  an  external  galea,  while  the 
exopodite  is  called  the  maxillary  palp. 

(3.)  The  last  pair  of  oral  appendages  or  second  maxilla  are  partially 
fused,  and  form  what  is  called  the  labium.  The  lower  and  upper  joints 
of  their  fused  protopodites  are  called  submentum  and  mentum  ;  the 
endopodites  on  each  side  are  double  as  in  the  first  maxillae,  and  consist 
of  internal  lacinia  and  external  paraglossa  ;  the  exopodites  are  called 
the  labial  palps. 

The  three  pairs  of  thoracic  legs  consist  of  many  joints, 
are  usually  clawed  and  hairy  at  their  tips,  and  vary  greatly 
according  to  their  uses.  Think,  for  instance,  of  the  hairy 
feet  by  aid  of  which  the  fly  runs  up  the  smooth  window  pane, 
of  the  muscular  limbs  of  grasshoppers,  of  the  lank  length  of 
those  which  characterise  "daddy-long-legs,"  of  the  pollen 


FIG.  100. — Joints  of  Cockroach's  Leg. 

C,  Coxa  ;  T,  trochanter  ;  F,  femur  ;  Tz,  tibia  ;  Ta,  tarsus  with 
terminal  claws. 

baskets  on  bees,  of  the  oars  of  water  beetles.  In  identifying 
insects  from  a  book  it  is  needful  to  recognise  the  joints  of 
the  legs  by  the  names  which  entomologists  have  transferred 
to  them  from  human  osteology,  viz.,  the  superior  coxa  with 
projecting  trochanter,  the  stout  femur,  the  tibia,  and  finally 
numerous  tarsal  joints. 

Wings. 

Wings  are  flattened  hollow  sacs,  which  grow  out  from  the 
two  posterior  segments  of  the  thorax.  They  are  moved  by 
muscles,  and  traversed  by  "veins"  or  "  nervures,"  which 
include  air-tubes,  nerves,  and  vessel-like  continuations  of 
the  body  cavity.  Most  insects  have  two  pairs,  but  many 
sluggish  females  and  parasites  like  lice  and  fleas  have  lost 
them.  On  the  other  hand,  there  is  no  reason  to  believe 


WINGS. 


309 


that  the  very  simplest  wingless  insects,  known  as  Collembola 
and  Thysanura,  ever  had  wings. 

There  are  many  interesting  differences  in  regard  to  wings  in  the 
various  order  of  Insects.  Thus,  in  beetles,  the  front  pair  form  wing  covers 
or  elytra,  in  the  little  bee  parasites — Strepsiptera — they  are  twisted 
rudiments,  in  flies  the  posterior  pair  are  small  knobbed  stalks  (halteres 
or  balancers),  in  bees  the  wings  on  each 
side  are  hooked  together.  When  the 
insect  is  at  rest,  the  wings  are  usually 
folded  neatly  on  the  back  ;  but  dragon 
flies  and  others  keep  them  expanded, 
butterflies  raise  them  like  a  single  sail 
on  the  back,  moths  keep  them  flat. 
Many  wings  bear  small  scales  or  hairs 
and  are  often  brightly  coloured.  Pro- 
fessor Eimer  maintains  that  the  arrange- 
ment of  the  nervures  and  the  colouring 
of  butterfly  wings  are  certain  marks  of 
the  progress  and  relationships  of  species. 
It  is  well  known  that  the  colours  also 
vary  with  sex,  climate,  and  surroundings. 
Most  interesting  are  those  cases  in  which 
the, colours  of  an  insect  harmonise  ex- 
actly with  those  of  its  habitat,  or  make 
it  a  mimetic  copy  of  some  more  success- 
fully protected  neighbour. 

As  to  the  origin  of  wings,  this  at  least 
should  be  remembered,  that  in  many 
cases  they  are  of  some  use  in  respiration 
as  well  as  in  locomotion.  Seeing  that 
the  power  of  flight  is  evidently  an  accom- 
plishment which  the  original  insects  did 
not  possess,  the  theory  seems  plausible 
that  wings  were  originally  respiratory 
outgrowths,  which  by-and-by  became 
useful  for  aerial  locomotion.  This  view 
is  consistent  with  an  idea,  which  grows  in  favour  with  evolutionists, 
that  new  organs  develop  by  the  predominance  of  some  new  function  in 
organs  which  had  some  prior  significance.  Moreover,  we  can  fancy 
that  an  increase  in  respiratory  efficiency  brought  about  by  the  out- 
growths in  question  would  quicken  the  whole  life,  and  would  tend  to 
raise  insects  into  the  air,  just  as  terrestrial  insects  can  be  made  to  frisk 
and  jump  when  placed  in  a  vessel  with  relatively  more  oxygen  than 
there  is  in  the  atmosphere.  Finally,  we  must  note  that  the  aquatic 
larvae  of  some  insects,  e.g.,  May  flies,  have  a  series  of  respiratory  out- 
growths from  the  sides  of  the  abdomen,  the  so-called  "tracheal  gills," 
which  in  origin  and  appearance  are  like  young  wings. 

Insects  excel  in  locomotion.     "  They  walk,  run,  and  jump 
with  the  quadrupeds ;  they  fly  with  the  birds ;  they  glide 


FIG.  1 01. — Young  May 
fly  or  Ephemerid.  (After 
EATON.) 

Showing  tracheal  gills,  and 
wings  appearing  in  front  of 
them. 


3io        PERIPATUS,    MYRIOPODS,   AND  INSECTS. 

with  the  serpents,  and  they  swim  with  the  fish."  They  beat 
the  elastic  air  with  their  wings,  and  though  there  cannot  be 
so  much  complexity  of  movement  as  in  birds  where  the 
individual  feathers  move,  the  insect  wing  is  no  rigid  plate, 
and  its  up  and  down  motions  are  complex.  They  can  soar 
rapidly,  but  their  lightness  often  makes  horizontal  steering 
difficult.  The  wind  often  helps  as  well  as  hinders  them ; 
thus  the  insects  which  fly  in  and  out  of  the  windows  of 
express  trains  are  probably  in  part  sucked  along.  Marey 
calculates  the  approximate  number  of  wing  strokes  per 
second  at  330  for  the  fly,  240  for  the  humble  bee,  190  for  the 
hive  bee,  no  for  the  wasp,  28  for  the  dragon  fly,  9  for  a 
butterfly.  It  has  been  found  that  for  short  distances  a  bee 
can  out-fly  a  pigeon. 

Skin. 

As  in  other  Arthropods,  the  epidermis  (or  hypodermis) 
of  Insects  forms  a  firm  cuticle  of  chitin,  which  in  the 
exigencies  of  growth  has  sometimes  to  be  moulted.  This 
cuticle  is  often  finely  marked,  so  that  the  animal  seems 
iridescent,  and  there  are  many  different  kinds  of  scales, 
hairs,  and  spines.  Chitin  is  not  favourable  to  the  develop- 
ment of  skin  glands,  but  most  insects  have  "  salivary  glands," 
opening  in  or  near  the  mouth,  bees  have  wax-making  glands 
opening  on  the  abdomen,  aphides  have  "  honey-dew"  tubes, 
not  a  few  have  poison  bags,  and  many  larvae  besides  silk- 
worms have  organs  from  which  are  exuded  the  threads  of 
which  a  cocoon  is  made. 

Muscular  System. 

In  very  active  animals  like  Insects,  we  of  course  find  a 
highly  developed  set  of  rapidly  contracting  striped  muscles. 
These  work  the  wings,  the  legs,  and  the  jaws.  The  result- 
ing movements  have  this  further  significance  that  they  help 
in  the  respiratory  interchange  of  gases,  and  in  the  circula- 
tion of  the  blood. 

Nervous  System. 

It  is  often  remarked  as  marvellous  that  ants  and  bees, 
with  brains  smaller  than  pin  heads,  should  be  so  clever. 
The  more  we  know  about  an  ant,  "  the  more  the  wonder 
grows,  so  small  a  head  should  carry  all  it  knows,"  or  seems 


SENSORY  STRUCTURES.  311 

to  know.  But  these  statements  imply  forgetfulness  of  the 
relative  size  of  brain  to  body,  and  tend  moreover  to  exagger- 
ate the  importance  of  mere  size.  The  complexity  of  a  brain 
is  the  important  fact,  not  its  size,  and  there  is  no  doubt  that 
the  cleverer  insects  (ants,  bees,  and  wasps),  have  more  com- 
plex brains  than  the  others.  As  in  other  Arthropods,  the 
nervous  system  consists  (a)  of  a  dorsal  brain  or  supra- 
cesophageal  ganglionic  mass,  and  (fr)  of  a  double  ventral 
nerve  cord  with  a  number  of  paired  ganglia  of  which  the 
most  anterior  (the  sub-cesophageal)  are  linked  to  the  brain 
by  a  ring  commissure  around  the  gullet,  and  (c)  of  nerves 
given  off  from  the  various  ganglia  to  the  sense  organs,  the 
alimentary  canal,  and  the  other  organs.  In  many  of  the 
higher  insects  the  ganglia  of  the  ventral  nerve  cord  are  in 
some  degree  concentrated,  and  the  adults  are  usually  more 
centralised  than  the  larvae. 

Sensory  Structures. 

Animals  so  much  alive  as  Insects,  and  in  surroundings  so 
stimulating  as  many  of  them  enjoy,  have  naturally  highly 
developed  sense  organs. 

Two  compound  eyes  are  present  on  the  head  of  all  adults 
except  the  primitive  Collembola,  the  degenerate  lice,  the 
likewise  parasitic  fleas,  and  blind  insects  which  live  in  caves 
or  other  dark  places.  Each  eye  contains  a  large  number  of 
similar  elements,  in  each  of  which  we  can  distinguish  (i)  a 
cuticular  or  corneal  facet;  (2)  a  glassy  lens-like  portion;  (3)  a 
retinal  portion  in  association  with  which  are  fibres  from  the 
optic  nerve,  and  there  are  also  pigmented  cells  between  the 
elements  (cf.  p.  259). 

Simple  eyes  or  ocelli  are  present  in  addition  to  the  com- 
pound eyes  in  the  adults  of  many  insects,  e.g.,  ants,  bees, 
and  wasps  ;  they  occur  without  the  accompaniment  of  com- 
pound eyes  in  Collembola,  lice,  and  fleas,  and  they  are 
usually  the  only  eyes  possessed  by  larvae.  They  have  only 
one  lens  (monomeniscous),  whereas  the  compound  forms 
have  many  lenses  (polymeniscous).  Their  structure  varies 
greatly,  and  their  use  is  very  uncertain. 

Auditory  (or  chordotonal)  organs  have  been  found  in  all  orders  of 
Insects  (except  as  yet  the  Thysanoptera),  and  occur  both  in  the  larvae  and 
in  the  adults.  Their  essential  structure  is  as  follows  : — A  nerve  ends  in 


312        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

a  centre  or  ganglion  near  the  skin,  some  of  the  cells  of  this  ganglion  grow 
out  into  long  sensitive  rods  enclosed  in  a  tiny  sheath,  the  rods  are  directly 
or  indirectly  connected  with  the  epidermis  above  them.  *'  They  are  found 
in  groups  of  2-200  in  various  parts  of  the  body,  antennae,  palps,  legs, 
wings,  in  the  halteres  of  Diptera,  and  upon  the  dorsal  aspect  of  the  abdo- 
men." Quite  different  from  these,  and  occurring  in  flies  alone,  on  the 
hind  end  of  the  larva,  or  at  the  base  of  the  adult's  feelers,  are  little  bags 
with  fluid  in  which  clear  globules  float.  We  do  not  know  how  much  or 
how  little  Insects  hear,  but  the  "song"  of  male  Cicadas  and  crickets 
does  not  fall  on  deaf  ears. 

In  addition  to  the  "eyes"  and  "ears"  there  are  innervated  hairs 
(tactile,  tasting,  olfactory)  on  the  antennae  and  mouth  parts  of  many 
insects.  Not  a  few  have  been  shown  to  possess  a  diffuse  or  dermatoptic 
sense,  by  which,  for  instance,  they  can,  when  blinded,  find  their  way 
out  of  a  dark  box. 

Many  Insects  produce  sounds  which  often  express  a  variety  of  emo- 
tions. We  hear  the  whirr  of  rapidly  moving  wings  in  flies,  the  buzz  of 
leaf-like  structures  near  the  openings  of  the  air-tubes  in  many  Hymenop- 
tera,  the  scraping  of  legs  against  wing  ribs  in  grasshoppers,  the  chirping 
of  male  crickets  which  rub  one  wing  against  its  neighbour,  the  piping  of 
male  Cicadas  which  have  a  complex  musical  instrument,  the  voice  of  the 
death's-head  moth  which  expels  air  forcibly  from  its  mouth.  The  death 
watch  taps  with  his  head  on  wooden  objects,  as  if  knocking  at  the  door 
behind  which  his  mate  may  be  hidden.  In  some  cases  the  sounds  are 
simply  automatic  reflexes  of  activity ;  in  many  cases  they  serve  as  allur- 
ing love  calls,  and  they  may  also  serve  as  expressions  of  fear  and  anger, 
or  as  warning  alarms. 

Alimentary  System. 

The  diet  of  Insects  is  very  varied.  Some,  such  as  locusts, 
are  vegetarian,  and  destroy  our  crops  ;  others  are  carnivor- 
ous (we  need  not  specify  the  homoeopathist's  leech)  and 
suck  the  blood  of  living  victims,  or  devour  the  dead ;  the 
bees  flit  in  search  of  nectar  from  flower  to  flower,  while  the 
ant  lion  lurks  in  his  pit  of  sand  for  any  unwary  stumbler  ;  the 
termites  gnaw  decaying  wood ;  some  ants  keep  aphides  as 
cows  ("  vaccae  formicarum,"  Linnaeus  called  them),  whose 
sweet  juices  they  lick ;  and  a  great  number  of  larvae  devour 
the  flesh  and  vegetables  in  which  they  are  born. 

It  is  important  to  have  some  vivid  idea  of  the  diversity  of 
diet,  for  the  many  modifications  of  mouth  organs,  in  beetle 
and  bee,  in  caterpillar  and  butterfly,  as  well  as  differences  in 
the  alimentary  canal  itself,  are  associated  with  the  way  in 
which  the  insect  feeds. 

For  purposes  of  classification,  the  following  distinctions  in  regard  to 
the  mouth  organs  are  very  useful : — 


ALIMENTARY  SYSTEM.  313 

(a)  The  mouth  parts  may  be  similar  in  all  stages  of  life,  and  adapted 

for  biting.     In  this  case  the  term  MENOGNATHA  (i.e.,  per- 
manently jawed)  is  applied  : — 

e.g.,  to  earwigs,  dragon  flies,  the  cockroach  order  (Orthop- 
tera),  the  beetle  order  (Coleoptera). 

(b)  The  mouth  parts  may  be  similar  in  all  stages  of  life,  and  adapted 

for  sucking.     In  this  case  the  term  MENORHYNCHA  (i.e.,  per- 
manently with  a  sucking  proboscis)  is  applied  : — 
e.g.,  to  bugs  of  all  sorts  (Rhynchota  or  Hemiptera). 

(c)  The  mouth  parts  may  be  adapted  for  biting  in  the  larva,  for  suck- 

ing in  the  adult.     In  this  case  the  term  METAGNATHA  (t.e., 
with  changed  jaws)  is  applied  : — 
e.g.,  to  butterflies  and  moths. 

The  alimentary  canal  consists  of  fore  gut,  mid  gut,  and 
hind  gut,  of  which  the  mid  gut  is  endodermic  and  the  result 
of  the  original  gastrula  cavity  (archenteron),  whereas  the 
other  two  regions  are  fore  and  hind  invaginations  of  the 
ectoderm,  and  therefore  lined  by  a  chitinous  cuticle. 

The  fore  gut  conducts  food,  and  includes  mouth  cavity, 
pharynx,  and  oesophagus,  the  latter  being  often  swollen  into 
a  storing  crop,  or  continued  into  a  muscular  gizzard  with 
grinding  plates  of  chitin. 

The  mid  gut  is  digestive  and  absorptive,  often  bearing  a 
number  of  glandular  outgrowths  or  caeca,  and  varies  in 
length  (in  beetles  at  least)  in  inverse  proportion  to  the 
nutritive  and  digestible  quality  of  the  food. 

The  hind  gut  is  said  to  be  partly  absorptive,  but  is 
chiefly  a  conducting  intestine,  often  coiled  and  terminally 
expanded  into  a  rectum  with  which  glands  are  frequently 
associated. 

In  association  with  the  alimentary  canal  are  various  glands  : — 

(a)  The  salivary  glands,  which  open  in  or  near  the  mouth.  They 
are  usually  paired  on  each  side,  and  provided  with  a  reservoir. 
They  arise  as  invaginations  of  the  ectoderm  near  the  mouth. 
Their  secretion  is  mainly  diastatic  in  function,  i.e.,  it  changes 
starchy  material  into  sugar  by  means  of  a  ferment.  Along 
with  these  may  be  ranked  the  "spinning  glands"  of  cater- 
pillars, &c.,  which  also  open  at  the  mouth.  They  secrete 
material  which  hardens  into  the  threads  used  for  the  cocoon. 
(b}  From  the  beginning  of  the  mid  gut,  blind  outgrowths  sometimes 
arise  (in  some  Orthoptera,  &c. ),  which  are  apparently  diges- 
tive. They  are  sometimes  called  pyloric  caeca.  In  other 
cases  (some  beetles)  there  may  be  more  numerous  and  smaller 
glandular  outgrowths  on  the  external  wall  of  the  mid  gut. 
(c)  From  the  hind  gut  arise  numerous  fine  Malpighian  tubes,  which 
are  certainly  excretory  in  function. 


3H        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

Respiratory  System. 

The  body  of  an  insect  is  traversed  by  a  system  of  air 
tubes  (tracheae),  which  open  laterally  by  special  apertures 
(stigmata),  and  by  means  of  numerous  branches  conduct 
the  air  to  all  the  recesses  of  the  tissues.  In  animals  which 
breathe  by  gills  or  lungs  the  blood  is  carried  to  the  air ;  in 
insects  the  air  permeates  the  whole  body.  But  how  does 
the  air  pass  in  and  out  ?  In  part,  no  doubt,  there  is  a  slow 
diffusion  ;  in  part,  the  movements  of  the  wings  and  legs  will 
help;  but  there  are  also  special  expiratory  muscles.  We 
see  their  action  when  we  watch  a  drone  fly  panting  on  a 
flower.  Inspiration  is  passive,  as  in  birds,  and  depends  on 
the  elasticity  of  the  skin  and  of  the  tracheal  walls;  expiration 
is  active,  and  depends  on  special  muscles.  These  are 
chiefly  situated  in  the  abdomen,  but  in  some  beetles  (at 
least)  they  are  also  present  in  the  metathorax. 

The  tracheae  seem  to  arise  as  tubular  ingrowths  of  skin, 
and,  primitively,  each  segment  probably  contained  a  distinct 
pair.  But  their  number  has  been  reduced,  and  many  are 
often  connected  into  a  system.  With  the  doubtful  excep- 
tion of  one  of  the  primitive  Collembola,  and  the  certain 
exception  of  caterpillars,  no  insects  have  any  tracheal 
openings  in  the  head  region.  There  are  rarely  more  than 
two  pairs  in  the  thorax,  there  are  often  six  to  eight  pairs  in 
the  abdomen,  the  maximum  total  is  ten  pairs.  Each  trachea 
is  kept  tense  throughout  the  greater  part  of  its  course  by 
internal  chitinous  thickenings,  which  apparently  have  a 
spiral  course.  The  branches  of  the  tracheae  penetrate 
into  all  the  organs  of  the  body,  carrying  oxygen  to  every 
part.  The  very  efficient  respiration  of  insects  must  be 
kept  in  mind  in  an  appreciation  of  the  general  activity  of 
their  life. 

As  the  conditions  of  larval  life  are  often  different  from  those  of  the 
adult  insects,  the  mode  of  respiration  may  also  differ  in  details. 

In  insects  without  marked  metamorphosis,  and  even  in  some  beetles 
in  which  the  metamorphosis  is  complete,  the  young  insect  and  the  adult 
both  breathe  by  tracheae  with  open  stigmata.  Both  are  said  to  be 
"  holopneustic." 

When  the  larvae  live  in  water,  the  tracheal  system  is  closed,  other- 
wise the  creatures  would  drown.  This  closed  condition  is  termed 
"apneustic."  These  larvse  (of  dragon  flies,  May  flies,  and  some  others) 


CIRCULATORY  SYSTEM.  315 

breathe  by  "  tracheal  gills  " — little  wing-like  outgrowths  from  the  sides 
of  the  abdomen,  rich  in  tracheae — or  by  tracheal  folds  within  the  rectum, 
in  and  out  of  which  water  flows.  In  either  case,  an  interchange  of  gases 
between  the  tracheae  and  the  water  takes  place.  In  adult  aerial  life, 
the  tracheae  of  the  body  acquire  stigmata,  and  the  insect  becomes 
"  holopneustic." 

In  most  insects  with  complete  metamorphosis,  the  larva  (e.g.,  cater- 
pillar or  grub)  has  closed  stigmata  on  the  last  two  segments  of  the 
thorax  (those  which  will  bear  wings),  but  there  is  a  pair  of  open  stigmata 
on  the  prothorax.  In  the  adult  the  reverse  is  true. 

There  are  some  other  modifications,  for  instance  what  obtains  in  the 
parasitic  larvae  of  some  flies,  e.g.,  gad  flies.  In  these  the  stigmata  are 
open  only  at  the  end  of  the  body.  In  all  cases,  however,  the  stigmata 
of  the  adult  are  already  present  as  rudiments  in  the  larva,  though  they 
may  not  open  till  adolescence  is  over. 

Circulatory  System. 

As  the  respiratory  system  is  very  efficient,  establishing 
the  possibility  of  gaseous  interchange  between  the  inmost 
recesses  of  the  body  and  the  external  medium,  it  is  natural 
that  the  blood  vascular  system  should  not  be  highly 
developed.  Within  a  dorsal  part  of  the  body  cavity,  known 
as  the  pericardium,  the  heart  lies,  swayed  by  special  muscles. 
It  is  a  long  tube,  usually  confined  to  the  abdomen,  usually 
of  eight  chambers,  with  paired  valvular  openings  on  its 
sides,  through  which  blood  enters  from  the  pericardium. 
The  blood  is  driven  forwards,  the  posterior  end  of  the 
heart  being  closed,  and  there  is  usually  an  anterior  aorta  or 
main  blood  vessel.  But,  for  the  most  part,  the  blood  cir- 
culates in  spaces  within  what  is  commonly  called  the  body 
cavity.  Such  a  circulation  is  often  described  as  lacunar. 
The  blood  may  be  colourless,  yellow,  red,  or  even  greenish, 
and,  in  some  cases,  haemoglobin,  the  characteristic  blood 
pigment  of  Vertebrates,  has  been  detected.  The  cells  of 
the  blood  are  amoeboid. 

Body  Cavity. 

One  is  apt  to -use  this  term  in  two  senses — for  the  primitive  body 
cavity  or  ccelome,  and  for  the  apparent  body  cavity  of  the  adult.  In 
discussing  the  development  of  Peripatus,  Sedgwick  notes  the  following 
characteristics  of  a  true  ccelome : — It  is  a  cavity  which  ( I )  does  not 
communicate  with  the  vascular  system  ;  (2)  does  communicate  by  neph- 
ridial  pores  with  the  exterior ;  (3)  has  the  reproductive  elements  developed 
on  its  lining ;  (4)  develops  either  as  one  or  more  diverticula  from  the 


316        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

primitive  enteron  (or  gut),  or  as  a  space  or  spaces  in  the  unsegmented 
or  segmented  mesoderm.     Now,  in  Arthropods  the  apparent  body  cavity 


FIG.  1 02. — Diagrammatic  cross  section  of  an  Inverte- 
brate, with  a  primary  body  cavity  (be)  which  is  shaded. 
(After  ZIEGLER.  ) 

ec.,  Ectoderm;  bl.,  bladder  of  nephridium  (as  in  Crustaceans);  ex.) 
excretory  duct ;  gn.,  genital  organ  ;  «.,  ventral  nerve  cord  ;  g.,  gut:  yC, 
funnels  of  nephridia  (as  in  worms). 

of  the  adult  is  not  a  true  coelome,  it  consists  of  a  set  of  secondarily 
derived  vascular  spaces  ;  it  has  been  called  a  pseudoccele  or  a  haemoccele. 


r  '/?          -s- 


-b' 


FIG.  103. — Diagrammatic  cross  section  of  an  Inverte- 
brate, with  a  secondary  and  a  primary  body  cavity.  (After 
ZIEGLER.) 

ec.,  Ectoderm;  s.bc.,  secondary  body  cavity  (as  in  Lamellibranchs) ; 
g.,  gut  ;  b.c.,  primary  body  cavity  (shaded);  ex.,  excretory  aperture  of 
nephridium  ;  gn.,  genital  organ  ;  «.,  ventral  nerve  ganglia. 

The  true  coelome  of  Arthropods  is  very  much  restricted  in  the  adult,  all 
the  more  so  that  most  Arthropods  (e.g.,  Insects)  have  no  distinct 
nephridia. 


EXCRETORY  AND  REPRODUCTIVE  SYSTEMS.    317 

But  the  apparent  body  cavity  in  which  the  organs  lie,  and  in  which 
the  blood  circulates,  is  well  developed  in  Insects.  It  includes,  inter  alia, 
a  peculiar  fatty  tissue,  which  seems  to  be  a  store  of  reserve  material, 
which  is  especially  large  in  young  insects  before  metamorphosis,  and  is 
also  interesting  as  one  of  the  seats  of  "phosphorescence  "  in  those  insects 
which  glow. 

Excretory  System. 

Although  no  structures  certainly  homologous  with  neph-~~ 
ridia  have  yet  been  demonstrated  in  Insects,  the  excretory 
system  is  well  developed.  From  the  hind  gut  (p^Ctodaeum), 
and  therefore  of  ectodermic  origin,  arise  fine  tubes,  or  in 
some  cases  solid  threads,  which  extend  into  the  apparent 
body  cavity.  Their  number  varies  from  two  (in  some 
Lepidoptera  for  instance)  to  one  hundred  and  fifty  (in  the 
bee).  They  twine  about  the  organs  in  the  abdominal  cavity, 
and  their  excretory  significance  is  inferred  from  the  fact  that 
they  contain  uric  acid. 

Reproductive  System. 

Among  Insects  the  sexes  are  always  separate  and  often 
different  in  appearance.  The  males  are  more  active,  smaller, 
and  more  brightly  coloured  than  the  females.  Darwin 
referred  the  greater  decorativeness  of  the  males  to  the 
sexual  selection  exercised  by  the  females.  The  handsomer 
variations  succeeded  in  courtship  better  than  their  rivals. 
Wallace  referred  the  greater  plainness  of  females  to  the 
elimination  of  the  disadvantageous^  conspicuous  in  the 
course  of  natural  selection.  There  may  be  truth  in  both 
views,  but  both  require  to  be  supplemented  by  the  con- 
sideration, in  part  accepted  by  Wallace,  that  the  "  secondary 
sexual  characters"  of  both  sexes  are  the  natural  and  necessary 
expressions  of  their  respectively  dominant  constitutions. 


3i8         PERIPATUS,    MYRIOPODS,   AND  INSECTS. 


Reproductive  Organs. 


MALE. 


FEMALE. 


The  paired  ovaries  usually  consist  of 
many  small  tubes  (ovarioles). 

Two  ducts  (oviducts),  conducting  the 
ova  (perhaps  in  part  comparable 
to  nephridia). 

An  unpaired  terminal  region  or 
vagina,  paired  and  with  two 
apertures  inEphemerids;  usually 
formed  from  an  external  invag- 
ination  meeting  the  united  ends 
of  the  oviducts. 

Near  or  from  the  vagina,  a  re- 
ceptaculum  seminis  for  storing 
spermatozoa  received  from  a 
male  during  copulation. 

Various  accessory  glands,  e.g.,  those 
which  secrete  the  material  sur- 
rounding the  eggs. 

Sometimes  a  special  bursa  copulatrix 

in  the  vagina. 

Often  external  hard  pieces,  e.g.,  ovi- 
positor. 


The  paired  testes  usually  consist  of 
many  small  tubes. 

Two  ducts  (vasa  deferentia),  con- 
—  ducting  spermatozoa  (perhaps  in 
part  comparable  to  nephridia). 

An  unpaired  terminal  and  ejaculatory 
ductf  pT.red  and  with  two  aper- 
tures in  Epi^meYids  on]v ;  some- 
times formecr  !by  a  union  of  the 
vasa  deferentia,  sometimes  by  an 
external  invagination  meeting 
the  vasa  deferentia. 

From  the  vasa  deferentia  or  from  the 
ejaculatory  duct,  a  paired  or  un- 
paired seminal  vesicle  for  storing 
spermatozoa. 

Various  accessory  glands,  whose 
secretion  sometimes  unites  the 
spermatozoa  into  packets  or 
spermatophores. 

Sometimes  a  copulatory  penis. 

Often  external  hard  pieces. 


Some  Peculiarities  in  Reproduction. 

Many  Insects,  such  as  aphides,  silk  moth,  and  queen  bee,  are  exceed- 
ingly prolific.  The  queen  termite  lays  thousands  of  eggs  "at  the  rate 
of  about  sixty  per  minute  "  ! 

The  store  of  spermatozoa  received  by  the  female,  and  kept  within  the 
receptaculum  seminis,  often  lasts  for  a  long  time, — for  two  or  three  years 
in  some  queen  bees.  Sir  John  Lubbock  gives  the  remarkable  instance 
of  an  aged  queen  ant,  which  laid  fertile  eggs  thirteen  years  after  the  last 
union  with  a  male. 

Parthenogenesis,  or  the  development  of  ova  which  are  unfertilised, 
occurs  normally,  for  a  variable  number  of  generations,  in  two  Lepidop- 
tera  and  one  beetle,  in  some  coccus  insects  and  aphides,  and  in  certain 
saw  flies  and  gall  wasps.  It  occurs  casually  in  the  silk  moth  and  several 
other  Lepidoptera,  seasonally  in  aphides,  in  larval  life  in  some  midges 
(Miastor,  Chironomus),  and  partially  or  "voluntarily"  when  the  queen 
bee  lays  eggs  which  become  drones.  Parthenogenetic  ova  (in  water 
fleas,  Rotifers,  £c.),  are  believed  to  form  only  one  polar  body;  the  egg 
which  becomes  a  drone  forms  two  as  usual,  but  the  case  of  the  bee  is  in 
several  respects  exceptional. 

A  few  insects  hatch  their  young  within  the  body,  or  are  "viviparous." 
This  is  the  case  with  parthenogenetic  summer  aphides,  a  few  flies,  the 
little  bee  parasites  Strepsiptera,  and  a  few  beetles. 


DEVELOPMENT  OF   THE  OVUM.  319 

Development  of  the  Ovum. 

The  tubes  which  compose  the  ovaries  and  lead  into  the 
oviducts  begin  as  thin  filaments,  the  ends  of  which  are 
usually  connected  on  each  side.  Those  thin  filaments 
consist  of  indifferent  germinal  cells,  all  of  them  potential 
ova,  and  of  mesodermic  epithelial  cells,  which  form  the 
ovarian  tubes,  &c.,  and  are  connected  anteriorly  to  the 
pericardial  wall. 

But  in  most  cases  only  a  minority  of  these  cells  become 
ova,  the  others  become  nutritive  cells,  which  are  absorbed  by 
the  ova,  and  follicle  cells  which  line  the  walls  of  the  ovarian 
tubes  and  help  to  furnish  the  egg  shells. 

There  may  be,  indeed,  ovarian  tubes  without  nutritive 
cells  (e.g.,  in  Orthoptera),  and  then  each  tube  is  simply  a 
bead-like  row  of  ova,  which  become  larger  and  larger  as 
they  recede  from  the  thin  terminal  filaments  and  approach 
the  oviducts.  In  other  cases,  the  bead-like  row  consists 
of  ova  alternating  with  clumps  of  nutritive  cells  (e.g.,  in 
Hymenoptera  and  Lepidoptera).  In  other  cases,  the  nutri- 
tive cells  mostly  remain  in  the  terminal  region,  but  their 
products  pass  down  to  the  receding  ova. 

As  there  are  numerous  ovarian  tubes  in  each  ovary, 
and  as  the  same  process  of  oogenesis  is  going  on  in  each, 
numerous  eggs  are  ready  for  liberation  at  the  same  time, 
and  are  simultaneously  discharged  into  the  oviduct  of  each 
side. 

The  eggs  are  large  and  contain  much  yolk.  In  relatively 
few  cases  yolk  is  almost  absent,  as  for  example,  in  the  sum- 
mer eggs  of  the  Aphides,  which  are  hatched  within  the  body, 
and  in  some  forms  where  the  young  are  endoparasitic.  The 
ovum  is  surrounded  by  a  vitelline  membrane,  and  also  by 
a  firm  chitinous  shell,  secreted  by  the  follicular  cells,  which 
is  often  sculptured  in  a  characteristic  manner.  This  shell 
is  pierced  by  one  or  more  minute  holes  (mtcropyles). 
Through  a  micropyle  the  spermatozoon  finds  entrance, 
sometimes  (as  in  the  cockroach)  after  moving  round  and 
round  the  shell  in  varying  orbits. 

Development. 

The  ripe  egg  usually  consists  of  a  central  yolk-containing  mass,  sur- 
rounded by  a  thin  sheath  of  protoplasm.  As  is  usual  for  Arthropods, 


320        PERIPATUS,    MYR10PODS,   AND  INSECTS. 

the  segmentation  is  peripheral  or  centrolecithal.  The  central  nucleus 
divides  up  into  several  nuclei,  which,  being  united  by  protoplasmic 
cords,  form  for  a  time  a  central  syncytium.  Later,  these  nuclei  emigrate 
into  the  peripheral  protoplasm,  which  segments  around  them,  thus  a 
peripheral  layer  of  similar  epithelial  cells  is  formed.  Some  of  the  nuclei 
may  be  left  behind  in  the  central  yolk  to  form  the  yolk  nuclei,  or,  what 
is  probably  the  more  primitive  condition,  these  are  formed  by  subsequent 
immigration  from  the  blastoderm. 

The  next  process  is  the  appearance  of  differentiation  among  the  similar 
cells  of  the  blastoderm.  Over  a  special  area — the  ventral  plate — (cf. 
Astacus]  the  cells  increase  in  number  and  become  cylindrical  in  shape  ; 
over  the  rest  of  the  egg  the  cells  flatten  out  and  become  much  thinner. 
In  the  middle  of  the  ventral  plate,  a  slight  groove  is  formed  by  rapid 


FIG.  104.- 


-Diagram  of  Insect  embryo, 
and  HEIDER.) 


(From  KORSCHELT 


A  transverse  section  before  union  of  amnion  folds,  and  a  longitudinal 
median  section  after  union  of  amnion  folds  ;  a,  anterior  pole  of 
ovum  ;  a',  anterior  end  of  blastoderm  ;  am,  amnion  ;  s,  serosa  ;  a.c, 
amniotic  cavity  ;  /,  posterior  pole  of  ovum  ;  ec,  ectoderm  ;  //,  lower 
germinal  layer  ;  y,  yolk. 

multiplication  of  the  cylindrical  cells.  This  represents  the  disguised 
gastrulation,  the  open  roof  of  the  groove  being  the  much  elongated 
blastopore.  The  surrounding  cylindrical  cells  unite  over  this  open  roof, 
the  groove  usually  flattens  out,  and  thus  we  have  formed  a  two  layered 
germinal  streak  which  spreads  forwards  and  backwards  over  the  egg, 
and  early  exhibits  externally  transverse  division  into  segments.  The 
upper  layer  is  the  ectoderm,  the  lower  includes  the  rudiments  of  both 
mesoderm  and  endoderm. 


METAMORPHOSIS   OF  INSECTS.  321 

Meanwhile  another  very  important  event  has  taken  place.  We  saw 
that  while  the  cells  of  the  ventral  plate  increased  in  depth,  the  remain- 
ing cells  flattened  out  laterally  ;  at  the  point  where  the  two  kinds  of 
cells  unite,  on  either  side  of  the  ventral  plate,  a  double  fold  arises.  The 
two  folds  unite  over  the  surface  of  the  ventral  plate,  forming  a  mem- 
branous arch  over  it.  The  internal  fold  is  called  "  amniotic,"  the 
outer  "  serous,"  from  their  resemblance  to  the  similar  envelopes  in  the 
embryos  of  higher  Vertebrates.  The  folds  take  no  direct  part  in  the 
development  of  the  embryo. 

We  must  now  return  to  the  germinal  streak.  The  gastrula  groove 
may  persist  as  a  tube  after  closure  of  the  blastopore,  but  it  is  usually 
compressed  by  the  ectoderm,  or  never  exists  as  a  distinct  cavity.  The 
greater  part  of  the  lower  stratum  of  the  germinal  streak  consists  of 
mesoderm.  This  becomes  divided  into  successive  segments  at  each 
side,  each  containing  a  primitive  ccelomic  cavity,  perhaps  continuous 
with  the  gastrula  cavity.  The  endoderm  arises  as  paired  clusters  of 
cells,  found  only  at  the  anterior  and  posterior  ends  of  the  primitive 
streak.  These  clusters  increase  rapidly  and  form  long  endodermal 
streaks  which  curve  downwards  so  as  to  enclose  the  yolk.  The  streaks 
meet  and  fuse,  first  ventrally  and  later  dorsally,  thus  constituting  the 
mid  gut.  The  yolk  nuclei  previously  mentioned  have  meanwhile 
increased  rapidly,  forming  yolk  cells  which  absorb  the  yolk.  These 
cells  are  included  in  the  endodermic  mid  gut,  and  there  break  up.  As 
the  endoderm  grows  round  the  yolk,  it  is  accompanied  by  a  layer 
(splanchnic)  of  the  mesoblast.  Fore  and  hind  gut  are  formed  by 
invaginations  which  fuse  with  the  mid  gut. 

In  the  later  stages  of  development  the  primitive  ccelomic  pouches 
lose  their  cross  partitions,  become  filled  with  meserichymatic  cells,  and 
practically  obliterated.  The  body  cavity  of  the  adult  is  formed  by  the 
appearance  of  lacunae  in  the  cells  of  the  mesenchyme. 

The  tracheoe  arise  as  segmentally  repeated  invaginations  of  the  ecto- 
derm. The  openings  of  the  invaginations  form  the  stigmata.  From 
the  hind  gut  arise  the  Malpighian  tubules,  which  are  therefore  ecto- 
dermic.  The  development  of  the  other  organs  is  similar  to  that  of  the 
Crustacea. 

In  summarising  the  development  of  Insecta,  one  must 
specially  note  the  peripheral  segmentation,  the  formation  of 
the  two-layered  germinal  streak,  the  presence  of  an  over 
arching  blastodermic  fold,  the  segmentation  of  the  meso- 
derm, and  the  formation  of  the  mid  gut  by  the  union  of 
endodermic  bands. 

Metamorphosis  of  Insects. 

(i.)  In  the  lowest  Insects — namely,  in  the  old-fashioned 
wingless  Thysanura  and  Collembola,  the  hatched  young  are 
miniature  adults.  By  gradual  growth,  and  after  several  moult- 
ings,  they  attain  adult  size. 

21 


322        PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

Similarly  the  newly  hatched  earwigs,  young  of  cockroaches 
and  locusts,  of  lice,  aphides,  termites,  and  bugs,  are  very 
like  the  parents,  except  that  they  are  sexually  immature, 
and  that  there  are  no  wings,  which  indeed  are  absent  from 
some  of  the  adults. 

These  insects  are  called  ametabolic,  i.e.,  they  exhibit  no 
marked  change  or  metamorphosis. 

(2.)  In  Cicadas  there  are  slight  but  most  instructive 
differences  between  larvae  and  adults.  The  adults  live 
among  herbage,  the  young  on  the  ground,  and  the  diversity 
of  habit  has  associated  differences  of  structure,  as  in 
the  burrowing  fore  legs  of  the  larva.  Moreover,  the  larva 
acquires  the  characters  of  an  adult  after  a  quiescent  period 
of  pupation. 

The  differences  between  larva  and  adult  are  more  striking 
in  May  flies,  dragon  flies,  and  the  related  Plecoptera  (e.g., 
Perla\  for  in  these  the  larvae  are  aquatic,  with  closed 
respiratory  apertures,  with  tracheal  gills  or  folds,  while 
the  adults  are  winged  and  aerial,  and  breathe  by  open 
tracheae. 

These  insects  are  called  hemimetabolic,  i.e.,  they  have  a 
partial  or  incomplete  metamorphosis. 

(3.)  Very  different  is  the  life  history  of  all  other  sets  of 
Insects — ant  lions,  caddis  flies,  flies,  fleas,  butterflies  and 
moths,  beetles,  ants,  and  bees.  From  the  egg  there  is 
hatched  a  larva  (maggot,  grub,  or  caterpillar),  which  lives  a 
life  very  different  from  the  adult,  and  is  altogether  unlike 
it  in  form.  The  larva  feeds  voraciously,  grows,  rests,  and 
moults.  Having  accumulated  a  rich  store  of  reserve 
material  in  its  "  fatty  body,"  it  finally  becomes  for  some 
time  quiescent,  as  a  pupa,  nymph,  or  chrysalis,  often  within 
the  shelter  of  a  cocoon.  During  this  period  there  are  great 
transformations;  wings  bud  out,  appendages  of  the  adult 
pattern  are  formed,  reconstruction  of  other  organs  is 
effected.  Finally,  out  of  the  pupal  husk  emerges  a 
miniature  winged  insect  of  the  adult  or  imago  type. 

These  insects  are  called  holometabolic,  i.e.,  they  exhibit  a 
complete  metamorphosis. 

Two  kinds  of  larvae  occur  among  insects,  (a)  In  many 
ametabolic  and  hemimetabolic  forms,  the  larva  is  somewhat 
like  one  of  the  lowly  Thysanuran  insects  (Campodea),  and  is 


INTERNAL  METAMORPHOSIS.  323 

therefore  called  campodeiform.  It  has  the  regions  of  the 
body  well  defined,  three  pairs  of  locomotor  thoracic  limbs, 
and  mouth  parts  adapted  for  suction,  (fr)  The  other  type  is 
worm-like  or  cruciform,  e.g.)  the  caterpillars  of-  butterflies  and 
moths,  with  three  pairs  of  limbs  ;  the  more  modified  grubs  of 
bees,  &c.,  with  distinct  head,  but  without  limbs ;  and  the 
degenerate  maggots  of  flies,  &c.,  not  only  limbless,  but  with 
an  ill-defined  head.  But  the  caterpillar  has  often  several 
pairs  of  abdominal  pro-legs,  which  may  be  homologous  with 
legs,  and  other  abdominal  appendages  are  known  on  the 
larvae  of  other  insects,  and  even  in  the  embryos  of  some 
whose  larvae  are  campodeiform.  These  facts  make  it  likely 
that  the  primitive  form  had  many  legs. 

The  larvae  of  Insects  vary  enormously  in  habit  and  in  structure,  and 
exhibit  numerous  adaptations  to  conditions  of  life  very  different  from 
those  of  the  parent.  Thus  caterpillars,  which  are  usually  plump  and 
tense,  so  that  a  peck  from  a  bird's  bill  may  cause  them  to  bleed  to  death, 
even  if  no  immediate  destruction  befall  them,  are  protectively  adapted 
in  many  different  ways.  Their  colours  are  often  changed  in  harmony 
with  those  of  their  surroundings,  some  palatable  forms  are  saved  by 
their  superficial  resemblance  to  those  which  are  nauseous,  a  few  strike 
"  terrifying  attitudes,"  others  are  like  pieces  of  plants. 

But  for  our  purpose  it  is  perhaps  more  important  to  recall  the 
differences  between  the  respiration  of  some  larvae  and  that  of  the  adult, 
between  the  apneustic  larva  of  the  dragon  fly  and  the  holopneustic 
winged  tyrant.  Likewise  of  great  importance,  and  supplying  a  basis  for 
classification,  are  the  changes  in  connection  with  the  mouth  organs.  The 
main  facts  may  be  summarised  in  a  terse  sentence  from  the  monumental 
work  of  Rolleston  and  Hatchett  Jackson  "  Forms  of  Animal  Life," 
Oxford,  1888).  "  The  mouth  parts  may  be  similar  in  all  stages  of  life, 
and  then  are  either  adapted  for  biting  (Menognatha^  i.e.,  jaws  persistent), 
or  for  sucking  (Menorkyncha,  i.e.,  proboscis  persistent),  or  else  they  are 
adapted  in  the  larva  for  biting,  in  the  adult  for  sucking,  the  change 
commencing  in  the  pupa,  and  rarely  affecting  the  larval  ^^(Metagnatha, 
i.e.,  jaws  changed)." 

Internal  Metamorphosis. 

In  Insects  with  no  marked  metamorphosis,  or  with  an 
incomplete  one  merely,  the  organs  of  the  larva  develop 
gradually  into  those  of  the  adult.  But  in  Insects  with 
complete  metamorphosis,  there  is  a  marvellous  internal 
reconstruction  during  the  later  larval,  and  especially  during 
the  quiescent  pupal  stage.  Most  of  the  larval  organs  are 
disrupted  and  partially  absorbed  by  amoeboid  cells,  their 
debris  being  used  in  building  new  structures.  Parts  of  larval 


324         PERIPATUS,    MYRIOPODS,    AND  INSECTS. 

organs  which  have  not  been  highly  specialised  form  the 
foundations  of  new  adult  structures.  Of  special  importance 
are  certain  ingrowths  of  the  larval  skin  (the  epi-  or  hypo- 
dermis)  which  form  what  are  called  "  imaginal  discs,"  from 
which  arise  the  wings,  legs,  and  epidermis  of  the  imago  or 
perfect  insect.  The  reconstruction  is  very  thorough  ;  most 
of  the  musculature,  much  of  the  tracheal  system,  part  of  the 
mid  gut,  &c.,  are  gradually  replaced  by  the  corresponding 
organs  of  the  adult.  Yet  there  is  no  abruptness ;  the 
absorption  and  replacement  of  organs  is  perfectly  gradual. 

BIONOMICS. 

The  average  insect  is  active,  but  between  orders  (e.g., 
ants,  bees,  and  wasps  versus  aphides,  coccus  insects,  and 
bugs),  between  nearly  related  families,  between  the  sexes 
(e.g.,  male  and  female  cochineal  insect),  between  caterpillar 
and  pupa,  we  read  the  constantly  recurrent  antithesis  between 
activity  and  passivity. 

The  average  length  of  life  is  short.  Queen  bees  of  five 
years,  queen  ants  aged  thirteen,  are  rare  exceptions.  In 
many  cases  death  follows  as  the  rapid  nemesis  of  repro- 
duction. But  though  the  adult  life  is  often  very  short, 
the  total  life  may  be  of  considerable  length,  witness  some 
Ephemerids  which  in  their  adult  life  of  winged  love-making 
may  be  literally  the  flies  of  a  day,  while  their  aquatic  larval 
stages  may  have  lived  for  two  years  or  more. 

The  relation  between  the  annual  appearance  of  certain 
insects  and  that  of  the  plants  which  they  visit,  the  habits 
of  hibernation  in  the  adult  or  larval  state,  the  occasional 
"  dimorphism "  between  winter  and  summer  broods  of 
butterflies  should  be  noticed. 

The  prolific  multiplication  of  many  insects  may  lead  to 
local  and  periodic  increase  in  their  numbers,  but  great 
increase  is  limited  by  the  food  supply  and  the  weather,  by 
the  warfare  between  insects  of  different  kinds,  by  the 
numerous  insects  parasitic  on  others,  by  the  appetite  of 
higher  animals, — fishes,  frogs,  ant-eaters,  insectivores,  and, 
above  all,  birds. 

There  is  a  great  variety  of  protective  adaptation.  The 
young  of  caddis  flies  are  partially  masked  by  their  external 


BIONOMICS.  325 

cases  of  pebbles  and  fragments  of  stem  ;  many  caterpillars 
and  adult  insects  harmonise  with  the  colour  of  their  environ- 
ment ;  leaf  insects,  "  walking  sticks,"  moss  insects,  scale 
insects,  have  a  precise  resemblance  to  external  objects  which 
must  often  save  them  ;  a  humming  bird  moth  closely  resem- 
bles a  humming  bird ;  many  palatable  insects  and  larvae 
have  a  mimetic  resemblance  to  others  which  are  nauseous  or 
otherwise  little  likely  to  be  meddled  with.  Many  insects 
may  be  saved  by  their  hard  chitinous  armour,  by  their  dis- 
gusting odour  or  taste,  by  their  deterrent  discharges  of 
repulsive  formic  acid,  &c.,  by  simulation  of  death,  by  active 
resistance  with  effective  weapons. 

Many  flowers  depend  for  cross  fertilisation  upon  insects 
which  carry  the  pollen  from  one  to  another.  Many  insects 
depend  for  food  on  the  nectar  and  pollen  of  flowers.  Thus 
many  flowers  and  insects  are  mutually  dependent.  But 
many  insects  injure  plants,  and  many  plants  exhibit  structures 
which  tend  to  save  them  from  attack.  On  the  other  hand, 
there  may  be  "  partnerships  "  between  insects  and  plants — 
witness  the  "  myrmecophilous "  (ant  loving)  plants  which 
shelter  a  bodyguard  of  ants,  by  whom  they  are  saved  from 
unwelcome  visitors.  And  again,  the  formation  of  galls  by 
some  insects  which  lay  their  eggs  in  plants,  and  the  insect 
catching  proclivities  of  some  carnivorous  plants,  should  be 
remembered. 

Most  insects  are  terrestrial  and  aerial;  the  majority  live  in 
warm  and  temperate  countries,  but  they  are  represented 
almost  everywhere,  even  above  the  snow  line,  in  arctic 
regions,  in  caves.  Even  on  the  sea  the  "  Challenger "  ex- 
plorers found  the  pelagic  Halobates,  a  genus  of  bugs.  The 
distribution  of  Insects  is  mainly  limited  by  food  supplies 
and  climate,  for  their  powers  of  flight  are  often  great,  and 
their  opportunities  of  passive  dispersal  by  the  wind,  floating 
logs,  &c.,  are  by  no  means  slight. 

Many  insects  are  more  or  less  parasitic,  either  externally 
as  adults,  e.g.,  fleas,  lice,  bird  lice,  plant  lice,  &c.,  or  inter- 
nally as  larvae,  e.g.,  the  maggots  of  gad  flies  on  cattle,  and  a 
great  number  of  borers  within  plants. 

We  need  only  mention  Hessian  fly,  Phylloxera,  Colorado 
beetle,  Weevils,  Locusts,  to  suggest  many  more  which  are  of 
much  economic  importance  as  injurious  insects.  On  the 


326         PERIPATUS,   MYRIOPODS,   AND  INSECTS. 

other  hand,  our  indebtedness  to  hive  bee  and  silk  moth,  to 
cochineal  and  lac  insects,  to  those  which  destroy  injurious 
insects,  and  to  those  which  carry  pollen  from  flower  to 
flower,  is  obvious. 

Finally,  we  must  at  least  mention  that  in  ants,  bees, 
wasps,  and  termites  we  find  illustration  of  various  grades  of 
social  life,  and  marvellous  exhibitions  of  instinctive  skill  as 
well  as  some  intelligence. 

Pedigree. 

Insects  must  have  appeared  relatively  early,  for  remains 
of  a  cockroach-like  form  have  been  found  even  in  Silurian 
strata.  The  higher  forms  with  complete  metamorphosis 
appear  much  later  (e.g.,  beetles  in  the  Carboniferous  ages), 
but  it  seems  that  the  Palaeozoic  Insects  were  mostly 
generalised  types,  prophetic  of,  rather  than  referable  to,  the 
modern  orders. 

As  to  the  pedigree  of  Insects,  the  wingless  Collembola 
and  Thysanura  are  doubtless  primitive.  They  lead  us  back 
to  some  of  the  less  specialised  Myriopods  (e.g.,  Scolopen- 
drella\  back  further  to  Peripatus,  which  helps  to  link  the 
Tracheate  to  the  Annelid  series. 

But  though  the  primitive  wingless  insects,  the  simple  types 
of  Myriopods,  and  Peripatus  represent  ascending  steps  in 
evolution,  what  the  actual  path  has  been  we  do  not  know. 


CHAPTER    XV. 

ARACHNOIDEA   AND    PAL^OSTRACA. 

Class  ARACHNOIDEA — Spiders,  Scorpions,  Mites,  &c. 

The  class  Arachnoidea  is  far  from  being  a  coherent  unity. 
Its  subdivisions  are  numerous  and  diverse,  and  a  statement 
of  general  characters  is  consequently  difficult. 

The  anterior  segments,  about  seven  in  number ',  are  fused  into 
a  cephalothorax,  which  bears  six  pairs  of  appendages.  The 
most  anterior  of  these  appendages  may  be  turned  in  front  of  the 
mouth,  but  there  are  no  pre-oral  outgrowths  like  the  antennce 
of  Insects  and  Myriopods.  The  first  two  pairs  of  appendages 
(chelicer<z  and pedipalps)  generally  have  to  do  with  seizing  and 
holding  the  food  ;  the  others  are  walking  legs.  But  although 
six  pairs  occur  in  most,  there  may  be  more  or  less.  The 
abdomen  is  generally,  but  not  always,  without  appendages  ;  it 
may  be  segmented  or  unsegmented;  it  is  generally  distinct  from, 
but  may  be  fused  to,  the  cephalothorax.  A  plate-like  internal 
skeleton,  called  the  endosternite,  is  often  present.  Respiration 
may  be  by  tubular  trachece,  or  by  lung  books  (chambered 
trachea  ?),  or  by  both,  and  many  would  include  the  Branchiate 
Paheostraca  along  with  Arachnoidea.  In  the  tracheate  forms 
there  are  never  more  than  four  pairs  of  stigmata.  An  elon- 
gated dorsal  heart  usually  lies  in  the  abdomen.  The  position 
of  the  genital  aperture  or  apertures  is  usually  on  one  of  the 
anterior  abdominal  segments.  Except  Tardigrada,  all  have 
separate  sexes. 

Order  i.  SCORPIONID^E 

Scorpions  are  elongated  Arachnids,  restricted  to  warm 
countries,  lurking  under  stones  or  in  holes  during  the  day, 


328 


ARACHNO1DEA   AND  PALMOSTRACA. 


but  active  at  night.  The  Scorpio  afer  of  the  East  Indies 
attains  a  length  of  6  inches,  but  most  are  much  smaller. 
They  feed  on  insects,  spiders,  and  other  small  animals.  The 
"  tail,"  with  the  venomous  sting  at  its  tip,  is  usually  curved 
over  the  anterior  part  of  the  body,  and  can  reach  forward  to 
kill  the  prey  caught  by  the  anterior  appendages,  or  can  be 


FIG.  105. — Scorpion. 

ch^  Chelicerae  ;  //,  pedipalps ;  o,  genital  operculum  ;  /.,  pectines  ; 
s,  stigma  of  a  lung  book  on  the  prae-abdomen  ;  st,  sting  or  post 
anal  piece. 

suddenly  straightened  to  strike  backwards.  When  man  is 
stung,  the  poison  seems  to  act  chiefly  on  the  red  blood 
corpuscles,  and  though  never  or  very  rarely  fatal,  may  cause 
much  pain.  It  has  been  said  that  scorpions  commit  suicide 
when  surrounded  by  fire  or  otherwise  fatally  threatened,  but 


SCORPIONS.  329 

it  has  been  answered  that  they  do  not  sting  themselves,  that 
they  could  not  if  they  would,  and  that,  even  if  they  could, 
the  poison  would  have  no  effect ! 

The  body  is  divided  into  (i)  a  cephalothorax  or  "pro- 
soma  "  of  six  segments,  whose  terga  fuse  into  a  carapace,  and 
(2)  an  abdomen  which  includes  a  broad  seven-segmented 
"  mesosoma,"  and  a  narrow  five-segmented  "  metasoma." 
At  the  end  of  the  latter  there  is  a  post-anal  curved  spine  or 
"telson,"  containing  a  paired,  compressible  poison  gland 
opening  at  the  sharp  tip.  There  is  a  strong  cuticle  of  chitin, 
and  also  an  interesting  internal  piece  of  skeleton  (the  endo- 
sternite),  partly  chitinoid,  but  also  resembling  fibro-cartilage, 
which  lies  in  the  cephalothorax  above  the  nerve  cord,  and 
serves  for  the  insertion  of  muscles. 

The  appendages  are — 

(i.)  Small  three-jointed  chelate  chelicerre  or  fakes  just  above  the 
mouth,  used  in  holding  prey. 

(2.)  Large,  six-jointed,  chelate  pedipalps.  These  seize  the  prey  ; 
their  basal  joints  help  in  mastication,  and  in  some  cases  they  produce 
rasping  sounds. 

(3-6.)  Four  pairs  of  seven-jointed,  non-chelate  walking  legs.  The 
basal  joints  of  the  first  two  pairs  help  in  connection  with  the  mouth. 

Apparently  equivalent  to  a  first  pair  of  abdominal  appendages  is  a 
small  notched  plate  or  operculum  which  covers  or  bears  the  genital 
aperture  or  apertures. 

Apparently  of  the  nature  of  appendages  are  the  comb-like,  probably 
tactile,  pectines  on  the  second  abdominal  segment. 

Six  other  pairs  of  abdominal  appendages  are  present  in  the  embryo, 
but  they  abort. 

The  nervous  system  consists  of  a  dorsal  brain,  a  ring 
round  the  gullet,  and  a  ventral  nerve  cord.  The  simple 
eyes  situated  on  the  carapace  are  innervated  from  the 
brain,  the  first  six  appendages  from  the  collar  and  the  sub- 
oesophageal  ganglion.  Behind  the  latter  there  are  seven 
ventral  ganglia  in  the  eleventh  to  seventeenth  segments 
inclusive. 

Scorpions  seize  small  animals  with  their  pedipalps,  hold 
them  close  to  the  small  mouth  by  their  chelicerae,  sting  them 
if  need  be,  and  suck  their  blood  and  juices.  The  pharynx 
serves  as  a  suction  pump,  a  narrow  gullet  leads  to  a  slight 
enlargement,  into  which  a  pair  of  salivary  glands  open  ;  from 
the  narrow  mid  gut  several  large  digestive  outgrowths  arise ; 


330  ARACHNOIDEA   AND  PAL&OSTRACA. 

then  follow  one  or  two  pairs  of  Malpighian  tubes ;  the  hind 
gut  ends  in  a  ventral  anus  beneath  the  base  of  the  sting. 
The  narrowness  of  the  gut  may  be  associated  with  the  fluid 
nature  of  the  food. 

The  body  cavity  is  for  the  most  part  filled  up  with  organs, 
muscles,  and  connective  tissue.  A  pair  of  coxal  glands,  per- 
haps excretory  and  nephridial,  but  apparently  closed  in  the 
adult,  lie  near  the  base  of  the  last  two  walking  legs. 

The  blood  contains  amoeboid  corpuscles  and  the  respira- 
tory pigment  haemocyanin.  An  eight-chambered  heart, 
within  a  pericardium,  lies  along  the  back  of  the  mesosoma. 
It  gives  off  lateral  arteries  from  the  posterior  end  of  each  of 
its  chambers,  is  continued  backwards  in  a  posterior  aorta, 
and  forwards  in  an  anterior  aorta.  The  latter  supplies  the 
head,  and  divides  into  two  branches  encircling  the  gullet 
and  reuniting  in  a  ventral  artery  above  the  nerve  cord.  From 
capillaries  the  blood  is  gathered  into  a  ventral  venous  sinus, 
is  purified  in  the  lung  books,  and  thence  returns  by  veins  to 
the  pericardium,  finding  its  way  by  valved  lateral  openings 
(ostia)  into  the  anterior  end  of  each  heart  chamber. 

On  the  ninth  to  twelfth  segments  lie  slit-like  stigmata,  the 
openings  of  four  pairs  of  lung  books.  Each  lung  book  is 
like  a  little  purse  with  numerous  (over  a  hundred)  compart- 
ments. Air  fills  the  much-divided  cavity,  and  blood  circul- 
ates in  the  lamellae,  which  form  the  partitions.  These  lung 
books  or  pulmonary  sacs  are  believed  by  some  to  be 
chambered  or  plaited  tracheae,  while  Professor  Ray  Lankes- 
ter  regards  them  as  invaginated  modifications  of  gill  books 
such  as  Limulus  possesses. 

The  testes  consist  of  two  pairs  of  longitudinal  tubes, 
united  by  cross  bridges ;  the  vas  deferens,  with  a  terminal 
copulatory  modification,  opens  under  the  operculum  on  the 
first  abdominal  segment.  The  ovary  consists  of  three  longi- 
tudinal tubes,  united  by  cross  ducts,  and  two  oviducts  open 
on  the  under  surface  of  the  genital  operculum. 

Fertilisation  is  internal ;  the  ova  begin  their  development 
in  the  ovary,  and  complete  it  in  the  oviduct.  The  segmenta- 
tion is  discoidal,  the  ova  are  hatched  within  the  mother. 
The  young,  thus  born  "  viviparously,"  are  like  miniature 
adults,  and  adhere  for  some  time  after  birth  to  the  body  of 
the  mother. 


SPIDERS.  331 

The  race  of  scorpions  is  of  very  ancient  origin,  for  one 
has  been  found  in  Silurian  strata,  and  others  nearly  resem- 
bling those  now  alive  are  found  in  the  Carboniferous. 

Examples. — Scorpio,  Euscorpius,  Buthus,  Androctonus. 

Order  2.     PSEUDOSCORPIONID^;.     "  Book  Scorpions,"  e.g., 
Chelifer,  Chernes. 

Minute  animals,  most  abundant  in  warm  climates,  under  bark,  in 
books,  under  the  wing  covers  of  insects,  &c.  They  are  like  miniature 
scorpions,  but  without  the  long  tail  and  sting.  Their  food  probably 
consists  of  the  juices  of  insects  ;  the  chelicerse  are  minute  suckers,  the 
pedipalps  like  those  of  scorpions.  The  abdomen  is  broad,  with  10-11 
segments.  They  breathe  by  tubular  tracheae,  and  have  spinning  glands. 

Order  3.    PEDI PALPI.    "  Whip  Scorpions,"  e.g.,  Thelyphonus,  Phrynus. 

Small  animals,  found  in  warm  countries.  The  abdomen  is  depressed, 
well-defined  from  the  thorax,  and  has  11-12  segments.  The  chelicerae 
are  simply  clawed,  but  are  poisonous  ;  the  pedipalps  are  simply  clawed 
or  else  truly  chelate.  The  first  pair  of  limbs  are  like  antennae.  Respira- 
tion by  two  pairs  of  lung  sacs.  In  J^helyphomts  there  is  a  long  terminal 
whip. 

Order  4.     PHALANGID^:  (or  OPILIONINA).     "  Harvest-men,"  e.g., 
Phalangium. 

The  small  spider-like  u  harvest  men  "  are  noted  for  their  extremely 
long  legs,  by  which  they  stalk  slowly  along  avoiding  the  glare  of  day. 
The  broad  six-segmented  abdomen  is  not  distinct  from  the  thorax  ;  the 
chelicerse  are  chelate  ;  the  pedipalps  are  like  legs.  Respiration  by 
tubular  tracheae.  The  harvest  men  are  sometimes  called  daddy-long  legs, 
but  we  reserve  that  name  for  the  crane  fly  (Tipula  oleraced).  Nor  are 
they  to  be  confused  with  the  troublesome  "  harvest  bugs  "  (Trombidium 
holosericeii-ni},  which  are  "minute  red  mites.  The  harvest  men  do  not 
trouble  us,  but  feed  on  small  insects. 

Order  5.     SOLPUGID^E  or  SOLIFUG^,  e.g.,  Galeodes  or  Solpuga. 

Active,  pugnacious,  venomous,  nocturnal  little  animals,  found  in  the 
warmer  parts  of  the  earth.  The  head  and  abdomen  are  distinct  from 
the  thorax.  The  thorax  has  three  segments,  the  abdomen  nine  or  ten. 
The  chelicerae  are  chelate,  the  pedipalps  like  long  legs.  The  respiration 
is  by  means  of  tubular  tracheae.  The  segmentation  of  the  thorax  is 
remarkable. 

Order  6.  ARANEID^E.     Spiders. 

Spiders  are  found  almost  everywhere  upon  the  earth,  and 
a  few  are  at  home  in  fresh  water.  Most  of  them  live  on  the 
juices  of  insects,  and  many  form  webs  in  which  their  victims 
are  snared.  They  may  be  divided  according  to  habit  into 


332  ARACHNOIDEA   AND  PAL&OSTRACA. 

the  wanderers  who  spin  little,  and  the  sedentary  forms  who 
spin  much.  The  body  consists  of  an  unsegmented  cephalo- 
thorax  and  a  soft  unsegmented  abdomen,  separated  by  a 
narrow  waist.  The  chitinous  cuticle  varies  in  hardness, 
hairiness,  and  colouring  ;  it  has  as  usual  to  be  moulted  as 
the  spider  grows.  Thus  the  young  garden  spider  moults 
eight  times  in  its  first  year. 

There  are  six  pairs  of  appendages  : — 

(i.)  The  two  jointed  chelicerae  or  falces,  whose  terminal  joint  bends 
down  on  the  other  in  "  sub-chelate  "  fashion,  and  is  perforated  by  the 
duct  of  a  poison  gland. 

(2.)  The  leg-like,  usually  six-jointed,  non-chelate  pedipalps,  whose 
basal  joint  helps  in  mastication,  while  the  terminal  joint  in  the  male 
expands  as  a  reservoir  for  the  spermatozoa  and  serves  as  a  copulatory 
organ. 

(3-6.)  Four  pairs  of  terminally  clawed  walking  legs.  The  most 
anterior  pair  are  much  used  as  feelers.  In  the  embryo  there  are  four 
pairs  of  abdominal  appendages  which  abort. 

The  nervous  system  is  of  the  usual  Arthropod  type,  but 
shows  much  centralisation.  Thus  the  ventral  ganglia  are 
fused  into  one  large  centre  in  the  cephalothorax,  a  condition 
comparable  to  that  in  crabs.  There  are  two  or  three  rows 
of  simple  eyes  on  the  cephalothorax,  whose  focal  distance  is 
very  short,  spiders  trusting  most  to  their  exquisite  sense  of 
touch  by  which  they  discriminate  the  various  vibrations  on 
a  web  line.  The  senses  of  smell,  hearing,  and  taste  are 
also  present,  but  little  is  known  in  regard  to  the  organs. 

Body  cavity,  endosternite,  and  coxal  glands  generally 
resemble  those  of  scorpions. 

The  spider  usually  sucks  the  blood  and  juices  of  its  prey, 
and  behind  the  gullet  lies  a  powerfully  suctorial  region, 
strengthened  by  chitinous  plates,  and  worked  by  muscles. 
From  the  small  mid  gut  arise  five  pairs  of  long  caeca,  a  pair 
running  forwards  and  a  pair  passing  into  the  bases  of  each 
pair  of  legs  and  then  back  again.  These  caeca  sometimes 
anastomose.  Further  back,  the  mid  gut  gives  off  numerous 
digestive  outgrowths  which  fill  a  large  part  of  the  abdomen. 
Their  secretion  digests  proteids.  Terminally  there  is  a 
large  cloaca,  and  where  the  intestine  joins  this,  four  ex- 
cretory Malpighian  tubes  are  given  off. 

A  three-chambered  heart,  containing  colourless  blood, 
lies  within  a  pericardium  near  the  dorsal  surface  of  the 


SPIDERS. 


333 


abdomen.  It  gives  off  an  anterior  and  a  posterior  aorta 
and  lateral  vessels  ;  and  the  circulation  corresponds  in 
general  to  that  of  the  scorpion. 

In  a  few  forms,  united  as  Tetrapneumones,  respiration  is 
effected  by  four  "  lung  books  "  ;  the  large  bird-catching 
Mygale  is  an  example.  In  the  vast  majority  (Dipneumones) 
there  are  two  "  lung  books,"  and  tubular  tracheae  in  addition. 
The  stigmata  of  the  lung  books  lie  on  the  anterior  ventral 
surface  of  the  abdomen  ;  the  tracheae  open  posteriorly  near 
the  spinnerets,  or  just  behind  the  openings  of  the  lung 
books,  or  at  both  places. 

The  spinnerets  (4-6)  lie  posteriorly,  a  little  in  front  of  the 
anus.  They  are  movable  organs,  perforated  by  numerous 

(often  many  hundred) 
fine  tubes  which  pro- 
ject as  "  spinning 
spools."  The  tubes  are 
connected  with  numer- 
ous compressible  glands 
secreting  liquid  silk. 
There  are  various  kinds 
of  glands,  both  the 
amount  and  nature  of 
the  silk  secretion  be- 
ing under  the  spinner's 
control. 

The  males  are  usually 
smaller  and  often  more 
brightly  coloured  than 
their  mates.  From  the  paired  testes  in  the  anterior  part 
of  the  abdomen,  two  vasa  deferentia  pass  to  a  common 
aperture  beside  the  openings  of  the  lung  books.  From 
the  paired  ovary  two  oviducts  likewise  arise  and  open 
into  a  uterus,  whose  external  aperture  is  surrounded  in 
the  mature  female  by  a  complex  genital  armature  or  epi- 
gynium.  Here  also  in  most  females  are  the  openings  of 
two  receptacula  seminis,  in  which  the  sperms  received  from 
a  male  are  stored,  and  from  which  they  pass  by  a  pair  of 
internal  ducts  to  the  oviducts,  there  to  fertilise  the  ova. 
Allusion  has  already  been  made  to  the  fact  that  the  sperms 
of  the  male,  after  emission,  may  be  stored  up  in  the  last 


FIG.  106. — Section  of  Lung  book. 
(After  MACLEOD.) 

d,  Dorsal ;  z/,  ventral ;  /,  lamellae  ;  /,  pos- 
terior ;  a,  anterior  ;  d.c,  dorsal  chamber  ;  JT, 
posterior  wall  ;  st,  stigma ;  ch,  one  of  the 
interlamellar  chambers. 


334  ARACHNOIDEA   AND  PAL&OSTRACA. 

joint  of  the  palps.  The  ova  are  usually  surrounded  by 
silken  cocoons,  which  are  carried  about  by  the  mother  or 
carefully  hidden  in  nooks  or  nests. 

Spinning. — Compression  of  the  spinning  glands  causes  a  flow  of  liquid 
silk  through  the  fine  spools  of  the  spinnerets.  The  extremely  thin  fila- 
ments from  each  spinneret  unite  into  a  thread,  and  the  thread  of  one 
spinneret  is  often  combined  with  that  from  the  others.  In  this  way  a  com- 
pound thread  of  exquisite  fineness,  though  rivalled  by  a  quartz  fibre,  is 
produced,  but  two  or  four  separate  threads  are  often  exuded  at  the 
same  time.  Before  beginning  to  "spin,"  the  spider  often  presses  the 
spinnerets  against  the  surface  to  which  the  thread  is  to  adhere,  and 
draws  the  filaments  out  by  slowly  moving  away.  Often,  however,  the 
filaments  ooze  out  quite  apart  from  any  attachment.  The  legs  are  also 
much  used  in  extending  and  guiding  the  thread,  and  some  spiders  have 
on  the  hind  legs  a  special  comb  of  stiff  hairs. 

One  of  the  most  important  ways  in  which  the  secreted  threads  are 
used  is  in  forming  a  web.  The  common  garden  spider  (Epeira)  makes 
a  web  which  is  a  beautiful  work  of  unconscious  art,  and  very  effective 
as  a  snare  for  insects.  The  spider  first  forms  "  foundation  lines  "  around 
the  selected  area  ;  it  then  swings  across  the  area  with  the  first  "  ray  " 
which  it  fixes  firmly  ;  another  and  another  is  formed,  all  intersecting 
in  one  centre.  Secondly,  it  starts  from  the  centre,  and  moves  from  ray 
to  ray  in  a  long  wide  spiral  gradually  outwards,  leaving  a  strong  spiral 
thread  as  it  goes.  Thirdly,  the  spider  moves  in  a  closer  spiral  from 
the  circumference  inwards,  biting  away  the  former  spiral,  replacing 
it  by  another,  which  is  viscid  and  adhesive.  It  is  to  this  that  the  web 
chiefly  owes  its  power  of  catching  insects  which  light  there.  There  is 
usually  a  special  thread  running  to  the  adjacent  hole  or  nest,  and  the 
entire  fabric  is  marvellously  sensitive,  for  the  spider  feels  rather  than 
sees  when  a  victim  is  caught. 

The  spun  threads  are  used  in  many  other  ways.  They  line  the  nest, 
.and  form  cocoons  for  the  eggs.  They  often  trail  behind  the  spiders  as 
they  creep ;  they  greatly  assist  locomotion,  and  are  used  in  marvellous 
feats  of  climbing.  Small  and  young  spiders  often  stand  on  tip-toe  on 
the  top  of  a  fence,  secrete  a  parachute  of  threads,  and  allow  them- 
selves to  be  borne  by  the  wind.  The  fallen  threads  are  known  as 
gossamer. 

Courtship. — The  males  are  usually  much  smaller  than  the  females. 
It  is  calculated  that  the  disproportion  is  sometimes  such  as  would  be 
observed  if  a  man  6  feet  high  and  150  pounds  in  weight  were  to  marry 
a  giantess  of  75-90  feet  high,  200,000  pounds  in  weight.  It  may  be 
that  the  smallness  of  the  males  is  mainly  due  to  the  fact  that  they  are 
males  ;  others  explain  it  by  saying  that  the  smaller  the  males  are,  the 
less  likely  they  are  to  be  caught  by  their  frequently  ferocious  mates. 
It  is  difficult,  however,  to  understand  how  this  characteristic  smallness, 
though  perhaps  advantageous  and  likely  to  be  favoured  by  natural 
selection,  can  be  entailed  on  the  male  offspring  only.  But  this  difficulty 
in  regard  to  inheritance  is  one  which  besets  many  similar  interpretations. 

The  males  are  often  more  brilliantly  coloured  than  the  females,  per- 
haps, again,  because  they  are  males,  though  what  the  physiological 


SPIDERS.  335 

connection  between  the  male  constitution  and  bright  colours  in  this  case 
is  we  cannot  tell  till  the  nature  of  the  pigments  is  known.  Wallace  has 
spoken  of  the  frequent  brilliancy  of  males  as  due  to  their  greater 
vitality,  and  refers  the  relative  plainness  common  in  females  to  their 
greater  need  for  protection.  Darwin  referred  the  greater  decorativeness 
of  males  to  the  fact  that  those  which  varied  in  this  direction  found 
favour  in  the  eyes  of  their  mates,  were  consequently  more  successful  in 
reproduction,  and  thus  tended  to  entail  brilliancy  on  their  male 
successors.  But  we  naturally  ask  how  the  brilliancy  began,  and  how 
its  enhancement  is  transmitted  to  males  alone.  In  the  "  Evohition  of 
Sex"  Professor  Geddes  and  I  have  recognised  that  sexual  selection 
may  help  to  establish  the  brilliancy  of  males,  and  that  natural  selection 
may  help  to  keep  the  females  plain,  but  have  also  sought  to  associate 
decorative  and  other  differences  between  the  sexes  with  the  more 
fundamental  qualities  of  maleness  and  femaleness. 

I  have  introduced  this  subject  here,  because  it  affords  a  pleasant 
interlude  in  our  systematic  survey,  and  because  it  serves  to  illustrate 
some  of  the  problems  of  evolution. 

Two  American  observers,  Mr.  and  Mrs.  Peckham,  have  made  a 
series  of  studies  on  the  courtship  of  spiders  more  careful  than  any  others 
of  the  kind. 

They  find  "  no  evidence  that  the  male  spiders  possess  greater  vital 
activity  ;  on  the  contrary,  it  is  the  female  that  is  the  more  active  and 
pugnacious  of  the  two."  They  find,  "  no  relation,  in  either  sex,  between 
development  of  colour  and  activity ;  the  Lycosidae,  which  are  among 
the  most  active  of  all  spiders,  having  the  least  colour  development,  while 
the  sedentary  orb- weavers  show  the  most  brilliant  hues."  "  In  the 
numerous  cases  where  the  male  differed  from  the  female  by  brighter 
colours  and  ornamental  appendages,  these  adornments  were  not  only  so 
placed  as  to  be  in  full  view  of  the  female  during  courtship,  but  the 
attitudes  and  antics  of  the  male  spider  at  that  time  were  actually  such 
as  to  display  them  to  the  fullest  extent  possible."  "The  males  were 
much  more  quarrelsome  in  the  presence  of  the  females,  and  to  a  great 
extent  lost  their  tendency  to  fight  when  the  mating  season  was  over." 

The  courtship  is  prolonged  and  elaborate,  the  females  are  not  only  coy 
but  often  savage.  The  male's  love-making  is  often  cut  short  by  his  death 
at  the  hands  or  chelicerse  of  his  desired  mate.  Of  course  we  must  be 
careful  not  to  exaggerate  the  subtlety  of  the  mental  processes  involved 
in  the  courtship  of  animals  ;  we  must  also  beware  of  regarding  it  too 
crudely. 

"  The  fact  that  in  Attidse  the  males  vie  with  each  other  in  making  an 
elaborate  display,  not  only  of  their  grace  and  agility,  but  also  of  their 
beauty,  before  the  females ;  and  that  the  females,  after  attentively 
watching  the  dances  and  tournaments  which  have  been  executed  for 
their  gratification,  select  for  their  mates  the  males  which  they  find  most 
pleasing,  points  strongly  to  the  conclusion  that  the  great  differences  in 
colour  and  in  ornament  between  the  males  and  females  of  these  spiders 
are  the  result  of  sexual  selection." 

It  is  still,  however,  quite  possible  that  the  colouring  and  decorations 
may  have  arisen  as  natural  outcrops  of  the  male  constitution,  the  charac- 
teristics of  which  are  by  no  means  limited  to  greater  vitality  or  activity. 


336  ARACHNOIDEA   AND  PAL^OSTRACA. 

Classification  of  Spiders. 

1.  Tetrapneumones. 

Four  lung  books  and  no  tracheae. 

My  gale,  a  large  lurking  spider  which  has  been  known  to  kill 
small  birds,  but  usually  eats  insects.  Atypus,  Cteniza,  and 
others  make  neat  trap  door  nests. 

2.  Dipneumones. 

Two  lung  books  and  tracheae  as  well. 

Here  are  included  the  web  spinners,  e.g.,  Epeira,  wolf  spiders, 
e.g.,  Lycosa,  Tarantula,  the  latter  with  poisonous  qualities 
which  have  been  much  exaggerated  ;  jumping  spiders,  the 
family  Attidse,  e.g.,  Attus  salticus.  The  common  house 
spider  is  Tegenaria  domestica ;  the  commonest  garden 
spider  is  Epeira  diademata.  Argyroneta  aquatica  fills  an 
aquatic  silken  nest  with  bubbles  caught  at  the  surface. 

Order  7.    ACARINA.    Mites  and  Ticks,  e.g.,  Cheese  mite  ( Tyroglyphus). 

Mites  are  minute  Arachnoids  inclined  to  parasitism.  They  occur  in 
the  earth  or  in  water,  salt  and  fresh,  or  on  animals  and  plants.  They 
feed  on  the  organisms  they  infest  or  upon  organic  debris. 

The  abdomen  is  fused  with  the  cephalothorax,  both  are  unsegmented. 
According  to  the  mode  of  life,  the  mouth  parts  are  adapted  for  biting  or 
for  piercing  and  sucking.  Respiration  may  be  simply  through  the  skin  ; 
in  the  majority  there  are  tracheae  with  two  stigmata.  A  heart  seems 
usually  absent,  but  it  is  present  in  Gamasus.  Many  of  the  young  have 
only  three  pairs  of  legs  when  hatched,  but  soon  gain  another  pair. 
When  some  mites  are  starved  or  desiccated,  and  to  some  extent  die, 
certain  cells  in  the  body  unite  within  a  cyst,  and  are  able  in  favourable 
conditions  to  regrow  the  animal. 

Examples — 

(a)  Without  tracheae.     Cheese  mite  (Tyroglyphus'].     Itch  mite 

(Sar  copies  scabiei],  causing  a  loathsome  disease.  S.  canis 
causes  "mange"  in  dogs.  Follicle  mite  (Demodex  folli- 
cu  forum),  common  in  the  hair  follicles  of  man  and 
domestic  animals.  Gall  mites  (Phytoptus],  on  plants. 

(b)  With  tracheae.      Harvest  mites  (Trombidium),  minute  para- 

sites often  troublesome  in  summer.  What  is  often  called 
the  red  spider  ( Tetrarhyncus  telearius],  spins  webs,  and 
lives  socially  under  leaves.  Water  mites,  e.g.,  Hydrachna, 
on  water  beetles,  and  Atax,  on  gills  of  fresh  water  mussels. 
Beetle  mites  (Gamasus),  often  found  on  carrion  beetles. 
Ticks  (Ixodes],  on  dogs,  cattle,  &c. 

Aberrant  Orders  or  Classes. 
Order  8.    LINGUATULIDA.    Pentastomum  tanioides. 

This  strange  animal  is  parasitic  in  the  nasal  and  frontal  cavities  of 
the  dog  and  wolf.  It  is  worm-like  in  form,  externally  ringed,  without 
any  oral  appendages,  but  with  two  pairs  of  movable  hooks  near  the 


PAL^OSTRACA.  337 

mouth.     There  are  no  sense  organs  nor  tracheae.     The  sexes  are  separate, 
the  males  smaller  than  the  females. 

Embryos  within  egg  cases  pass  from  the  nostrils  of  the  dog.  If  they 
happen  to  be  swallowed  by  a  rabbit  or  a  hare,  or  it  may  be  some  other 
mammal,  the  embryos  hatch  in  the  gut  and  penetrate  to  liver  or  lung. 
There  they  encyst,  moult,  and  undergo  metamorphosis.  -  The  final  larval 
form  is  not  so  unlike  an  Arachnoid  as  the  adult  is.  Liberated  from 
its  encystment,  it  moves  about  within  its  host,  but  will  not  become  adult 
or  sexual  unless  its  host  be  eaten  by  dog  or  wolf.  There  are  a  few  other 
species  occurring  in  Reptiles,  Apes,  and  even  man,  but  their  history  is 
not  adequately  known. 

Order  9.  TARDIGRADA.     Water  Bears  or  Sloth  animalcules,  e.g., 
Macrobiotus. 

Microscopic  animals,  sometimes  found  about  the  damp  moss  of  swamps 
or  even  in  the  roof  gutters  of  houses.  The  body  is  somewhat  worm-like, 
with  four  pairs  of  clawed  limbs  like  little  stumps,  with  mouth  parts 
resembling  those  of  some  mites,  and  adapted  for  piercing  and  sucking. 
There  is  no  abdomen.  There  is  a  food  canal,  a  brain  and  a  ventral 
chain  of  four  ganglia,  sometimes  even  a  pair  of  simple  eyes,  but  no 
respiratory  or  vascular  organs.  They  are  the  only  hermaphrodite 
Arachnoids,  if  they  are  Arachnoids.  The  eggs,  which  are  developed  in 
the  cast  skin  of  the  parent,  undergo  total  segmentation  but  little  is 
known  in  regard  to  their  development. 

The  water  bears  are  said  to  have  great  powers  of  successfully  resisting 
desiccation,  but  perhaps  it  is  the  enclosed  eggs  which  do  so,  developing 
rapidly  when  favourable  conditions  return. 

Some  authorities  dignify  (8)  and  (9)  as  classes;  for  reasons  of  practical 
expediency  I  continue  to  call  them  orders. 


Class  PAL^OSTRACA. 

The  three  following  orders, — Xiphosura,  Eurypterina,  and 
Trilobita  may  be  united  under  this  title.  They  live  or  lived 
in  water,  and  have  or  had  gills  in  association  with  the  limbs. 
The  recently  discovered  antennae  of  Trilobites,  together 
with  the  markedly  biramose  character  of  some  of  their 
limbs,  suggests  an  affinity  with  Crustacea,  but,  on  the  other 
hand,  the  affinities  of  the  Xiphosura  seem  to  be  distinctly 
Arachnoid. 

Order  i.  XIPHOSURA. 

There  is  one  living  genus,  the  King  Crab  or  Horse-shoe 
Crab  (Limulus). 

22 


338  ARACHNOIDEA  AND  PAL^OSTRACA. 

The  King  Crab  lives  at  slight  depths  off  the  muddy  or 
sandy  shores  of  the  sheltered  bays  and  estuaries  of  North 
America,  from  Maine  to  Florida,  in  the  West  Indies,  and 
also  on  the  Molucca  Islands,  &c.,  in  the  far  East.  The 
body  consists  of  a  vaulted  cephalothorax  shaped  like  a 
horse-shoe,  and  an  almost  hexagonal  abdomen  ending  in  a 
long  spine.  Burrowing  in  the  sand,  Limulus  arches  its 
body  at  the  joint  between  cephalothorax  and  abdomen,  and 
pushes  forward  with  legs  and  spine.  It  may  also  walk 
about  under  water,  and  even  rise  a  little  from  the  bottom. 
It  is  a  hardy  animal,  able  to  survive  exposure  on  the  shore 
or  even  some  freshening  of  the  water.  Its  food  consists 
chiefly  of  worms. 

The  King  Crab  is  interesting  in  its  structure  and  habits,  and  also 
because  it  is  the  only  living  representative  of  an  old  race.  Since  Ray 
Lankester  published  in  1881  a  famous  paper  entitled  "Limulus  an 
Arachnid,"  it  has  been  generally,  though  not  unanimously  recognised, 
that  the  King  Crab's  relationships  among  modern  animals  are  with 
Arachnoidea,  not  with  Crustacea. 

The  hard,  horse-shoe-shaped  chitinous  cephalothoracic  shield  is 
vaulted,  but  the  internal  cavity  is  much  smaller  than  one  would  at 
first  sight  suppose ;  the  well-defined  abdomen  shows  some  hint  of 
being  divisible  into  meso-  and  metasoma  ;  the  long  sharp  spine  is  (like 
the  scorpion's  sting)  a  post-anal  telson. 

On  the  concave  under  surface  of  the  cephalothorax,  there  are  six  (or 
seven)  pairs  of  limbs,  as  in  spiders  and  scorpions  : — 

(i)  A  little  pair  of  3-jointed  chelicerse  in  front  of  and  bent 
towards  the  mouth.  (They  are  chelate  in  the  female, 
simply  clawed  in  the  male.) 

(2-6)  Five  pairs  of  6-jointed  walking  legs,  the  bases  of  which 
surround  the  mouth  and  help  in  mastication.  The  last  of 
these  ends  in  two-flat  plates,  which  help  in  digging.  The 
other  appendages  are  usually  chelate,  except  the  first  in 
the  male. 

(7)  Then  follows  on  the  abdomen  a  double  "operculum"  over- 
lapping the  rest.  The  genital  apertures  lie  on  its  posterior 
surface.  Some  refer  this  operculum  to  the  cephalothorax. 

(8-12)  Under  the  operculum  lie  five  pairs  of  flat  plates  bearing 
remarkable  respiratory  organs  ("gill  books").  These 
appendages  show  hints  of  the  exopodite  and  endopodite 
structure  characteristic  of  Crustaceans.  At  any  rate  in 
the  young  they  serve  also  as  swimming  organs. 

As  in  the  scorpion,  there  is  an  internal  skeletal  structure,  or  endo- 
sternite,  lying  between  the  gullet  and  the  nerve  ring,  serving  for  the 


KING   CRAB. 


339 


attachment  of  muscles.     It  should  be  noted,  however,  that  an  analogous 
structure  occurs  in  Apus  and  some  other  Crustaceans. 

The  Nervous  System. — The  supra-cesophageal  brain  gives  off  nerves 
to  the  eyes.  United  to  the  brain  are  two  ganglionated  and  transversely 
connected  commissures  forming  a  long  oval  cesophageal  ring,  giving  off 
nerves  to  the  limbs,  and  continued  into  a  ganglionated  abdominal  cord. 
Ensheathing  ring,  ventral  cords,  and  some  of  the  nerves  are  numerous 
blood  vessels. 

There  are  two  "compound"  eyes  lying  towards  the  sides  of  the 
cephalothoracic  shield,  and  in  front  of  these  two  more  median  simple 
eyes.  The  compound  eyes  are  covered  by  a  layer  of  chitin  continuous 
with  that  of  the  shield,  and  the  various  eye  elements  are  so  remarkably 

distinct  from  one  another,  that 
the  eye  might  be  called  a  group 
of  simple  eyes. 

The  Food  Canal. — Worms  and 
the  like  seized  by  some  of  the 
pincers,  are  partly  masticated 
by  the  bases  of  the  five  pos- 
terior cephalothoracic  legs.  The 
mouth  leads  into  a  suctorial 
pharynx,  with  chitinous  folds ; 
thence  the  fore  gut  bends  up- 
wards and  forwards  into  a  crop. 
Separated  from  this  by  a  valve 
is  the  mid  gut  which  extends 
along  the  cephalothorax  and 
abdomen,  and  in  the  former 
bears  two  pairs  of  large  yellow 
hepato-pancreatic  outgrowths. 
The  hind  gut  is  short  and  ends 
in  front  of  the  base  of  the  spine. 
Two  large  reddish  glands  lie 
in  the  cephalothorax,  and  open 
in  young  forms  at  the  bases  of 
the  fifth  appendages.  They  also 
open  internally,  and  may  be 
compared  with  the  coxal  glands 
of  spider  and  scorpion,  with  the 
shell  gland  of  Entomostraca,  and 
with  nephridia  (?). 

The  Vascttlar  System.  — The 
heart  lies  within  a  pericardium 
and  is  partially  divided  into 
eight  chambers,  with  eight  pairs 
of  valved  ostia.  Haemocyanin  is 
present  as  usual  as  the  respira- 
tory pigment  of  the  blood,  and  there  are  oval  corpuscles.  From  an 
anterior  aorta,  like  that  of  the  scorpion,  two  vessels  are  given  off  which 
bend  backward,  unite  with  lateral  arteries  from  each  chamber  of  the 
heart,  and  form  a  collateral  vessel  on  each  side  of  the  heart.  These 


FIG.  107.  — Limulus  or  King  Crab. 

ch.,  Chelicarae  ;  op.,  operculum  ; 
a.,  anus. 


340 


ARACHNOIDS  A  AND  PALMOSTRACA. 


unite  in  a  posterior  dorsal  artery.  From  the  anterior  aorta  two  other 
branches  unite  in  a  ring  around  the  nerve  collar  which  gives  off  vessels 
to  the  limbs,  and  is  continued  backwards  around  the  nerve  cord.  From 
capillaries,  the  blood  is  gathered  into  a  ventral  venous  sinus,  whence 
it  passes  to  the  respiratory  organs,  and  thence  to  the  pericardium  and 
heart. 

The  Respiratory  Organs  or  gill  books  are  borne  by  the  last  five  append- 
ages. Each  looks  like  a  much  plaited  gill,  or  like  a  book  with  over  a 
hundred  hollow  leaves.  Their  leaf-like  folds  are  externally  washed  by 
the  water,  within  them  the  blood  flows.  The  leaves  of  the  gill  books 
are  compared  to  the  leaves  of  the  lung  books  of  scorpions.  If  this 
homology  ib  correct  the  gill  books  are  evaginations,  the  lung  books 
invaginations,  of  the  skin. 

The  Reproductive  System. — The  males  are  smaller  than  the  females. 
The  testes  are  very  diffuse,  the  two  vasa  deferentia  open  on  the  internal 
surface  of  the  operculum,  and  the 
spermatozoa,    which    are    vibratile, 
are    shed    into    the    water.      The 
ovaries   form   two   much    branched 
but   connected   sacs ;    the  oviducts 
are    separate,    and    enlarge    before 
they  open  beneath  the  operculum. 

Spawning  occurs  in  the  spring 
and  summer  months.  The  ova 
and  spermatozoa  are  deposited  in 
hollows  near  high  water  mark. 
Some  of  the  early  stages  of  de- 
velopment, still  imperfectly  known, 
present  considerable  resemblance 
to  corresponding  stages  in  the  scor- 
pion. In  the  larvae,  both  cephalo- 
thorax  and  abdomen  show  signs  of 
segmentation,  but  these  disappear. 
The  spine  is  represented  only  by 
a  very  short  plate,  and  the  larva 
presents  a  striking  superficial  re- 
semblance to  a  Trilobite. 

It  seems  likely  that  Limulus  is  linked  to  the  extinct  Eurypterids  by 
some  fossil  forms  known  as  Hemiaspidae,  e.g.,  Hemiaspis,  Belinurus. 


FIG.  108. — Young  Limulus. 
(After  WALCOTT.) 


Order  2.  EURYPTERINA  ( =  Merostomata),  e.g.,  Eurypterus. 

Gigantic  extinct  forms  found  from  Ordovician  to  Carboniferous  strata. 
The  body  is  divided  into  head,  thorax,  and  abdomen.  The  head  is 
small  and  unsegmented.  The  thorax  is  composed  of  six  distinct 
segments,  the  abdomen  of  six  with  a  terminal  telson,  which  was  some- 
times a  pointed  spine,  sometimes  paddle  shaped.  There  is,  however, 
some  doubt  as  to  the  exact  nomenclature  of  the  regions.  On  the  head 
are  borne  six  pairs  of  appendages  of  varying  shape,  two  lateral  com- 
pound eyes,  and  two  median  ocelli.  On  the  ventral  surface  of  the 
thorax,  there  are  five  pairs  of  gills  covered  by  flat  plates,  of  which  the 


TRILOBITA.  341 

most  anterior  pair  are  very  large,  and  form  the  so-called  operculum 
(cf.,  Limulus}.  The  surface  of  the  body  was  covered  with  scales. 
Some  of  the  Eurypterids  reached  a  length  of  six  feet. 

This  order  is  sometimes  placed  near  the  Crustacea,  but  the  general 
opinion  seems  to  be  that  which  links  them  through  Limulus  to 
Arachnoids. 

Order  3.  TRILOBITA.     Trilobites,  e.g. ,  Cafymene,  Phacops,  Asaphus. 

Extinct  forms  chiefly  found  in  Cambrian  and  Ordovician  strata,  but 
extending  up  to  the  Carboniferous.  The  body  as  found  is  divisible  into 
three  parts, — the  unsegmented  head  shield,  often  prolonged  backwards 
at  the  angles  ;  the  flexible  thorax  of  a  varying  number  of  segments  ;  the 
unsegmented  abdomen,  or  pygidium.  A  median  longitudinal  ridge,  or 
rachis,  divides  the  body  into  three  longitudinal  portions. 

Traces  of  limbs  are  only  rarely  preserved.  In  the  head  region  there 
are  four  pairs,  apparently  simple.  Antennae  have  been  recently  found 
in  this  region.  The  thorax  and  abdomen  were  furnished  with  biramose 


FIG.  109. — Vertical  cross  section  of  a  Trilobite,  Calymene. 
(After  WALCOTT.  ) 

2.,  Intestine  ;  s.,  shield  ;  L.,  endopodite  ;  e.,  exopodite  ; 
b.)  epipodial  parts. 

appendages  with  long  jointed  endopodite,  short  exopodite,  and  a  gill  (or 
epipodite  ?  )  of  varying  shape.  In  the  abdominal  region,  the  gills  were 
perhaps  rudimentary. 

Trilobites  are  often  found  rolled  up  in  a  way  that  reminds  one  of  some 
wood  lice.  So  abundant  are  they  in  some  rocks  that  even  their  develop- 
ment has  been  studied  with  some  success. 

The  limbs  seem  to  be  more  like  those  of  Crustaceans  than  those 
of  Arachnoids,  and  the  recent  discovery  of  antennae  accentuates 
the  resemblance  ;  but  the  marked  affinities  with  Limulus t  accord- 
ing to  the  views  of  most  authorities,  justify  the  continued  asso- 
ciation with  Arachnoids.  It  is,  perhaps,  most  logical  to  regard 
Trilobites  as  an  offshoot  from  a  stock  ancestral  to  both  Arachnoids 
and  Crustaceans. 


342  ARACHNOIDEA   AND  PALMOSTRACA. 

Incertce  Sedis. 
PANTOPODA  or  PYCNOGONID^. 

These  are  marine  Arthropods,  sometimes  called  sea  spiders.  Their 
affinities  are  uncertain,  but  perhaps  they  may  be  ranked  between 
Crustaceans  and  Arachnoids.  Many  climb  about  seaweeds  and  hydroids 
near  the  shore,  but  some  live  at  great  depths.  The  body  consists  of  an 
anterior  proboscis,  a  cephalothoracic  region  with  three  fused  and  three 
free  segments,  and  an  unsegmented  rudimentary  abdomen.  In  most 
there  are  seven  pairs  of  appendages,  into  five  of  which  outgrowths  of 
the  mid  gut  extend.  The  sexes  are  separate,  and  the  males  usually 
carry  the  eggs  attached  to  the  third  pair  of  appendages.  The  larvae  are 
at  first  unsegmented,  with  three  pairs  of  appendages. 

Examples  : — Pycnogonum,  Nymphon,  Ammothea. 


CHAPTER    XVI. 

MOLLUSCA.     - 

Classes  I.  AM  PH  IN  EUR  A — A  small  class  of  bilaterally  symmetrical 
forms,  e.g.,  Chiton.  2.  GASTEROPODA,  e.g.,  Snails.  3.  SCAPHO- 
PODA — A  small  class,  of  which  the  best  known  is  Dentalium. 
4.  LAMELLIBRANCHIATA — Bivalves.  5.  CEPHALOPODA— Cuttle- 
fishes. 

THE  series  of  Molluscs  stands  in  marked  contrast  to  that  of 
Arthropods,  for  the  body  of  the  Mollusc  is  unsegmented, 
and  there  are  no  appendages.  The  general  habit  of  life  is 
also  very  different,  for,  although  there  are  active  Molluscs 
and  sluggish  Arthropods,  it  is  true  as  an  average  statement 
that  Molluscs  are  sluggish  and  Arthropods  active.  Though 
the  pedigree  is  unknown,  there  does  not  seem  to  be  any 
possible  ancestry  for  Molluscs  less  remote  than  the  stock 
from  which  Turbellarians  and  other  unsegmented  "  worms  " 
have  sprung. 

GENERAL  CHARACTERS. — Molluscs  are  unsegmented  and 
without  appendages.  The  symmetry  is  fundamentally 
bilateral,  but  this  is  lost  in  most  Gasteropods.  The  "foot" 
— a  muscular  protrusion  of  the  ventral  surface,  is  very 
characteristic  ;  it  usually  serves  for  locomotion,  but  is  much 
modified  according  to  habit.  Typically,  a  projecting  dorsal 
fold  of  the  body  wall  forms  a  mantle,  or  pallium  (Fig.  i  io,  c.\ 
Which  often  secretes  a  single  or  bilobed  shell  covering  the 
viscera  ;  but  both  mantle  and  shell  may  be  absent.  /  There  are 
three  chief  pairs  of  ganglia — cerebrals,  pedals,  and  pleurals, 
with  connecting  commissures,  and  often  with  accessory  ganglia, 
especially  two  viscerals  on  a  loop  connecting  the  pleurals 
(Figs,  1 10,  1 1 8).  Except  in  Lamellibranchs,  in  which  the 
head  region  is  degenerate,  there  is  in  the  mouth  a  chitinous 


344 


MOLLUSC  A. 


ribbon  or  radula,  usually  bearing  numerous  small  teeth,  and 
moved  by  special  muscles,  the  whole  structure  being  known  as 
the  odontophore.  A  portion  of  the  true  body  cavity  or  ccelome 


9f 

FIG.  no. — Ideal  Mollusc.     (After  RAY  LANKESTER.  ) 

m.,  Mouth  ;  g.c.,  cerebral  ganglia  ;  c.,  edges  of  mantle  skirt  ;  z.g., 
duct  of  right  digestive  gland  ;   s.,  pericardial  cavity  ;  f.,  edges  of 


usually  persists  as  the  pericardium  at  least  (Fig.  no,  s.\  and 
communicates  with  the  exterior  through  the  nephridium  or 
nephridia.  The  vascular  system  is  almost  always  well 


of      £ 

FIG.  in. — Stages  in  Molluscan  development. 

Z>.,  Larva  of  Heteropod  (after  Gegenbaur);  sk.,  shell  covering 
visceral  hump  ;  v.,  velum  ;  f.,  foot. 

£.,  Larva  of  Atlanta  (after  Gegenbaur);  v.,  velum;  s/i.,  shell; 
f.,  foot ;  op.,  operculum. 

developed,  but  part  of  the  circulation  is  in  most  cases  through 
ill  defined  spaces  or  lacunce.  Respiratory  organs  are  most 
typically  represented  by  a  pair  of  vascular  processes  of  the 


AMPHINEURA. 


345 


body  wall  (ctenidia  or  gills),  but  one  or  both  of  these  may 
be  absent.  I  At  the  base  of  the  gills  there  is  generally  an  olfac- 
tory organ  or  osphradium.  )  Frequently  there  are  two  larval 
stages,  the  Trocho sphere,  which  resembles  the  same  stage  in 
some  Annelids,  and  the  more  characteristic  Veliger ;  but  the 
development  is  often  direct. 

Class  I.  AMPHINEURA. 
Syn.  GASTEROPODA  ISOPLEURA,  e.g.,  Chiton. 

GENERAL  CHARACTERS. — The  Amphineura  are  marine 
Molluscs,  more  or  less  elongated  in  form,  with  bilateral 
symmetry.  They  are  often  ranked  along  with  Gasteropods. 

The  mouth  is  anterior,  the  anal 
and  nephridial  apertures  are  pos- 
terior. The  mantle,  which  bears 
cuticular  spicules,  covers  at  least 
a  great  part  of  the  body.  I  The 
nervous  system  consists  of  a  cere- 
bral commissure  and  two  paired 
longitudinal  cords,  with  ganglionic 
cells,  but  at  most  very  feeble  ganglia, 
which  run  the  whole  length  of  the 
body.  Of  these  paired  cords  the 
pedals  are  connected  by  numerous 
cross  commissures,  and  the  vis- 
cerals  or  pallials  are  united  pos- 
teriorly by  a  commissure  above  the 
rectum.  The  bilateral  symmetry  is  shown  internally,  e.g.,  in 
the  paired  nephridia,  auricles,  and  genital  ducts.  The  class  is 
of  ancient  origin,  dating  from  the  Silurian.  There  are  two 
orders : — Polyplacophora,  e.g..  Chiton,  and  Aplacophora, 
e.g.,  Neomenia. 

ist  Order  of  AMPHINEURA,  POLYPLACOPHORA  (Chitonicke). 

The  members  of  this  order,  represented  on  British  coasts  by  several 
species  of  Chiton,  are  sluggish,  usually  vegetarian,  animals,  occurring 
from  the  shore  to  great  depths.  The  foot  is  generally  as  long  as  the 
body;  the  mantle  covers  the  back  and  bears  eight  shell  plates  (Fig.  112), 
perforated,  in  many  cases  at  least,  by  numerous  sensory  organs,  which 
may  be  in  part  optic  ;  numerous  gills  lie  in  a  regular  row  along  a  groove 
on  each  side  between  the  mantle  and  the  foot. 


FIG.  112. — Chiton. 
/        PRE"TRE.  ) 


(After 


346 


MOLLUSC  A. 


In  most  cases  the  eight  shell  plates  are  jointed  on  one  another,  and 
the  animal  can  roll  itself  up.  The  uncovered  parts  of  the  mantle  bear 
spicules.  \  Ganglia,  in  the  strict  sense,  are  scarcely  developed,  but  there 
is  a  supra-oesophageal  ganglionic  commissure  from  which  the  visceral  and 
pedal  cords  extend  backwards  along  the  whole  length  of  the  body. 
There  are  no  special  sense  organs  on  the  head,  which  is  but  slightly 


cpc"- 


FIG.  113. — Dorsal  view 
of  nervous  system  of 
Chiton.  (After  PEL- 

SENEER.) 

c.,  Cerebral  commissure; 
£•.,  gut  (above  all  the  com- 
missures except  cerebral  and 
supra-rectal);  pa.,  pallial  or 
visceral  loop  with  supra- 
rectal  commissure  (s.r.  c.)\ 
p.,  pedal  nerves  united 
by  numerous  transverse 
branches ;  s.g.,  stomato- 
gastric  commissure;  s.r. , 
subradular  commissure;  /., 
labial  commissure  ;  v. ,  vis- 
ceral commissure. 


Fig.  114. — Proneomenia.     Ner- 
vous   system      (From     HUB- 

RECHT. ) 

e.g.)  Cerebral  ganglia;  slg.,  sub- 
lingual  ;  a.p.g.,  anterior  pedal  ;  p-p.g.i 
posterior  pedal ;  p,v.g.,  posterior  vis- 
cerals  ;  si.,  sublingual  connectives; 
cpc.)  cerebro- pedal  connective ;  pe., 
longitudinal  pedal  nerves  ;  la.,  longi- 
tudinal lateral  nerves. 


differentiated  ;  but  the  pallial  sense  organs  are  usually  numerous  and 
varied.  A  twisted  gut  runs  through  the  body,  surrounded  by  a  diffuse 
digestive  gland.  There  is  a  radula  in  the  mouth.  The  heart  is  median 
and  posterior,  and  consists  of  a  ventricle  and  2-8  auricles.  Numerous 
gills  lie  in  a  regular  row  along  a  groove  on  each  side  between  the  mantle 
and  the  foot.  There  are  two.  symmetrical  nephridia  opening  posteriorly. 


GASTEROPODA.  347 

The  sexes  are  separate  ;  a  single  reproductive  organ  extends  dorsally 
between  gut  and  intestine  almost  the  whole  length  of  the  body ;  the 
genital  ducts  are  paired  and  open  posteriorly  in  front  of  the  excretory 
apertures.  The  ova  with  chitinous  spiny  shells  are  usually  retained  for 
some  time  by  the  female  between  the  mantle  and  the  gills.  The 
segmentation  is  holoblastic,  and  a  gastrula  is  formed  by  invagination. 

2nd  Order  of  AMPHINEURA,  APLACOPHORA,  e.g.,  Neonienia, 
Proneomenia,  and  Ch&toderma. 

The  members  of  this  order  are  worm-like  animals,  in  which  the 
mantle  envelops  the  whole  body  and  bears  numerous  spicules  but  no 
shell.  There  are  two  families,  Neomeniidse  and  Chretodermidae. 

Of  Neomeniidse,  six  genera  are  known.  They  have  a  longitudinal 
pedal  groove,  an  intestine  without  distinct  digestive  gland,  two  nephridia 
with  a  common  aperture,  and  hermaphrodite  reproductive  organs.  The 
ChDetodermidoe,  represented  by  one  genus  Chcetoderma,  are  cylindrical 
in  form,  without  a  pedal  groove,  with  a  radula  bearing  one  tooth,  with 
a  distinct  digestive  gland,  and  with  two  nephridia  opening  separately 
into  a  posterior  cavity,  which  also  contains  two  gills.  The  sexes  are 
separate. 

There  seem  to  be  more  than  merely  superficial  resemb- 
lances between  these  simple  Molluscs  and  such  worm 
types  as  Turbellarians.  It  seems  justifiable  to  speak  of  the 
Amphineura  as  primitive  Molluscs,  but  the  Aplacophora 
are  perhaps  rather  degenerate  than  primitive. 

Class  II.     GASTEROPODA,  e.g.,  Snail,  Whelk,  Limpet. 

GENERAL  CHARACTERS. — Gasteropods  are  more  or  less 
asymmetrical  Molluscs.  The  head  region,  which  is  well 
developed,  remains  symmetrical,  and  so  does  the  foot,  which  is 
typically  a  flat  creeping  organ.  But  the  visceral  mass  or 
hump,  with  its  mantle  fold,  is  more  or  less  twisted  forwards 
and  to  the  right.  Thus  the  pallial,  anal,  nephridial,  and 
genital  apertures  usually  lie  on  the  right  side,  more  or  less 
anteriorly.  A  further  asymmetry  is  shown  by  the  twisting  of 
the  morphologically  right  gill  to  the  left  side,  while  the  original 
left  gill  is  usually  lost.  Similarly,  one  of  the  nephridia,  pro- 
bably that  which  is  morphologically  the  left,  tends  to  disappear, 
and  in  most  cases  only  one  persists — topographically  on  the  left 
side.  The  main  torsion  must  be  distinguished  from  the  spiral 
twisting  which  the  visceral  hump  often  exhibits,  and  from  the 
frequently  associated  spiral  coiling  of  the  univalve  shell. 
Moreover,  a  superficial  secondary  bilateral  symmetry  tends  to 


348  MOLLUSC  A. 

be  acquired  by  free  swimming  forms,  e.g.,  Heteropods.  The 
foot  usually  contains  a  mucus  gland,  and  tends  to  be  divided 
into  three  regions — the  pro-,  me  so-,  and  meta-podium.  There 
is  a  single  reproductive  organ  and  genital  duct. 

A  type  of  GASTEROPODA — The  snail  (Helix). 

Mode  of  Life. 

The  common  garden  snail  (H.  aspersd)  and  its  larger 
neighbour  species  (H.  pematia),  rare  in  England  but 
abundant  on  the  Continent,  are  so  like  one  another,  except 
in  size,  that  the  same  description  will  serve  for  both.  They 
are  thoroughly  terrestrial  animals,  breathing  air  directly 
through  a  pulmonary  chamber.  They  drown  (slowly)  when 
immersed  in  water.  Their  food  consists  of  leaves  and 
other  parts  of  plants,  but  they  sometimes  indulge  in  strange 
vagaries  of  appetite.  They  are  hermaphrodite,  but  their 
sexual  relations  are  by  no  means  simple.  The  breeding 
time  is  spring,  and  the  eggs  are  laid  in  the  ground.  In 
winter  snails  bury  themselves,  usually  in  companies,  cement 
the  mouths  of  their  shells  with  hardened  mucus  and  a  little 
lime,  and  fall  into  a  state  of  "latent  life"  in  which  the 
heart  beats  feebly.  In  such  a  state  they  have  been  known 
to  survive  for  years. 

General  Appearance. 

A  snail  actively  creeping  shows  a  well  developed  head, 
with  two  pairs  of  retractile  horns  or  tentacles,  of  which  the 
longer  and  posterior  bear  eyes.  The  foot,  by  the  muscular 
contraction  of  which  the  animal  creeps,  is  very  large ;  it 
leaves  behind  it  a  trail  of  mucus.  The  viscera  protrude, 
as  if  ruptured,  in  a  dorsal  hump,  which  is  spirally  coiled  and 
protected  by  the  spiral  shell.  On  slight  provocation  the 
the  animal  retracts  itself  within  its  shell,  a  process  which 
drives  air  from  the  mantle  cavity,  and  thus  promotes 
respiration.  Around  the  mouth  of  the  shell  is  a  very  thick 
mantle  margin  or  collar,  by  which  the  continued  growth  of 
the  shell  is  secured.  On  the  right  side  of  the  expanded 
animal,  close  to  the  anterior  edge  of  the  shell,  there  is  a  large 
aperture  through  which  air  passes  into  and  out  of  the 
mantle  cavity.  Within  the  same  aperture  is  the  terminal 


THE   SNAIL.  349 

opening  of  the  ureter.  The  food  canal  ends  slightly  below 
and  to  the  right  of  the  pulmonary  aperture.  All  the  three 
openings  are  close  together.  The  anterior  termination  of 
ureter  and  food  canal  is  one  of  the  results  of  the  twisting  of 
the  visceral  mass  forwards  to  the  right.  But  still  further 
forward,  at  the  end  of  a  slight  groove  which  runs  along  the 
right  side  of  the  neck,  indeed  quite  close  to  the  mouth,  is 
the  genital  aperture.  Lastly,  an  opening  just  beneath  the 
mouth  leads  into  the  large  mucus  gland  of  the  foot. 

The  Shell. 

The  shell,  a  right-handed  spiral,  is  a  cuticular  product  made 
and  periodically  enlarged  by  the  mantle  margin.  Chemically 
it  consists  of  carbonate  of  lime  and  an  organic  basis 
(conchiolin).  The  outermost  layer  is  coloured,  without 
lime,  and  easily  rubbed  off :  the  median  layer  is  thickest, 
and  looks  like  porcelain ;  the  innermost  layer  is  pearly. 
The  twisted  cavity  of  the  shell  is  continuous,  and  the 
viscera  extend  to  the  uppermost  and  oldest  part. 

As  the  shell  is  gradually  made,  the  inner  walls  of  the  coils  form  a 
central  pillar  (columella),  as  on  a  staircase,  and  to  this  the  animal  is 
bound  by  a  strong  (columellar)  muscle.  Many  Gasteropods  bear  a 
horn-like  shell  lid  (operculum)  on  their  foot,  but  Helix  has  none ;  the 
"epiphragm"  with  which  the  shell  is  sealed  in  winter,  consists  of 
hardened  mucus,  plus  phosphate  and  a  smaller  quantity  of  carbonate 
of  lime.  It  is  formed  very  quickly  from  the  collar  region  when  cold 
weather  sets  in,  has  no  organic  connection  with  the  animal,  such  as 
binds  an  operculum  to  the  foot  of  the  whelk,  and  is  loosened  off  in  the 
mildness  of  spring. 

External  Appearance  after  the  Shell  is  Removed. 

If  the  shell  is  removed  carefully,  so  that  nothing  is  broken 
except  the  columellar  muscle,  many  structures  can  be  seen 
without  any  dissection.  The  skin  of  the  head  and  foot 
should  be  contrasted  (a)  with  the  thick  collar  of  the 
mantle;  (b}  with  the  loose  roof  of  the  mantle  or  pulmonary 
chamber;  (c)  with  the  exceedingly  delicate,  much  stretched, 
and  always  protected  skin  of  the  visceral  hump.  It  is 
important  to  realise  that  the  snail  has  an  "enlargement  of  the 
liver  "  and  a  great  rupture-like  hump  of  viscera  on  the  dorsal 
surface,  that  this  has  been  coiled  spirally,  and  that  there  is 
the  yet  deeper  torsion  forward  to  the  right. 


350  MOLLUSC  A. 

A  great  part  of  the  hump  consists  of  the  greenish  brown 
digestive  gland,  in  which  the  bluish  intestine  coils  behind 
the  mantle  chamber;  on  the  left  lies  the  triangular  and 
greyish  kidney ;  the  whitish  reproductive  organ  lies  in  the 
second  last  and  third  last  coil  of  the  spiral. 

The  Skin. 

The  skin  varies  greatly  in  thickness.  It  consists  of  a 
single  layered  epidermis  and  a  more  complex  dermis, 
including  connective  tissue  and  muscle  fibres.  There  are 
numerous  cells  from  which  mucus,  pigment,  and  lime  are 
secreted;  those  forming  pigment  and  lime  are  especially 
abundant  on  the  collar,  where  they  contribute  to  the  growth 
of  shell. 

Muscular  System. 

The  most  important  muscles  are — (a)  those  of  the  foot; 
(&)  those  which  retract  the  animal  into  its  shell,  and  are  in 
part  attached  to  the  columella  ;  (c)  those  which  work  the 
radula  in  the  mouth;  (d)  the  retractors  of  the  horns;  and  (e) 
the  retractor  of  the  penis.  The  muscle  fibres  usually  appear 
unstriated.  There  is  much  connective  tissue,  some  of  the 
cells  of  which  contain  glycogen,  pigment,  and  lime. 

Nervous  System. 

This  is  concentrated  in  a  ring  around  the  gullet.  Careful 
examination  shows  that  this  ring  consists  dorsally  of  a  pair 
of  cerebral  ganglia,  connected  ventrally  with  a  pair  of 
pedals  and  a  pair  of  pleuro-viscerals,  which,  according  to 
some  authorities,  have  a  median  abdominal  ganglion  lying 
between  them  (Fig.  118). 

The  cerebrals  give  off  nerves  to  the  head,  e.g.,  to  the 
mouth,  tentacles,  and  otocysts,  and  also  two  nerves  which 
run  to  a  pair  of  small  buccal  ganglia,  lying  beneath  the 
junction  of  gullet  and  buccal  mass.  The  pedals  give  offnerves 
to  the  foot,  the  viscerals  to  the  mantle  and  posterior  organs. 

Sense  Organs. 

An  eye,  innervated  from  the  brain,  is  situated  on  one  side 
of  the  tip  of  each  of  the  two  long  horns.  It  is  a  cup 
invaginated  from  the  epidermis,  lined  posteriorly  by  a  single 


ALIMENTARY  SYSTEM.  351 

layer  of  pigmented  and  non-pigmented  retinal  cells,  filled 
with  a  clear  vitreous  body  perhaps  equivalent  to  a  lens, 
and  closed  in  front  by  a  transparent  "  cornea,"  and 
strengthened  all  round  by  a  firm  "  sclerotic."  How  much 
a  snail  sees  we  do  not  know,  but  it  detects  swift  move- 
ments. Though  the  eye  is  by  no  means  very  simple, 
the  snail  soon  makes  another  if  the  original  be  lost,  and 
this  process  of  regeneration  has  been  known  to  occur  twenty 
times  in  succession. 

The  otocysts  appear  as  two  small  white  spots  on  the  pedal 
ganglia.  Each  is  a  sac  of  connective  tissue,  lined  by 
epithelium  which  is  said  to  be  ciliated  in  one  region,  contain- 
ing a  fluid  and  a  variable  number  of  oval  otoliths  of  lime, 
innervated  by  a  delicate  nerve  from  the  cerebral  ganglia. 

Though  no  osphradium  or  smelling  patch,  comparable  to 
that  which  occurs  at  the  base  of  the  gills  in  most  Molluscs, 
has  been  discovered  in  Helix,  the  snail  is  repelled  or 
attracted  by  odours ;  it  shrinks  from  turpentine,  it  smells 
strawberries  from  afar.  This  sense  of  smell  seems  to  be 
located  in  the  horns,  for  a  dishorned  snail  has  none.  The 
tips  of  both  pairs  of  horns  bear  sensory  cells  connected 
with  ganglionic  tissue  and  nerve  fibres  within. 

Other  sensory  cells,  probably  of  use  in  tasting,  lie  on  the 
lips  ;  and  there  are  many  others,  which  may  be  called  tactile, 
on  the  sides  of  the  foot,  and  on  various  parts  of  the  body. 
In  short,  the  snail  is  diffusely  sensitive. 

Alimentary  System. 

The  snail  files  the  leaves  of  plants  by  means  of  the  radula 
or  toothed  ribbon  which  lies  in  the  mouth,  and  it  grasps  the 
debris  with  its  lips. 

The  radula  is  a  long  strip  of  membrane,  bearing  several 
longitudinal  rows  of  minute  chitinoid  teeth.  It  rests  on  a 
cartilaginous  pad  on  the  floor  of  the  mouth  cavity,  and  is 
moved  (backwards  and  forwards,  and  up  and  down)  in  a 
curve,  by  protractor  and  retractor  muscles.  The  whole 
apparatus,  including  radula  teeth,  membrane,  and  pad,  is 
called  the  odontophore.  The  radula  wears  away  anteriorly, 
but  is  added  to  posteriorly  within  a  radula  sac  which 
projects  from  the  floor  of  the  buccal  cavity.  Its  action  on 


352  MOLLUSC  A. 

leaves  may  be  compared  very  roughly  to  that  of  a  file,  but 
its  movements  within  the  mouth  also  produce  a  kind  of 
suction  which  draws  food  particles  inwards.  In  this 
suction  the  muscular  lips  and  the  cilia  in  the  mouth 
cavity  assist. 

Altogether  apart  from  the  radula,  lying  on  the  upper 
surface  of  the  buccal  chamber,  sometimes  visible  when 
the  snail  opens  its  mouth,  is  a  hard,  crescent  shaped  jaw 
plate. 

The  ducts  of  two  large  salivary  glands  open  on  the 
dorsal  surface  of  the  buccal  cavity,  and  there  are  numerous 
distinct  glandular  cells  close  to  the  entrance  of  the  two 
ducts.  The  salivary  glands  are  large  lobed  structures, 
and  extend  far  backward  on  the  crop.  They  consist  of 
hundreds  of  glandular  cells  or  unicellular  glands,  which 
secrete  a  clear  fluid  stuff.  This  travels  up  the  ducts,  and 
is  forced,  in  part  at  least,  by  muscular  compression,  into 
the  buccal  cavity.  While  some  say  that  this  fluid  converts 
starch  into  sugar  (after  the  usual  fashion  of  saliva),  other 
authorities  deny  that  it  has  any  effect  upon  the  food. 
Similar  glands  are  found  in  all  Gasteropods,  while  they 
are  entirely  absent  in  Lamellibranchs.  In  some  boring 
Gasteropods  the  secretion  contains  2-4  per  cent,  of  free 
sulphuric  acid. 

The  gullet  extends  backward  from  the  buccal  cavity,  and 
expands  into  a  storing  crop ;  this  is  followed  by  a  stomach 
surrounded  by  the  digestive  gland;  thence  the  intestine 
extends,  and  after  coiling  in  the  visceral  hump,  passes 
forward  to  end  on  the  right  side  anteriorly  beside  the 
respiratory  aperture.  The  digestive  tract  is  muscular,  and 
in  part  ciliated  internally. 

A  large  part  of  the  visceral  spiral  is  occupied  by  the 
so-called  "  liver,"  a  digestive  gland  of  many  qualities, 
producing  juices  which  digest  all  kinds  of  food,  making 
glycogen,  storing  phosphate  of  lime,  and  containing  a 
greenish  pigment  called  enterochlorophyll.  It  is  possible 
that  the  phosphate  of  lime  may  be  used  to  form  the 
autumnal  epiphragm,  but  the  most  important  fact  is  that 
the  gland  is  more  than  a  "  liver,"  more  even  than  a 
"  hepato-pancreas,"  it  is  a  complex  digestive  gland,  pro- 
ducing several  digestive  ferments. 


VASCULAR  AND  RESPIRATORY  SYSTEMS.        353 

Vascular  System. 

The  blood  of  the  snail  contains  some  colourless  amoeboid 
cells,  and  a  respiratory  pigment  called  haempcyanin,  which 
gives  the  oxidised  blood  a  blue  tint,  and  is  very  common 
among  Molluscs. 

The  heart,  consisting  of  a  ventricle  and  an  auricle,  lies 
within  a  pericardial  chamber  on  the  dorsal  surface,  to  the 
left  side,  behind  the  mantle  cavity. 

From  the  ventricle  pure  blood  flows  by  cephalic  and 
visceral  arteries  to  the  head,  foot,  and  body,  passes  into 
fine  ramifications  of  these  arteries,  and  thence  into  spaces 
among  the  tissues.  Authorities  differ  as  to  the  existence  of 
capillaries,  but  the  distinction  between  these  and  narrow 
channels  is  of  no  physiological  importance.  From  spaces 
among  the  tissues  the  blood  is  collected  in  larger  venous 
spaces,  and  eventually  in  a  pulmonary  sinus  around  the 
mantle  cavity,  on  the  roof  of  which  there  is  a  network  of 
vessels.  There  the  blood  is  purified.  Most  of  it  returns 
directly  to  the  auricle  by  a  large  pulmonary  vein,  but  some 
passes  first  through  the  kidney. 

Respiratory  System. 

Most  Gasteropods,  e.g.,  the  dog  whelk  (Purpura\  the 
buckie  (Buccinum),  the  periwinkle  (Littorina),  breathe  by 
gills  covered  over  by  a  fold  of  the  mantle.  The  snail  being 
entirely  terrestrial  has  a  pulmonary  or  lung  cavity,  formed  by 
the  mantle  fold.  On  the  roof  of  this  cavity  the  blood 
vessels  are  spread  out.  Air  passes  into  and  out  of  the 
pulmonary  chamber  by  the  respiratory  aperture.  When  the 
animal  is  retracted  within  its  shell,  the  freshening  of  the  air 
in  the  pulmonary  chamber  takes  place  by  slow  diffusion,  but 
when  the  snail  extends  itself  at  full  length,  the  chamber  is 
rapidly  filled  with  air,  and  it  is  even  more  rapidly  emptied 
when  the  animal  withdraws  into  the  shell  again. 

Excretory  System. 

There  is  a  single  triangular  greyish  kidney  behind  the 
pulmonary  chamber,  between  the  heart  and  the  rectum.  It 
is  a  sac  with  plaited  walls,  and  excretes  nitrogenous  waste 
products,  which  pass  out  by  a  long  ureter  running  along  the 

23 


354  MOLLUSC  A. 

right  side  of  the  pulmonary  chamber,  and  opening  close 
beside  the  anus.  From  two  sources  the  kidney  is  supplied 
with  blood  (a)  from  the  pulmonary  chamber,  and  (b]  from  the 
heart  by  a  renal  artery.  As  in  most  other  Molluscs,  the 
kidney  communicates  by  a  small  aperture  with  that  part  of  the 
body  cavity  which  forms  the  pericardial  sac.  Thus,  as  in 
earthworm,  lobworm,  &c.,  the  coelome  has  a  nephridial 
connection  with  the  exterior. 

Reproductive  System. 

The  snail  is  hermaphrodite,  and  its  reproductive  organs 
exhibit  much  division  of  labour. 

(a)  The  essential  reproductive  organ  (the  ovotestis]  is  a 
whitish   body   near   the   apex   of   the   visceral    spire.       It 
consists  of  numerous  cylindrical  follicles,  in  each  of  which 
both  ova  and  spermatozoa  are  formed,  but  not  at  the  same 
time.     Simultaneous  formation  of  elements  so  different  is 
probably  very  rare. 

(b)  A  much  convoluted   hermaphrodite  duct  of  a  white 
colour  conducts  the  sex  cells  from  the  ovotestis,  and  leads 
to  the  base  of  a  large  yellowish  albumen  gland. 

(c)  This  tongue  shaped  albumen  gland  varies  in  size  with 
the  age  and  sexual  state  of  the  snail.     It  forms  gelatinous 
proteid  material,   which  envelops  and    probably  nourishes 
the  ova. 

(d)  The  ova  and  spermatozoa  pass  from  the  hermaphrodite 
duct  towards  the  head  along  a  common  duct,  but  not  at  the 
same  time.      Moreover  their   paths  are  different,   for   the 
portion   of  the  duct  down    which  the  ova  travel  is  much 
plaited,  while  the  path  which  the  spermatozoa  follow  is  a  less 
prominent  groove,  incompletely  separated  from  the  other. 
Both  paths  are  glandular,  and  the  glands  on  the  male  side 
are  often  called  prostatic. 

(e)  At   the   base  of  this   common  duct,   a  distinct  vas 
deferens  diverges  to  the  left  and  leads  into  a  muscular  penis, 
which  can  be  protruded  at  the  single  genital  aperture  and 
retracted  by  a  special  muscle.      Before  the   vas   deferens 
enters  the  penis,  a  long  process  m  flagellum  is  given  off.     It 
is  like  the  lash  of  a  whip,  and  is  as  long  as  the  common  duct. 
Within  it  a  spermatophore  is  partly  formed,  but  seems  to  be 
completed  in  the  penis.     This  spermatophore  is  laden  with 


REPRODUCTIVE  SYSTEM. 


355 


a  large  number  of  spermatozoa,  and  is  transferred  by  the 
penis  into  the  genital  aperture  of  another  snail. 

(/)  Continued  from  the  oviducal  side  of  the  common 
duct,  there  is  a  separate  ciliated  oviduct.  This  has  a  short 
course,  and  ends  in  the  common  genital  aperture.  Before 
it  reaches  this,  however,  the  oviduct  is  associated  with  two 
structures.  The  first  of  these  is  a  long  process,  as  long  as 
the  common  duct,  beside  which  it  runs,  in  appearance 


h.d. 


R 


d.g. 


m.g, 


FIG.  115. — Dissection  of  Helix  pomatia. 
LEU  CK  ART.) 


(Mainly  after 


n.r.,  Nerve  ring  ;  s.g.,  salivary  glands  on  the  crop  \f.,  foot ;  d.g., 
digestive  gland  opening  into  mid  gut ;  h.  heart;  k.  kidney ;  r.,  rectum ; 
R.  hermaphrodite  organ  in  terminal  part  of  digestive  gland  ;  h.d., 
hermaphrodite  duct;  a.g. ,  albumen  gland ;  c.d.,  common  duct,  with 
more  convoluted  oviducal  part ;  v.d.,  vas  deferens  entering  penis; 
_/?.,  flagellum  ;  r.s.  receptaculum  seminis,  with  a  branch  from  its 
duct ;  m.g.)  mucus  glands  ;  d.s.,  dart  sac. 

suggesting  the  flagellum,  but  expanding  at  its  free  end  into 
a  globular  sac — the  receptaculum  seminis.  It  is  into  this 
long  duct  and  sac  that  a  spermatophore  from  another  snail 
passes,  and  is  after  some  days  dissolved,  liberating  hundreds 
of  spermatozoa.  By  these  spermatozoa  the  ova  of  this  snail 


356  MOLLUSC  A. 

are  fertilised.  The  second  structure  associated  with  the 
female  duct  is  a  conspicuous  mucus  gland,  formed  of  two 
sets  of  finger-like  processes.  The  mucus  secretion  of  this 
gland  is  very  abundant  during  copulation,  and  as  it  contains 
not  a  little  lime,  it  is  possible  that  it  may  form  the  calcareous 
shells  of  the  eggs. 

(g)  Finally,  between  the  entrance  of  oviduct  and  penis 
into  the  terminal  aperture  there  lies  a  firm  cylindrical 
structure,  larger  than  the  penis  and  with  muscular  walls.  It 
is  the  Cupid's  Dart  Sac,  and  contains  a  pointed  calcareous 
arrow  (spiculum  amoris),  which  is  jerked  out  previous  to 
copulation.  The  dart  is  sometimes  found  adhering  to  the 
skin  of  a  snail,  and  after  copulation  the  sac  is  empty,  soon, 
however,  to  be  refilled. 

When  two  snails  pair,  the  genital  apertures  are  dilated, 

*-  * 

^Ec 


bl. 


big 


FIG.  1 1 6. — Diagram  of  larva  of  Paludina.     (After  ERLANGER.  ) 

£c.,  ectoderm;  En.,  endoderm;  v.  velum,  with  cilia;  g.>  gut 
cavity;  S.c.,  segmentation  cavity;  c.p. ,  coelome  pocket  from  gut; 
bl.g.,  blastopore  groove  closed,  except  at  bl.,  which  becomes  the 
anus. 

the  protruded  penis  of  one  is  inserted  into  the  aperture  of 
the  other,  and  the  transference  of  a  spermatophore  is  thus 
effected. 

The  eggs  are  laid  in  the  earth  in  June  and  July.  Each  is 
surrounded  by  gelatinous  material  acquired  in  the  oviduct, 
.  and  by  an  elastic  but  calcareous  shell. 

Segmentation  is  total  but  slightly  unequal.  As  the  snail 
is  a  terrestrial  Gasteropod,  there  is  no  trochosphere  larva  nor 
more  than  a  slight  hint  of  the  characteristic  Molluscan 
velum.  A  miniature  adult  is  hatched  in  about  three  weeks. 


CLASSIFICATION  OF    GASTEROPODA.  357 

The  study  of  development  may  be  more  profitably  followed 
in  the  pond  snail  Limnaus,  where  gastrula,  trochosphere,  and 
veliger  can  be  readily  seen. 

CLASSIFICATION  OF  GASTEROPODA. 

Sub-Class  I.  Streptoneura. 

(1)  Zygobranchia,  e.g.,  limpet. 

(2)  Azygobranchia,  e.g.,  whelk. 

Sub-Class  II.  Euthyneura. 

(1)  Opisthobranchia,  e.g.,  Aplysia. 

(2)  Pulmonata,  e.g.,  Snail. 

I.  Streptoneura  (i.e.,  with  twisted  nerves).  In  this  division  the 
torsion  of  the  body  has  twisted  the  visceral  nerve  loop  in  the 
form  of  a  figure  8.  The  sexes  are  separate. 

(1)  Zygobranchs.       Both    gills    may   persist,    or   both   may 

degenerate,  their  functions  being  then  discharged  by 
folds  of  the  mantle.     Both  nephridia  persist,  that  on 
the  right  side  being  the  larger,  and  serving  also  as  a 
genital  duct. 
Examples — 

Limpet  (Patella),  Ear  shell  (Haliotis). 

(2)  Azygobranchs.    The  morphologically  left  gill,  nephridium, 

and  osphradium  disappear,  those  morphologically  right 
persist  topographically  on  the  left.      It  may  also  be 
that  the  genital  duct  is  a  modification  of  the  morpho- 
logically left  nephridium. 
Examples — 

Periwinkle  (Littorind),  Buckie  (Buccinuni),  Dog 
whelk  (Purpura),  Cone  shells  (Conus),  Murex, 
lanthina ;  and  also  the  pelagic  Heteropods,  with 
foot  adapted  for  swimming,  e.g.,  Atlanta  (large  shell), 
Carinaria  (small  shell),  Pterotrachea  (no  shell). 

II.   Euthyneura   (i.e.,    with   straight   nerves).      The  torsion  of  the 
visceral  hump  is  less  complete,  and  the  visceral  loop  is 
untwisted.       All  are  hermaphrodite.      The  shell  is  often 
light,  and  may  be  absent  from  the  adult.     The  mantle  may 
also  be  much  reduced.     The  gill  (if  present)  is  behind  the 
heart  (Opisthobranch  condition),  whereas  in  the  Strepto- 
neura it  was  almost  always  in  front  (Prosobranch  condition), 
(l)  Opisthobranchia.     Small  visceral  hump,  rarely  any  shell 
in  the  adult ;   anus  behind,  and  heart  in  front  of  gill 
when  this  is  present. 

(a)    Tectibranchia.      Well    developed    mantle    fold    and 
functional  gill,  usually  with  delicate  shell. 

Bulla,  Aplysia,  Dolabella,  Umbrella, 
(d)  Nudibranchia.     Mantle   fold   rudimentary  and   shell 
absent  in  the  adult.    The  gill  is  either  much  modified 
or  absent. 

Triton,  Doris,  Eolis. 


358  MOLLUSC  A. 

As  Tectibranchia  must  also  be  included,  the 
Pteropoda,  the  Winged  Snails  or  Sea  Butterflies, 
which  have  become  modified  for  pelagic  life.  They 
have  a  secondarily  acquired  apparent  symmetry, 
and  swim  by  two  large  lateral  lobes  of  the  foot 
("  parapodia").  They  often  swim  actively  in 
shoals,  and  occur  in  all  seas.  They  afford  food  for 
whales,  &c. ,  and  the  shells  of  some  are  abundant  in 
the  ooze.  They  include  : — 

(a)  Thecosomata,  with  mantle  fold  and  shell,  diet  of 

minute   animal  or   vegetable  organisms,  closely 
related  to  Bulla  and  its  allies. 
Examples — Hyalea,  Cymbulia. 

(b)  Gymnosomata,  without  mantle  fold  or  shell  in  the 

adult.     Closely  allied  to  Aplysia  and  its  allies. 
Actively  carnivorous. 
Examples —  Clio,  Pneumoderma. 

(2)  Pulmonata.  Air  breathing,  without  gill,  the  edge  of  the 
mantle  has  fused  with  the  body  w^all,  forming  a 
pulmonary  chamber  with  a  small  aperture,  e.g.,  Helix 
(snail)  ;  Limax  (grey  slug) ;  Arion  (black  slug)  ; 
Limnceus,  Planorbis,  and  Ancylus  (common  fresh  water 
snails). 

GENERAL  NOTES  ON  GASTEROPODS. 

From  a  form  in  essentials  similar  to  Chiton,  except  for  its  eight  shells,. 
we  may  consider  that  the  Gasteropods  proper  have  been  developed. 
They  are  all  more  or  less  asymmetrical,  but  we  must  notice — (l)  that 
this  want  of  symmetry  does  not  affect  the  head  or  the  foot,  but  only  the 
dorsal  viscera,  which  are  more  or  less  twisted  round  to  the  right  side 
towards  the  head  ;  (2)  the  torsion  must  be  distinguished  from  the 
frequent  spiral  twisting  of  the  visceral  hump  and  of  the  shell  ;  (3)  the 
torsion  occurs  in  variable  degree,  and  some  forms,  especially  free 
swimmers,  have  a  superficial  symmetry. 

The  current  explanation  of  the  asymmetry,  which  has  been  recently 
elaborated  by  Lang,  is  as  follows  : — 

If  we  begin  with  a  form  something  like  a  Chiton,  but  with  a  simple 
shell,  we  must  suppose  the  head  and  foot  to  become  increasingly 
specialised,  and  at  the  same  time  to  acquire  an  increasing  freedom  of 
movement ;  during  the  process  the  viscera  will  tend  to  become  more 
and  more  limited  to  a  special  region  of  the  body,  and  a  "visceral 
hump  "  will  thus  be  formed.  The  shell  becomes  limited  to  this  region, 
but  the  contractility  of  head  and  foot,  which  enables  these  to  be  drawn 
into  the  shell,  must  be  correlated  with  the  increasing  size  and  complexity 
of  this  structure.  As,  however,  shell  and  visceral  hump  become  larger, 
they  become  too  heavy  to  be  carried  in  the  primitive  position  on  the 
back  of  the  animal,  and  incline  to  one  side.  There  is,  therefore,  a  one- 
sided pressure,  which  results  in  an  increased  growth  relatively  of  the 
opposite  side,  and  so  in  a  deep  seated  twisting,  which  brings  the 
originally  posterior  anus  to  an  anterior  position  near  the  mouth,  and 
produces  a  tendency  to  the  suppression  of  one  of  the  originally  paired 


MODE   OF  LIFE.  359 

gills,  nephridia,  &c.  According  to  Lang,  during  the  torsion  an  increased 
growth  of  the  upper  surface  of  the  visceral  mass  is  necessary  in  order  to 
avoid  rupture,  and  thus  the  superficial  spiral  coiling  is  produced  ;  this 
is  reflected  in  the  coiling  of  the  shell.  In  one  series  of  the  Gasteropods 
the  visceral  nerve  loop,  running  from  the  cerebral  and  pleural  to  the 
visceral  ganglia,  is  "caught  in  the  twist,"  and  twines  like  a  figure  8 
(Streptoneural  condition),  in  the  others  the  same  visceral  loop  is  short 
and  untwisted  (Euthyneural  condition).  In  both  groups  we  find  forms 
with  coiled  shells,  but  among  the  Etithyneura  there  is  a  tendency  to 
lose  the  shell,  the  visceral  hump  becoming  at  the  same  time  incon- 
spicuous, while  a  superficial  appearance  of  symmetry  is  produced.  The 
deep  seated  torsion  of  the  organs  is,  however,  still  retained. 

It  is  not  very  uncommon  to  find,  either  as  a  constant  occurrence  or  as 
an  occasional  variation,  spirally  coiled  shells  with  a  reversed  or  left 
handed  spiral.  In  some  of  these  cases  the  superficial  coiling  of  the 
visceral  hump,  as  well  as  the  deep  seated  torsion,  is  also  left  handed  ; 
but  in  others  we  find  that  the  internal  structure  retains  the  normal 
arrangement. 

Mode  of  Life. 

From  the  number  of  diverse  types  which  the  class  in- 
cludes, it  is  evident  that  few  general  statements  can  be 
made  about  the  life  of  Gasteropods.  We  are  safe  in  saying, 
however,  that  though  the  majority  are  sluggish  when  com- 
pared with  Crustaceans,  they  are  active  when  compared 
with  Lamellibranchs. 

The  locomotion  effected  by  the  contractions  of  the 
muscular  foot  is  usually  a  leisurely  creeping,  but  there  are 
many  gradations  between  the  activity  of  Heteropods  in  tha 
open  sea,  the  gliding  of  fresh  water  snails  (Limnceus)  foot 
upwards  across  the  surface  of  the  pool,  the  explorations  of 
the.  periwinkles  on  the  sand  of  the  shore,  and  the  extreme 
passivity  of  limpets  (Patella),  which  move  only  for  short 
distances  at  a  time  from  their  resting  places  on  the  rocks. 

Statistics  are  neither  interesting  nor  reliable,  for  there  is 
much  difference  of  opinion  as  to  the  limits  of  species  and 
varieties,  but  we  may  notice  that  the  number  of  terrestrial 
snails  and  slugs,  breathing  the  air  directly  by  means  of  a 
pulmonary  chamber,  is  estimated  at  over  6000  living  species, 
while  the  aquatic  Gasteropods  are  reckoned  at  about  10,000, 
most  of  which  are  marine.  Of  this  myriad,  about  9000  are 
Streptoneural,  the  relatively  small  minority  are  euthyneural 
opisthobranchs  and  nudibranchs,  with  light  shells  or  none. 
The  Heteropods  and  some  opisthobranchs  live  in  the  open 
sea  ;  the  great  majority  of  aquatic  Gasteropods  frequent  the 


36o 


MOLLUSC  A. 


shore  and  the  sea  bottom  at  relatively  slight  depths  ;  the 
deep  sea  forms  are  comparatively  few. 

Gasteropods  rarely  feed  at  such  a  low  level  as  bivalves  do, 
indeed  some  of  them  are  fond  of  eating  bivalves.  Most 
prosobranchs  (streptoneural),  with  a  respiratory  siphon  and 
a  shell  notch  in  which  this  lies,  are  carnivorous,  e.g.,  the 
buckies  (Bucdnum)  and  "  dog  whelks  "  (Purpura) ;  on  the 
other  hand,  those  without  this  siphon,  and  with  an  un- 
notched  shell  mouth,  feed  on  plants,  e.g.,  the  seaweed  eating 
periwinkles  (Littorina).  The  vege- 
tarian habits  of  most  land  snails 
and  slugs  are  known  to  all.  Many 
Gasteropods,  both  marine  and  ter- 
restrial, are  very  voracious  and  in- 
discriminate in  their  meals;  others 
are  as  markedly  specialists  or  epi- 
cures. Some  marine  forms,  partial 
to  Echinoderms,  have  got  over  the 
difficulty  of  eating  such  hard  food 
by  secreting  dilute  sulphuric  acid, 
which  is  said  to  change  the  car- 
bonate of  lime  in  the  starfish  into 
the  more  brittle  and  readily  pul- 
verised sulphate.  Only  a  few  Gas- 
teropods are  parasitic,  e.g.,  Eulima 
and  Sty  lifer  on  Echinoderms,  and 
the  extremely  degenerate  Entoconcha 
mirabilis, — within  the  Holothurian 
Synapta. 

Life  History. 

The  eggs  of  Gasteropods  are 
usually  small,  without  much  yolk, 
but  surrounded  by  a  jelly,  the  sur- 
face of  which  hardens.  In  the 
snail  and  in  some  others  there  is 
an  egg  shell  of  lime. 

Sexual  union  occurs  between  her- 
maphrodites as  well  as  between  separate  sexes,  and  ferti- 
lisation is  effected  inside  the  genital  duct.  Development 
sometimes  proceeds  within  the  parent,  but  in  most  cases 


FIG.  117. — Stages  in 
Molluscan  development. 

A ,  Blastula  of  Limpet  (after 
Patten) ;  B,  Gastrula  of  Palu- 
dina7rivtyara.(a.fterTdnniges)', 
v,  beginning  of  velum  ;  arc.) 
archenteron  ;  ;>z,  mesoderm 
cells ;  C,  later  stage  of  the 
same ;  z/,  velum ;  in,  mouth 
invagination  ;  arc.,  archen- 
teron ;  a,  anus  ;  f,  beginning 
of  foot ;  sh.g,  shell  gland. 


BIONOMICS.  361 

the  fertilised  eggs  are  laid  in  gelatinous  clumps,  or  within 
special  capsules.  The  free  swimming  lanthina  carries 
the  eggs  in  capsules  attached  to  a  large  raft-like  float 
towed  by  the  foot.  On  the  shore  one  often  finds  numer- 
ous egg  capsules  of  the  "buckie"  (Buccinum  undatum) 
united  in  a  ball  about  the  size  of  an  orange.  Under 
the  ledges  of  rock  are  many  little  yellowish  cups,  the 
egg  capsules  of  the  dog  whelk  (Purpura  lapillus).  In 
the  buckie  and  whelk,  and  in  some  other  forms,  there  is 
a  struggle  for  existence — an  infant  cannibalism — in  the 
cradle,  for  out  of  the  numerous  embryos  in  each  capsule 
only  a  few  reach  maturity,  those  that  get  the  start  eating  the 
others  as  they  develop. 

The  segmentation  of  the  ovum  is  total,  but  somewhat 
unequal ;  a  gastrula  is  formed  by  invagination  or  by  over- 
growth according  as  there  is  less  or  more  yolk ;  the  gastrula 
becomes  a  trochosphere  with  a  pre-oral  ring  of  cilia ;  the 
trochosphere  grows  into  a  veliger  with  a  lobed  ciliated 
cushion  or  velum,  a  visceral  dome,  a  dorsal  shell  gland 
which  soon  disappears,  and  an  incipient  ventral  foot.  In 
terrestrial  snails  like  Helix,  the  life  history  is  abbreviated. 
In  the  water  snail  Limnczus,  Ray  Lankester  has  detected 
the  persistence  of  the  velum  in  the  circumoral  lobes  of  the 
adult. 

Past  History. — As  the  earth  has  grown  older  the  Gasteropods  have 
increased  in  numbers.  A  few  have  been  disinterred  from  the  Cambrian 
rocks ;  thence  onwards  they  increase.  Most  of  the  Palaeozoic  genera  are 
now  quite  extinct,  but  many  modern  families  trace  their  genealogy  to  the 
Cretaceous  period.  Those  with  respiratory  siphons  were  hardly,  if  at 
all,  represented  in  Palaeozoic  ages,  and  the  terrestrial  air  breathers  are 
comparatively  modern.  Zoological  statisticians  estimate  the  number  of 
Gasteropods  at  23,000,  of  which  7000  are  extinct,  16,000  alive.  But 
besides  the  numerical  success  which  may  be  inferred  from  these  figures, 
it  is  important  to  notice  that  not  a  few  types  have  persisted  from  early 
ages. 

Bionomics. 

As  voracious  animals,  with  irresistible  raspers,  Gastero- 
pods commit  many  atrocities  in  the  struggle  for  existence 
and  decimate  many  plants.  Professor  Stahl  shows,  however, 
that  there  are  more  than  a  dozen  different  ways  in  which 
plants  are  saved  from  snails, — by  crystals,  acids,  ferments, 
&c.  ;  and  like  an  orthodox  Darwinian  he  regards  these 


362  MOLLUSC  A. 

plants  as  the  survivors  of  a  multitude,  which  did  not 
become  sufficiently  gritty  or  poisonous.  As  food  and  bait 
many  Gasteropods  are  very  useful;  their  shells  have  supplied 
tools  and  utensils  and  objects  of  delight  ;  the  juices  of 
Purpura  and  Murex  furnished  the  Tyrian  purple,  more 
charming  than  all  aniline. 

Class  III.    SCAPHOPODA. 

Very  different  from  Gasteropoda  are  the  Scaphopoda,  of  which 
Dentalium  (Elephant's  tooth  shell)  is  the  commonest  genus.  Their 
position  is  uncertain.  Some  place  them  nearer  bivalves,  others  nearer 
Cephalopods.  They  burrow  in  the  sand  at  considerable  depths  off  the 
coasts  of  many  countries.  The  mantle  has  originally  two  folds,  which 
fuse  ventrally,  and  the  shell  becomes  cylindrical,  like  an  elephant's  tusk. 
It  is  open  at  both  ends.  The  larger  opening  (directed  downwards  in 
the  sand)  is  anterior,  the  concave  side  of  the  shell  is  dorsal.  The  small 
cylindrical  head  bears  at  its  extremity  a  mouth  surrounded  by  numerous 
"tentacles,"  while  at  its  base  there  is  a  double  cluster  of  ciliated  con- 
tractile processes  possibly  representing  gills.  The  foot  is  long,  with 
three  small  terminal  lobes.  It  is  used  in  slow  creeping,  and  is  protruded 
at  the  anterior  opening.  There  are  cerebral  and  pleural  ganglia,  near 
one  another  in  the  head,  pedal  ganglia  in  the  foot,  and  a  long  untwisted 
visceral  loop  with  olfactory  ganglia  near  the  posterior  anus.  Sense 
organs  are  represented  by  otocysts  beside  the  pedal  ganglia.  There  is 
an  odontophore  with  a  simple  radula.  The  food  consists  of  minute 
animals.  There  is  no  heart,  but  colourless  blood  circulates  in  the  body 
cavity.  There  are  two  nephridial  apertures,  one  on  each  side  of  the 
anus  ;  the  nephridial  chamber  is  perforated  by  the  intestine.  The 
sexes  are  separate  ;  the  reproductive  organ  is  simple  and  dorsal  in 
position  ;  the  elements  pass  out  by  the  right  nephridium.  The  gastrula 
is  succeeded  by  a  free  swimming  stage,  in  which  there  is  a  hint  of  a 
velum  and  a  rudimentary  shell  gland. 

Examples. — Dentalium^  Entalium.  About  forty  widely  distributed 
species  are  known.  Dentalium  entale  occurs  off  British  coasts. 
The  genus  occurs  as  a  fossil  from  Carboniferous  (or  perhaps 
earlier)  strata  onward. 


Class  IV.  LAMELLIBRANCHIATA  or  BIVALVES. 

(Synonyms.  Acephala,  Conchifera,  Pelecypoda, 
Lipocephala,  &c.) 

Examples. — Cockles,  Mussels,  Clams,  and  Oysters. 

Lamellibranchs  are   bilaterally   symmetrical  Molluscs,    in 
which  the  body  is  compressed  from  side  to  side  and  the  foot 


LAMELLIBRANCHIA  TA.  363 

more  or  less  ploughshare-like.  The  head  (or  prostomiuni) 
region  remains  undeveloped,  without  tentacles  or  eyes ;  the 
mouth  is  without  radula,  horny  jaws,  or  salivary  glands,  but 
there  is  a  pair  of  labial  palps  on  each  side.  The  mantle  skirt 
is  divided  into  two  flaps,  which  secrete  the  two  valves  of  the 
shell,  now  lateral  instead  of  dorsal  in  position.  The  valves 
are  united  by  a  dorsal  elastic  ligament,  and  closed  by  two 
transverse  adductor  muscles  or  by  one.  Internal  bilateral 
symmetry  is  marked  by  the  paired  nature  and  disposition  of 
the  nephridia,  auricles,  gills,  digestive  and  reproductive  organs. 
The  gills  (ctenidia)  consist  of  numerous  gill  filaments  which 
typically  grow  together  into  large  plates  (hence  the  title 
Lamellibranch).  (  There  are  usually  three  pairs  of  ganglia  : 
(a)  cerebro-pleurals  in  the  head ;  (b)  pedals  in  the  foot ;  (c) 
viscerals  at  the  posterior  end  vf the  body.  The  heart  consists 
of  a  ventricle  and  two  auricles,  and  is  surrounded  by  a  peri- 
cardium which  is  cczlomic  in  origin,  and  communicates  with 
the  exterior  by  means  of  the  two  nephridia.  Reproductive 
organs  are  always  simple,  and  the  sexes  are  usually  separate. 
The  typical  development  includes  trochosphere  and  veliger 
stages.  Most  Lamellibranchs  feed  on  microscopic  organisms 
and  particles ;  the  distribution  is  very  wide  both  in  salt  and 
fresh  water  ;  the  average  habit  is  sedentary  or  sluggish. 

Type  of  LAMELLIBRANCHIATA  :  The  Fresh  water  Mussel 
(Anodonta  cygnea). 

The  fresh  water  mussel  lives  in  rivers  and  ponds.  It  lies 
with  its  head  end  buried  in  the  mud,  or  ploughs  slowly 
along  by  means  of  its  wedge-like  foot.  Its  food  consists  of 
minute  plants  and  animals,  which  are  wafted  in  at  the 
posterior  end  by  the  currents  produced  by  the  ciliated  gills. 

External  Appearance* 

The  bivalve  is  four  to  six  inches  long;  its  valves  are 
equal  and  united  in  a  dorsal  hinge  by  an  elastic  ligament, 
an  uncalcified  part  of  the  shell ;  on  the  ventral  surface 
when  the  valves  gape  the  foot  protrudes ;  the  anterior  end 
is  rounded,  the  posterior  end  is  more  pointed,  and  it  is 
there  that  the  water  currents  flow  in  (ventrally)  and  out 
(dorsally).  In  bivalves  the  ligament  is  generally  posterior 


364  MOLLUSC  A. 

to  the  dorsal  knob  or  umbo — the  oldest  part  of  the  shell — 
and  the  umbo  generally  points  towards  the  anterior  end. 
The  greenish-brown  soft  ("  horny ")  layer  of  the  shell  is 
often  worn  away  near  the  umbo  on  each  side,  and  then 
displays  the  median  "  prismatic  "  layer  of  lime.  Internally 
there  is  a  pearly  layer.  Lines  of  growth  on  the  shell  mark 
the  position  of  the  margin  in  former  years,  the  youngest 
part  being  obviously  at  the  edge. 

The  shell  is  a  cuticular  structure,  i.e.,  it  is  made  by  the 
epidermis  of  the  mantle.  !  It  consists  mostly  of  calcium 
carbonate  plus  an  organic  substance  called  conchiolin. 


Internal  Appearance. 

When  the  right  half  of  the  shell  is  folded  back,  the 
anterior  and  posterior  closing  muscles  being  carefully  cut 
close  to  the  gently  raised  valve,  the  mantle  folds  are  seen 
lining  the  shell  and  forming  posteriorly  the  ventral  inhalent 
and  dorsal  exhalent  lips.  The  ventral  lips  have  papillary 
processes.  (In  some  bivalves,  e.g.,  My  a,  the  lips  of  the 
mantle  are  prolonged  into  retractile  siphons.)  Internal  to 
the  mantle  two  gill  plates  lie  on  each  side ;  projecting  from 
between  these  is  the  foot,  muscular  ventrally,  softer  dorsally ; 
the  median  dorsal  pericardium  is  just  beneath  the  ligament ; 
the  ventricle  shines  through  its  walls,  and  the  dark-coloured 
kidneys  are  seen  through  its  floor.  Below  the  anterior 
closing  (adductor)  muscle,  the  large  mouth  will  be  found, 
bordered  beneath  by  two  lip  processes  (labial  palps)  on  each 
side;  above  the  posterior  closing  muscle  the  food  canal 
ends.  The  whole  space  between  the  two  mantle  flaps  is 
called  the  mantle  cavity,  and  it  is  divided  by  a  slight  parti- 
tion at  the  bases  of  the  gills  into  a  large  ventral  infra- 
branchial  chamber,  and  a  small  dorsal  supra-branchial 
chamber  which  ends  at  the  exhalent  orifice. 

On  the  valve  of  the  shell  folded  back,  are  seen  a  number 
of  concentric  (pallial)  lines,  marking  the  gradual  extension  of 
the  mantle  and  the  consequent  growth  of  the  shell.  A  few 
small  pearls  may  also  be  seen;  they  are  formed  by  the 
enclosure  of  some  minute  grains  of  sand  in  the  prismatic 
layer.  The  following  muscles  are  inserted  on  the  shell  and 
leave  impressions : — 


SKIN— MUSCULAR  SYSTEM.  365 

(a)  The  anterior  adductor. 
(&)  The  posterior  adductor. 

(c)  The    anterior    retractor    of    the    foot    continuous 

with  (a). 

(d)  The  protractor  of  the  foot  a  little  below  (a). 

(e)  The   posterior   retractor    of    the   foot    continuous 

with  (£). 

As  the  shell  grows  the  insertion  of  the  muscles  and  the 
attachment  of  the  mantle  change,  and  the  traces  of  this 
shifting  are  visible. 

Skin. 

There  is  much  ciliated  epithelium  about  Anodonta,  especi- 
ally on  the  internal  surface  of  the  mantle,  on  the  gills,  and  on 
the  labial  palps ;  and  little  pieces  cut  from  an  animal  incom- 
pletely dead  (e.g.,  from  the  oyster  which  many  of  us  swallow 
half  alive)  have  by  means  of  their  cilia  a  slight  power  of 
motion.  The  skin  of  the  foot  is  not  ciliated  but  glandular ; 
on  the  mantle  edge  sensitive  and  glandular  cells  are  abun- 
dant, but  usually  in  inverse  ratio  to  one  another. 

Muscular  System. 

The  shell  is  closed  and  kept  closed  by  the  action  of  the 
two  adductor  muscles.  When  these  are  relaxed  under 
nervous  control,  the  elasticity  of  the  hinge  ligament  opens 
the  valves.  A  book  with  an  elastic  binding,  stretched  when 
the  book  is  closed  by  clasps,  would  in  the  same  way  open 
when  unclasped.  It  is  easier  for  the  mussel  to  open  the 
valves  of  its  shell  than  to  keep  them  shut.  The  foot  is  a 
muscular  protrusion  of  the  ventral  surface,  under  the  control 
of  three  muscles — a  retractor  and  a  protractor  anteriorly, 
and  a  posterior  retractor.  Its  upper  portion  contains  some 
coils  of  gut  and  the  reproductive  organs ;  its  lower  region 
is  very  muscular.  /  The  protrusion  or  extension  of  this  loco- 
motor  organ  is  mainly  due  to  an  inflow  of  blood,  which  is 
prevented  from  returning  by  the  contraction  of  a  sphincter 
muscle  round  the  veins.  In  moving,  the  animal  literally 
ploughs  its  way  along  the  bottom  of  the  pond  or  river  pool, 
and  leaves  a  furrow  in  its  track.  The  muscle  fibres  are  of 
the  slowly  contracting  non-striped  sort. 


366 


MOLLUSC  A. 


Nervous  System. 

There  are  three  pairs  of  nerve  centres  : — 

(a)  Cerebro-pleural  ganglia,  lying  above  the  mouth  on 
each  side  on  the  tendon  of  the  anterior  retractor 
of  the  foot,  connected  to  one  another  by  a  com- 
missure, connected  to  the  two  other  pairs  of 
ganglia  (b)  and  (c\  by  long  paired  connectives, 
and  giving  off  some  nerves  to  mantle,  palps,  &c. 


FIG.  1 1 8. — Nervous  system  of  Molluscs. 

To. the  left  that  of  Anodonta;  to  the  right  that  of  Octopus  ;  in  the 
middle  that  of  Helix.  In  the  last  two  the  position  of  the  gullet  is 
shown. 

c.p.)  Cerebro  pleural  ganglia;  /.,  pedals;  v.,  yiscerals;  c.,  cere- 
brals; //.,  pleurals  ;  £.,  buccals;  s.,  stellate  ganglion. 

(b)  Pedal  ganglia,  lying  close  together  about  the  middle 

of  the  foot,  united  by  connectives  to  (a\  giving 
off  nerves  to  the  foot,  and  having  beside  them 
two  small  ear  sacs,  each  with  a  calcareous 
otolith,  and  with  a  nerve  said  to  be  derived 
from  the  connective  between  (a)  and  (b}. 

(c)  Visceral  ganglia  (also  called  parieto-splanchnic  or 

osphradial),  lying  below  the  posterior  adductor, 
connected  to  (a)  by  two  long  connectives,  and 
giving  off  nerves  to  mantle,  muscles,  &c.,  and 
to  a  patch  of  "  smelling  cells  "  at  the  bases  of 
the  gills. 


ALIMENTARY  AND    VASCULAR  SYSTEMS.        367 

Sense  Organs. 

Unlike  not  a  few  bivalves  which  have  hundreds  of  "eyes" 
on  the  mantle  margin,  Anodonta  has  no  trace,  of  any.  The 
ear  sac,  originally  derived  from  a  skin  pit,  is  sunk  deeply 
within  the  foot  and  is  of  doubtful  use.  The  "smelling 
patch"  or  " osphradium"  at  the  base  of  the  gills,  has 
perhaps  water  testing  qualities.  There  are  also  "  tactile  " 
cells  about  the  mantle,  labial  palps,  &c. 

Alimentary  System. 

The  mouth  lies  between  the  anterior  adductor  and  the 
foot,  and  beside  it  lie  the  ciliated,  vascular,  and  sensitive 
labial  palps,  two  on  each  side.  It  opens  immediately  into  the 
gullet,  for  the  pharynx  of  other  Molluscs,  with  all  its  associ- 
ated structures,  is  absent  in  Lamellibranchs.  The  short 
wide  gullet  leads  into  a  large  stomach  surrounded  by  the 
paired  digestive  gland  (hepato-pancreas  (?) ),  whose  juices 
are  partly  analogous  with  those  of  the  Vertebrate  liver  and 
pancreas.  Part  of  the  food  digested  by  these  juices  in  the 
stomach  is  compacted  in  autumn  into  a  "crystalline  style" — 
a  mass  of  reserve  food  stuffs,  and  similar  but  less  solid 
material  is  found  in  the  intestine.  On  this  supply  the 
mussel  tides  over  the  winter.  Some  authorities,  however, 
maintain  that  the  style  is  a  glandular  secretion,  protecting 
the  lining  of  the  gut  from  injury.  Similar  structures  are 
found  in  several  Gasteropods.  The  intestine,  which  has  in 
part  a  folded  wall  like  that  of  the  earthworm,  coils  about  in 
the  foot,  ascends  to  the  pericardium,  passes  through  the 
ventricle  of  the  heart,  and  ends  above  the  posterior  adductor 
at  the  exhalent  orifice. 

Vascular  System. 

The  heart  lies  in  the  middle  line  on  the  dorsal  surface, 
within  a  portion  of  the  body  cavity  called  the  pericardium, 
and  consists  of  a  muscular  ventricle  which  has  grown  round 
the  gut  and  drives  blood  to  the  body,  and  of  two  trans- 
parent auricles — one  on  each  side  of  the  ventricle — which 
receive  blood  returning  from  the  gills  and  mantle.  The 
colourless  blood  passes  from  the  ventricle  by  an  anterior 


368 


MOLLUSC  A. 


and  a  posterior  artery,  flows  into  ill-defined  channels,  is 
collected  in  a  "vena  cava"  beneath  the  floor  of  the  peri- 
cardium, passes  thence  through  the  kidneys,  where  it  loses 
nitrogenous  waste,  to  the  gills,  where  it  loses  carbonic  acid 
and  gains  oxygen,  and  returns  finally  by  the  auricles  to  the 
ventricle.  The  blood  from  the  mantle,  however,  returns 
directly  to  the  auricles  without  passing  through  kidneys  or 
gills,  but  probably  freed  from  its  waste  none  the  less.  The 
so-called  "organ  of  Keber"  consists  of  "pericardial  glands" 


s.t. 


FIG.  119. — Structure  of  Anodonta.     (After  RANKIN.) 

aa.,  Anterior  adductor;  c.p.g.,  cerebro-pleural  ganglia;  st., 
stomach^z/.,  ventricle^  with  an  auricle  opening  into  it ;  /&.,  kidney, 
above  which  the  posterior  retractor  of  foot ;  r.,  rectum,  ending  above 
posterior  adductor  ;  v.g.,  visceral  ganglia  with  connectives  (in  black) 
from  cerebro  pleurals ;  g.,  gut  coiling  in  foot ;  p.g.,  pedal  ganglia  in 
foot,  where  also  are  seen  branches  of  the  anterior  aorta  and  the 
reproductive  organs;  /./.,  labial  palps  behind  mouth. 

on  the  epithelium  of  the  pericardial  cavity.  They  seem 
to  be  connected  with  excretion.  Many  of  the  cells  lining 
the  blood  channels  secrete  glycogen,  the  principal  product 
of  the  Vertebrate  liver. 


RESPIRATORY  AND  EXCRETORY  SYSTEMS.      369 

Respiratory  System. 

Lying  between  the  mantle  flaps  and  the  foot  there  are  on 
each  side  two  large  gill  plates,  whence  the  title  Lamelli- 
branch.  They  are  richly  ciliated,  their  internal  structure  is 
like  complex  trellis  work,  their  cavities  communicate  with 
the  supra-branchial  chamber.  "  Ctenidia  "  they  are  often 
called,  because  they  are  more  than  gills ;  not  only  are  they 
surfaces  on  which  blood  is  purified  by  the  washing  water 
currents  (a  respiratory  function),  but  some  of  their  many 
cilia  waft  food  particles  to  the  mouth  (a  nutritive  function), 
and  in  the  females  the  outer  gill  plate  shelters  and  nourishes 
the  young  larvae  (a  reproductive  function).  The  water  may 
pass  through  the  gills  to  the  supra-branchial  chamber  and 
thence  out  again,  or  over  the  gills  to  the  mouth,  and  thence 
into  the  supra-branchial  chamber.  It  is  likely  that  the 
mantle  has  no  small  share  in  the  respiration. 

The  precise  structure  and  attachment  of  the  gill  plates  is  complex, 
but  it  is  important  to  understand  the  following  facts  : — (a)  a  cross 
section  of  the  two  gill  plates  on  one  side  has  the  form  of  a  W,  one  half 
of  which  is  the  outer,  the  other  the  inner  gill  plate  ;  (b]  each  of  these 
gill  plates  consists  of  a  united  series  of  gill  filaments,  which  descend 
from  the  centre  of  the  W  and  then  bend  up  again ;  (c)  adjacent  fila- 
ments are  bound  together  by  fusions  and  bridges  both  horizontal  and 
vertical,  so  that  each  gill  plate  becomes  like  a  complex  piece  of  basket 
work  ;  (d)  both  gill  plates  begin  by  the  downward  growth  of  filaments 
from  a  longitudinal  "  ctenidial  axis,"  the  position  of  which  on  cross 
section  is  at  the  median  apex  of  the  W ;  (e)  this  mode  of  origin,  and  the 
much  less  complex  gills  of  other  bivalves,  lead  one  to  believe  that  there 
is  on  each  side  one  gill,  consisting  of  two  gill  plates  formed  from  a  series 
of  united  and  reflected  gill  filaments.  On  the  gills  there  are  often 
parasitic  mites  (Atax], 

Excretory  System. 

The  paired  kidney,  which  used  to  be  called  the  "  organ 
of  Bojanus,"  lies  beneath  the  floor  of  the  pericardium. 
Each  half  is  a  nephridium  bent  upon  itself,  with  the  loop 
posterior,  the  two  ends  anterior.  The  lower  part  of  this 
bent  tube  is  the  true  kidney ;  it  is  dark  in  colour,  spongy  in 
texture,  and  excretes  guanin  and  other  nitrogenous  waste 
from  the  blood  which  passes  through  it.  It  has  an  internal 
opening  into  the  pericardium,  which  thus  communicates 
indirectly  with  the  exterior.  The  upper  part  of  the  bent 
tube,  lying  next  the  floor  of  the  pericardium,  is  merely  a 

24 


370  MOLLUSC  A. 

ureter.  It  conveys  waste  products  from  the  glandular  part 
to  the  exterior,  and  opens  anteriorly  just  under  the  place 
where  the  inner  gill  plate  is  attached  to  the  visceral  mass. 
As  already  mentioned,  the  "  pericardial  glands  "  probably 
aid  in  excretion,  and  possibly  the  same  may  be  said  of  the 
mantle. 

The  Reproductive  Organs. 

These  lie  in  the  upper  part  of  the  foot,  adjacent  to  the 
digestive  gland.  Ovaries  and  testes  occur  in  different 
animals,  and  the  two  sexes  are  distinguishable,  though  not 
very  distinctly,  by  the  greater  whiteness  of  the  testes  and  by 
slight  differences  in  the  shells.  The  females  are  easily 
known  when  the  larvae  begin  to  accumulate  in  crowds  in  the 
outer  gill  plates.  The  reproductive  organs  are  branched 
and  large  ;  there  are  no  accessory  structures ;  the  genital 
aperture  lies  on  each  side  under  that  of  the  ureter. 

Autumn  and  winter  months  seem  to  be  the  usual  periods 
of  reproduction.  The  ova  are  squeezed  out  of  the  foot, 
and  appear  to  be  moved  to  the  exhalent  region,  whence, 
however,  they  do  not  escape,  but  are  crowded  backward  till 
they  pass  into  the  cavity  of  the  outer  gill  plate.  At  some 
stage  they  are  fertilised  by  spermatozoa  drawn  in  by  the 
water  currents,  though  it  is  difficult  to  believe  that  this  is 
entirely  a  matter  of  chance.  Development  takes  place  in 
the  gill  cavity,  which  is  often  much  distended  with  larvae. 

Development  and  Life  history. 

The  development  of  Anodonta  differs  in  certain  details  from  that  of 
most  bivalves,  perhaps  in  adaptation  to  fresh  water  conditions. 
Moreover,  a  temporary  parasitism  of  the  larva  has  complicated  the 
later  stages. 

The  egg  cell  is  surrounded  by  a  vitelline  membrane,  and  attached  to 
the  wall  of  the  ovary  by  a  minute  stalk,  the  insertion  of  which  is  marked 
on  the  liberated  ovum  by  an  aperture  or  micropyle,  through  which  the 
spermatozoon  enters. 

Segmentation  is  total  but  unequal.  A  number  of  small  clear  yolkless 
cells  are  rapidly  divided  off  from  a  large  yolk-containing  portion,  which 
is  slower  in  dividing.  Eventually,  a  hollow  ball  of  cells  or  blastosphere 
results  (Fig.  120). 

On  the  posterior  dorsal  region  a  number  of  large  opaque  cells  form  an 
internally  convex  plate,  the  beginning  of  the  future  shell  sac.  A  pair  of 
large  cells  are  intruded  into  the  central  cavity,  and  begin  the  mesoderm. 

On  the  under  surface  posteriorly  there  is  a  slight  protrusion  of  ciliated 
cells  forming  a  ciliated  disc.  In  front  of  this,  at  an  unusually  late  stage, 


DEVELOPMENT  AND  LIFE  HISTORY. 


an  invagination  establishes  the  archenteron  and  the  embryo  becomes  a 
gastrula  (see  Fig.  120). 

The  shell  sac  forms  an  embryonic  shell,  and  many  of  the  mesoderm 
cells  combine  in  an  adductor  muscle.  The  mouth  of  the  gastrula  closes, 
and  a  definite  mouth  is  subsequently  formed  by  an  ectodermic  invagina- 
tion. Gradually  a  larva  peculiar  to  fresh  water  mussels,  and  known  as 
a  Glochidium,  is  built  up. 

The  Glochidium  has  two  triangular,  delicate,  and  porous  shell  valves, 
each  with  a  spiny  incurved  tooth  on  its  free  edge.  The  valves  clap 
together  by  the  action  of  the  adductor  muscle.  The  mantle  lobes  are 
very  small,  and  their  margins  bear  on  each  side  three  or  four  patches  of 
sensory  cells.  The  foot  is  not  yet  developed,  but  from  the  position 
which  it  will  afterwards  occupy  there  hang  long  attaching  threads  of 


FIG.  120. — Development  of  Anodonta.     (After  GCETTE.) 

1.  Section  of  blastosphere.     sd.,  shell  gland  ;  cd.,  ciliated  disc  ; 
£.,  beginning  of  endodermic  imagination.     Note  mesoderm  cells 
in  the  cavity. 

2.  Later  stage,     in.  mesoderm. 

3.  Embryonic  shell  has  appeared. 

J.  Glochidium  larva ;  note  byssus  threads,  and  teeth  on  shell 
ves. 

"  byssus,"  which  moor  the  larva.  If  it  manage  to  anchor  itself  on  the 
tail,  fins,  or  gills  of  a  fish,  the  Glochidium  shuts  its  valves  and  fixes  itself 
more  securely,  and  is  soon  surrounded  by  a  pathological  growth  of  its 
host's  skin. 

In  this  parasitic  stage  a  remarkable  metamorphosis  occurs.  The 
sensory  or  tactile  patches  not  unnaturally  disappear  ;  the  byssus  and  the 
embryonic  byssus  glands  vanish,  but  a  new  byssus  gland  (which  remains 


372  MOLLUSC  A. 

quite  rudimentary  in  Anodonta]  appears  ;  the  single  adductor  atrophies 
and  is  replaced  by  two  ;  the  foot  and  the  gills  make  their  appearance  ; 
the  embryonic  mantle  lobes  increase  greatly,  or  are  replaced  by  fresh 
growths ;  and  the  permanent  shell  begins  to  be  made. 

After  this  metamorphosis,  when  the  larva  has  virtually  become  a 
miniature  adult,  no  longer  so  liable  to  be  swept  away,  it  drops  from  its 
temporary  host  to  the  bottom  of  the  pond  or  river  pool. 

CLASSIFICATION. 

Lamellibranchs  are  often  classified  as  follows,  the  emphasis  being  laid 
on  the  adductor  muscles  : — 
Order  I.  ISOMYA.     Adductor  muscles  approximately  equal. 

Sub-order  I.  Integripallia.  The  mantle's  line  of  attachment  to 
the  shell  is  not  broken  by  a  sinus  into  which  inhalent  and 
exhalent  siphons  may  be  retracted,  but  in  most  these  siphons 
are  present.  Area  (Noah's  Ark  shell),  Unio  and  Anodonta 
(fresh  water),  Lticina,  Cyprina,  Cardium  (cockle),  Cyclas 
(fresh  water),  Tridacna  (the  largest  form). 

Sub-order  2.  Sinupallia.  The  mantle's  line  of  attachment  to 
the  shell  is  inflected  by  a  sinus  into  which  the  large  siphons  are 
retracted.  Venus,  Mya,  Saxicava  (a  boring  bivalve),  Solen 
(razor  shell),  Pholas  (borer),  Teredo  (ship  worm),  Aspergillum 
(watering  pot  shell). 

Order  2.   HETEROMYA.     The  anterior  adductor  is  much  smaller  than 
the   posterior,  and  siphons  are  rare.     Mytilus  (edible  mussel), 
Modiola  (horse  mussel),  Lithodoimis  (borer),  Dreissena. 
Order  3.    MONOMYA.       One   adductor,    no    siphon.      Ostrea   (oyster), 
Anemia,  Lima,  Pecten  (scallop). 

Pelseneer,  however,  lays  emphasis   on  the  nature  of  the  gills,  and 
classifies  as  follows  : — 
Order    i.    PROTOBRANCHIA.      The    gill    filaments,   arranged   in   two 

divergent  rows,  are  not  reflected,  e.g. ,  Nucula,  Solenomya. 
Order  2.   FILIBRANCHIA.    The  gill  filaments  lie  parallel  and  are  directed 

ventrally ;     they    are    reflected,     and    united    only   by   ciliated 

interfilamentar  bridges,  e.g.,  Anomia,  Area,  Mytilus. 
Order  3.   PSEUDOLAMELLIBRANCHIA.     The  gill  filaments  are  loosely 

connected  by  connective  or  vascular  bridges,  e.g. ,  Avicula,  Ostrea, 

Pecten. 
Order  4.  EULAMELLIBRANCHIA.      The  gill  filaments  are  bound  into 

plates,  as  in  Anodon,  Cardium,  Mya,   Venus. 
Order  5.    SEPTIBRANCHIA.     The   gills   form   a   remarkable   muscular 

septum   extending  round  the  foot   from   the   anterior   adductor 

to  the  separation  of  the  two  siphons.     Poromya  and  Cuspidaria. 

GENERAL  NOTES  ON  LAMELLIBRANCHS. 

The  organs  which  most  frequently  vary  in  other  bivalves,  as  com- 
pared with  Anodonta,  are  the  foot,  the  gills,  the  adductor  muscles,  and 
the  mantle  skirt.  The  foot  varies  much  in  size  and  shape  ;  in  Area  it 
has  a  flat  sole-like  surface  which  suggests  the  creeping  foot  of  Gastero- 


GENERAL  NOTES  ON  LAMELLIBRANCHS.         373 

pods  ;  the  pedal  gland  of  Gasteropods  is  often  represented  by  a  "byssus" 
gland,  which  secretes  attaching  threads,  well  seen  in  the  edible  mussel 
(Mytilus}.  In  oysters  the  foot  is  absent.  The  gills  show  an  interesting 
series  of  gradations,  from  a  slight  interlocking  of  separate  gill  filaments 
to  the  formation,  by  complicated  processes  of  "concrescence,"  of  plate- 
like  structures  such  as  those  of  Anodonta.  These  processes  are,  however, 
much  more  closely  related  to  the  method  of  nutrition  than  of  respiration, 
which,  indeed,  is  probably  largely  performed  by  the  mantle  skirt.  The 
mantle  skirt  is  often  united  to  a  greater  or  less  extent  inferiorly,  and  is 
often  prolonged  and  specialised  posteriorly  to  form  exhalent  and  inhalent 
"siphons."  These  siphons  sometimes  attain  a  considerable  length; 
they  occur  especially  in  forms,  such  as  Mya,  which  live  buried  in  sand 
or  mud,  or  which  burrow  in  wood  or  stone,  e.g. ,  Pholas.  The  variations 
of  the  adductor  muscles  afford  one  basis  for  classification. 

We  may  associate  with  the  sluggish  habits  and  sedentary  life  of 
bivalves  (i)  the  undeveloped  state  of  the  head  region  ;  (2)  the  largeness 
of  the  plate-like  gills  which  waft  food  particles  to  the  mouth ;  and  (3)  the 
thick  limy  shells.  We  may  reasonably  associate  these  and  other  facts  of 
structure  (e.g. ,  the  absence  of  head  eyes,  biting  or  rasping  organs)  with 
the  conditions  of  life,  without  being  able  to  say  very  precisely  what  the 
relation  is.  It  seems  to  some  not  improbable  that  sluggish  habits  have 
cumulative  and  manifold  results  in  the  course  of  generations ;  that 
structural  changes  produced  by  use  and  disuse  of  parts  may  have  con- 
stitutional consequences  which  affect  the  germ  cells,  that  is  to  say,  the 
offspring.  To  others  the  adaptations  seem  to  be  most  readily  explained 
as  the  result  of  the  natural  selection  of  indefinite  germinal  variations, 
which  arise  altogether  apart  from  function  or  environment.  In  thinking 
about  the  sluggishness  of  most  bivalves,  we  must  not  forget  that  the 
larval  trochospheres  and  veligers  are  very  active,  perhaps  almost  too 
active,  young  creatures. 

Habit. — Most  bivalves,  as  every  one  knows,  live  in  the  sea,  and  their 
range  extends  from  the  sand  of  the  shore  to  great  depths.  They  occur 
in  all  parts  of  the  world,  though  only  a  few  forms  like  the  edible  mussel 
(Mytilus  edtilis]  can  be  called  cosmopolitan.  Some,  such  as  oysters,  can 
be  accustomed  to  brackish  water.  The  fresh  water  forms  may  have 
found  that  habitat  in  two  ways — (a)  a  few  may  have  crept  slowly  up 
from  estuary  to  river,  from  river  to  lake  ;  Dreissena  polymorpha  has 
been  carried  on  the  bottom  of  ships  from  the  Black  Sea  to  the  rivers  and 
canals  of  Northern  Europe  ;  and  it  is  likely  that  aquatic  birds  have 
assisted  in  distributing  little  bivalves  like  Cyclas  ;  (6)  on  the  other  hand, 
it  is  more  probable  that  the  fresh  water  mussels  ( Unio,  Anodonta,  &c. ), 
are  relics  of  a  fauna  which  inhabited  former  inland  seas,  of  which  some 
lakes  are  the  freshened  residues. 

Between  the  active  Lima  and  Pecten,  which  swim  by  moving  their 
shell  valves  and  mantle  flaps,  and  the  entirely  quiescent  oyster,  which 
has  virtually  no  foot,  there  are  many  degrees  of  passivity,  but  most 
incline  towards  the  oyster's  habit.  Of  course,  there  is  much  internal 
activity,  especially  of  ciliated  cells,  even  in  the  most  obviously  sluggish. 
The  cockle  (Cardium)  uses  its  bent  foot  to  take  small  jumps  on  the 
sand ;  the  razor  fish  (Solen]  not  only  bores  in  the  sand,  but  may  swim 
backwards  by  squirting  out  water  from  within  the  mantle  cavity ;  many 


374  MOLLUSC  A. 

(e.g.,  Teredo,  Pholas,  Lithodoums,  Xylophaga],  bore  holes  in  stone  or 
wood,  but  we  do  not  certainly  know  how  ;  in  the  great  majority  the  foot 
is  used  for  slow  creeping  motion. 

The  food  consists  of  Diatoms  and  other  Algae,  Infusorians  and  other 
Protozoa,  minute  Crustaceans  and  organic  particles,  which  the  ciliary 
action  of  the  gills  carries  from  the  posterior  end  of  the  shell  to  the 
mouth.  The  bivalves  are  themselves  eaten  by  worms,  star  fishes,  gas- 
teropods,  fishes,  birds,  and  even  mammals. 

Life  History. — The  eggs  are  sometimes  laid  in  the  water,  either  freely 
or  in  attached  capsules,  or,  more  frequently,  they  are  fertilised  by  sper- 
matozoa drawn  in  with  the  inhalent  water,  and  are  subsequently  sheltered 
within  the  body  during  part  of  the  development.  In  the  Unionidae  the 
embryos  are  retained  within  the  cavities  of  the  outer  gills ;  in  Cyclas 
and  Pisidium  there  are  special  brood  chambers  at  the  base  of  the  gills. 
In  Cyclas  the  embryos  are  nourished  by  the  maternal  epithelial  cells. 
Segmentation  is  always  unequal ;  a  gastrula  may  be  formed  by  invagina- 
tion  or  by  overgrowth,  the  two  cases  being  connected  by  a  series  of 
gradations.  A  trochosphere  stage  is  more  or  less  clearly  indicated, 
being  most  obvious  in  cases  where  the  eggs  are  laid  in  the  water.  The 
free  swimming  trochosphere  becomes  a  veliger,  and  this  is  modified  into 
the  adult.  The  fresh  water  forms,  with  the  exception  of  Dreissenapoly- 
morpha  in  which  the  habit  is  recently  acquired,  do  not  possess  free 
swimming  larvae ;  this  must  be  regarded  as  an  adaptation. 

Past  History  of  Bivalves. — Even  in  Cambrian  rocks,  which  we  may 
call  the  second  oldest,  a  few  bivalves  have  been  discovered  ;  in  the 
Upper  Silurian  they  become  abundant,  and  never  fall  off  in  numbers. 
About  9000  extinct  and  5000  living  species  were  catalogued  some  years 
ago,  so  that  we  evidently  have  our  full  share  now.  Those  with  one 
closing  muscle  to  the  shell  seem  to  have  appeared  after  those  which  have 
two  such  muscles.  Those  which,  from  the  shell  markings,  seem  to  have 
had  an  extension  of  the  mantle  into  a  protrusible  tube  or  siphon,  were 
also  of  later  origin.  The  present  fresh  water  forms  were  late  of  appearing. 
Of  all  the  fossil  forms  the  most  remarkable  are  large  twisted  shells,  called 
Hippurites  (Rudistae),  whose  remains  are  often  very  abundant  in  deposits 
of  the  chalk  period. 

Class  V. — CEPHALOPODA.     Cuttlefish. 
Examples  : — Sepia ,  Octopus,  Loligo,  Naiitilus. 

The  Cephalopods  are  bilaterally  symmetrical  free  swimming 
Molluscs.  Part  of  the  foot  has  come  to  surround  the  head, 
and  is  divided  into  numerous  "arms"  bearing  tentacles  or 
slickers.  Another  part  forms  a  partial  or  complete  tube — the 
"siphon"  or  "funnel" — through  which  water  is  forcibly 
expelled  from  the  mantle  cavity,  driving  the  animal  backwards. 
The  muscular  mantle  flap  which  shelters  the  gills  is  posterior 
in  position  ;  the  visceral  hump  shows  no  trace  of  spiral  coiling, 
but  is  elongated  in  a  direction  anatomically  dorsal  and  posterior. 


TYPES   OF  CEPHALOPODA.  375 

though  it  may  point forwards  when  the  animal  propels  itself 
through  the  water.  Except  in  the  Pearly  Nautilus,  the  shell 
of  modern  forms  has  been  enclosed  by  the  mantle,  and  is,  in 
most  cases,  only  hinted  at.  There  is  a  very  distinct  head 
region,  furnished  with  eyes  and  other  sensitive  structures,  and 
the  mouth  has  strong  beak-like  jaws,  as  also  a  well  developed 
radula.  The  nervous  system  shows  considerable  specialisation, 
and  the  chief  ganglia  are  concentrated  in  the  head.  The  true 
body  cavity,  pericardium  of  other  Molluscs,  is  usually  well 
developed,  and  frequently  surrounds  the  chief  organs.  Except 
in  Nautilus,  /"/  communicates  with  the  exterior  by  the  nephridia. 
The  vascular  system  is  well  developed,  and,  except  in 
Nautilus,  there  are  accessory  branchial  hearts.  The  sexes  are 
separate.  Development  is  direct.  In  habit,  Cephalopods  are 
predominantly  active  and  predatory  ;  in  diet,  carnivorous. 


TYPES    OF    CEPHALOPODA. 
First  Type.     The  Common  CUTTLEFISH  (Sepia  officinalis). 

Mode  of  Life. 

This  common  cuttlefish  is  widely  distributed,  especially 
in  warmer  seas  like  the  Mediterranean.  Unlike  the  Octopus, 
which  usually  lurks  passively,  the  Sepia  is  an  active  swimmer; 
it  moves  head  foremost  by  working  the  fins  which  fringe 
the  body,  or  it  jerks  itself  energetically  backwards  by  the 
outgush  of  water  through  the  funnel.  It  likes  the  light,  and 
is  sometimes  attracted  by  lanterns.  The  beautiful  colours 
change  according  to  external  conditions  and  internal  emo- 
tions ;  and  a  plentiful  discharge  of  ink  often  covers  its  retreat 
from  an  enemy.  Its  food  includes  fish,  other  molluscs,  and 
crabs.  In  spring  the  female  attaches  her  encapsuled  eggs 
to  sea  weeds  and  other  objects,  and  often  comes  fatally  near 
the  shore  in  so  doing.  The  "  cuttles  "  are  caught  for  food 
and  bait.  The  "  cuttle  bone  "  and  the  pigment  of  the  ink 
bag  are  sometimes  utilised  by  man. 

External  Appearance. 

A  large  Sepia  measures  about  10  inches  in  length  and 
4  to  5  in  breadth ;  the  body,  fringed  by  a  fin,  is  shaped  like 
a  shield,  the  broad  end  of  which  bears  a  narrower  head, 


376  MOLLUSC  A. 

with  eight  short  and  two  long  sucker-bearing  arms.  Besides 
the  diffuse  pigment  cells,  there  are  bands  across  the  "  back." 
The  large  eyes,  the  parrot-beak-like  jaws  protruding  from 
the  mouth,  the  spout-like  funnel  on  the  neck,  and  the 
mantle  cavity  are  conspicuous. 

The  true  orientation  of  the  different  regions  in  Sepia  is 
not  obvious.  If  the  "arms"  surrounding  the  mouth  be 
divided  portions  of  the  anterior  part  of  the  "foot,"  the 
ventral  surface  is  that  on  which  the  animal  rests  when  we 
make  it  stand  on  its  head.  We  can  fancy  how  the  "foot" 
of  a  snail  might  grow  forward  and  surround  the  mouth,  so 
as  to  bring  that  into  the  middle  of  the  sole.  Then  the 
visceral  mass  has  been  elongated  in  an  oblique  dorso-posterior 
direction,  so  that  the  tip  of  the  shield,  directed  forward 
when  the  cuttle  jerks  itself  away  from  us,  represents  in 
anatomical  strictness  the  dorsal  surface  tilted  backwards. 
(As  above  noticed,  the  animal  may  also  swim  with  foot  and 
mouth  in  front.)  The  side  of  lighter  colour,  marked  by  the 
mantle  cavity  and  the  siphon  or  funnel,  is  posterior  and 
slightly  ventral :  the  banded  and  more  convex  side  on  which 
the  cerebral  ganglia  lie  in  the  head  region,  and  on  which 
the  shell  lies  concealed  in  the  visceral  region,  is  anterior 
and  slightly  dorsal. 

The  Skin. 

The  skin  contains  numerous  actively  changeful  pigment 
cells  or  chromatophores  lying  in  the  connective  tissue 
beneath  the  epidermis.  Each  cell  is  expanded  by  the 
contraction  of  fine  muscle  fibres  which  radiate  from  it,  and 
contracts  when  the  fibres  relax.  It  is  probable  that  these 
chromatophore  cells  have  some  protoplasmic  spontaneity  of 
their  own,  but  the  controlling  fibres  seem  to  be  affected  by 
nervous  impulses  from  the  central  ganglia.  As  the  cells 
dilate  or  contract  the  pigment  is  diffused  or  concentrated, 
and  the  colours  change.  The  animal's  beauty  is  further 
enhanced  by  numerous  "  iridocysts  "  or  modified  connective 
tissue  cells,  with  fine  markings  which  cause  iridescence. 

Muscular  System. 

The  cuttlefish  is  very  muscular,  notably  about  the  arms, 
the  mantle  flap,  and  the  jaws.  With  great  quickness  it 


SEPIA.  377 

seizes  its  prey  by  throwing  out  its  two  long  arms,  which  are 
often  entirely  retracted  within  pouches.  With  great  force  it 
jerks  itself  backwards  by  contracting  the  mantle  cavity,  and 
making  the  water  gush  out  through  the  pedal  funnel.  This 
mode  of  locomotion  is  very  quaint.  At  one  time  the  mantle 
cavity  is  wide,  and  you  can  thrust  your  fingers  into  its  gape ; 
when  about  to  contract,  this  gape  is  closed  by  a  strange 
double  hook  and  eye  arrangement ;  contraction  occurs,  and 
the  water,  no  longer  free  to  leave  as  it  entered,  gushes  out 
by  the  funnel,  the  base  of  which  is  within  the  mantle  cavity. 
Another  muscular  development  is  interesting,  that  of  the 
suckers  on  the  arms.  They  are  muscular  cups,  borne  on 
little  stalks  (unstalked  in  Octopus,  &c\  well  innervated,  and 
able  to  grip  with  a  tenacity  which  in  the  giant  cuttlefishes  is 
dangerous  even  to  men.  The  inner  edge  of  the  cup  margin 
is  supported  by  a  chitinoid  ring  bearing  small  teeth.  Each 
cup  acts  as  a  sucker,  in  a  fashion  which  has  many  analogues, 
for  a  retractor  muscle  increases  the  size  of  the  cavity  after 
the  margin  has  been  applied  to  some  object.  The  external 
pressure  is  then  greater  than  that  within  the  cup,  and  the 
little  teeth  keep  the  attachment  from  slipping. 

It  seems  likely  that  the  arms  represent  a  propodium,  and 
the  siphon  a  mesopodium,  and  a  valve  within  the  siphon  has 
been  compared  to  a  metapodium. 

Skeletal  System. 

An  internal  skeleton  is  represented  by  supporting  cartila- 
ginous plates  in  various  parts  of  the  body,  especially  (a)  in 
the  head,  round  about  the  brain,  arching  over  the  eyes, 
enclosing  the  "  ears  "  ;  (b)  at  the  bases  of  the  arms  ;  (c)  as  a 
crescent  on  the  neck ;  (d)  at  the  hook  and  eye  arrangement 
of  the  mantle  flap;  (e)  along  the  fringing  fins.  Ramified 
"  stellate  "  cells  lie  in  the  structureless  transparent  matrix  of 
the  cartilage. 

On  the  shore  one  often  finds  the  "cuttle  bone"  or 
sepiostaire,  which  is  sometimes  given  to  cage  birds  to  peck 
at  for  lime,  or  used  for  polishing  and  other  purposes.  It  lies 
on  the  dorsal  side  of  the  animal,  covered  over  by  the  mantle 
sac.  In  outline  it  is  somewhat  ellipsoidal,  thinned  at  the 
edges  like  a  flint  axe  head,  and  with  curved  markings  which 
indicate  lines  of  growth.  In  the  very  young  Sepia,  it  consists 


378  MOLLUSC  A. 

wholly  of  the  organic  basis  conchiolin,  but  to  this  lime  is 
added  from  the  walls  of  the  sac.  Between  the  plates  of  lime 
there  is  gas,  and  though  the  structure  may  give  the  cuttle 
some  stability,  it  is  probably  of  more  use  as  a  float. 

Internal  Appearance. 

When  we  cut  open  the  mantle  flap  and  fold  the  halves 
back,  we  at  once  see  the  two  plume-like  gills,  and  the  lower 
end  of  the  siphon.  The  dark  outline  of  the  ink  bag  followed 
along  towards  the  head  leads  our  eyes  to  the  end  of  the  food 
canal.  Near  this  are  the  external  apertures  of  the  two 
kidneys  and  of  the  genital  duct.  On  each  side  of  the  base 
of  the  funnel  lies  a  very  large  and  unmistakable  "  stellate  " 
ganglion.  Removing  the  skin  as  carefully  as  possible  over 
the  whole  visceral  region  between  the  gills,  and  taking  pre- 
cautions not  to  burst  the  ink  sac,  we  see  the  median  heart, 
the  saccular  kidneys,  contractile  structures  or  branchial  hearts 
at  the  base  of  each  gill,  and  the  essential  reproductive  organs 
near  the  apex  of  the  visceral  mass.  Disturbing  the  arrange- 
ment of  these  organs,  we  can  follow  the  food  canal  with  its 
stomach,  digestive  gland,  &c. 

Nervous  System. 

Three  pairs  of  ganglia  surround  the  gullet, — cerebral  on 
the  dorsal  and  anterior  side,  pedal  and  pleuro-visceral  on 
the  ventral  and  posterior  side  (Fig.  118). 

The  cerebral  ganglia  are  three  lobed,  and  are  connected  anteriorly  by 
two  commissures  with  a  "  supra-pharyngeal "  ganglion,  which  gives  off 
nerves  to  the  mouth  and  lips,  and  is  connected  also  with  an  "  infra  - 
pharnygeal"  ganglion.  The  cerebral  ganglia  are  also  connected  by 
short  double  commissures,  with  the  pedals  and  pleuro-viscerals  on  the 
ventral  side  of  the  gullet. 

The  following  chief  nerves  are  given  off  from  the  central  system  : — 

1 I )  The  very  thick  optic  nerves  are  given  off  from  the  commissures 

between  cerebrals  and  pleuro-viscerals,  and  lead  to  a  large 
optic  ganglion  at  the  base  of  each  eye. 

(2)  Ten  nerves  to  the  "arms"  are  given  off  by  the  pedal  ganglion, 

and  this  is  one  of  the  reasons  which  have  led  most  morpho- 
logists  to  regard  these  arms  as  portions  of  the  ' '  foot. " 

(3)  Two  large  nerves  from  the  more  ventral  portion  of  the  pleuro- 

visceral  ganglia  form  a  visceral  loop,  and  give  off  many 
branches  to  the  gills  and  other  organs.  From  the  pleural 
portion  arise  two  mantle  nerves,  each  of  which  ends  in  a 
large  stellate  ganglion. 


ALIMENTARY  SYSTEM.  379 

Sense  Organs. — The  eyes  are  large  and  efficient.  They  present  a 
striking  resemblance  to  those  of  Vertebrates,  and,  as  they  are  not  "  brain 
eyes,"  they  illustrate  how  superficially  similar  structures  may  be  developed 
in  different  ways  and  in  divergent  groups.  In  cuttlefishes,  the  eyes  lie 
on  the  sides  of  the  head,  protected  in  part  by  the  cartilage  surrounding 
the  brain,  and  in  part  by  cartilages  on  their  own  walls. 

The  eye  is  a  sensitive  cup  arising  in  great  part  from  the  skin.  Its 
internal  lining  is  a  complex  retina,  on  &&  posterior  surface  of  which  the 
nerves  from  the  optic  ganglion  are  distributed.  In  the  cavity  of  the  cup 
there  is  a  clear  vitreous  humour. 

The  mouth  of  the  cup  is  closed  by  a  lens,  supported  by  a  "  ciliary 
body."  The  lens  seems  to  be  formed  in  two  parts — an  outer  and  an 
inner  plano-convex  lens.  The  pupil  or  hole  in  front  of  the  lens  is 
fringed  by  a  contractile  iris. 

The  outer  wall  of  the  optic  cup  is  ensheathed  by  a  strong  supporting 
layer — the  sclerotic,  which  is  in  part  strengthened  by  cartilage,  covered 
by  a  silvery  membrane,  and  continued  into  the  iris. 

In  front  of  the  eye  there  is  a  transparent  cornea,  and  the  skin  also 
forms  protecting  lids. 

Round  about  the  optic  ganglion  there  is  a  strange  "white  body," 
which  seems  to  be  a  fatty  cushion  on  which  the  eye  rests. 

The  two  ear  sacs,  containing  a  spherical  otolith  and  a  fluid,  sometimes 
with  calcareous  particles,  are  enclosed  in  part  of  the  head  cartilage,  close 
by  the  pedal  ganglia.  The  nerves  seem  to  come  from  the  pedals,  but  it 
is  said  that  their  fibres  can  be  traced  up  to  the  cerebrals.  * 

A  ciliated  "olfactory  sac"  lies  behind  each  eye,  and  is  innervated 
from  a  special  ganglion  near  the  optic.  There  are  no  osphradia  of  the 
usual  type. 

Finally,  there  are  tactile  or  otherwise  sensitive  cells  on  various  parts 
of  the  body,  especially  about  the  arms. 

Alimentary  System. 

The  cuttlefish  eats  food  which  requires  tearing  and  chew- 
ing, and  this  is  effected  by  the  chitinous  jaws  worked  by 
strong  muscles,  and  by  the  toothed  radula  moving  on  a 
muscular  cushion.  The  mouth  lies  in  the  midst  of  the  arms, 
bordered  by  a  circular  lip.  Through  the  ganglionic  mass 
passes  the  narrow  gullet,  which  leads  into  the  globular 
stomach,  lying  near  the  dorsal  end  of  the  body.  The 
stomach  is  followed  by  a  caecum  or  pyloric  sac,  and  the 
intestine  curves  headwards  again  to  end  far  forward  in  the 
mantle  cavity.  There  do  not  seem  to  be  any  glands  on  the 
walls  of  the  food  canal,  the  stomach  has  a  hard  cuticle,  the 
digestion  which  takes  place  there  must  therefore  be  due  to 
the  digestive  juices  of  the  glandular  appendages.  Of  these 
the  most  important  is  usually  called  the  liver ;  it  is  bilobed, 
and  lies  in  front  of  the  stomach  attached  to  the  oesophagus. 


MOLLUSC  A. 


Its  two  ducts  conduct  the  digestive  juice  to  the  region  where 
stomach,  "pyloric  sac,"  and  intestine  meet,  and  these  ducts 
are  fringed  by  numerous  vascular  and  glandular  appendages 
which  are  called  "  pancreatic,"  though  in  reality  formed  as 
an  outgrowth  of  the  nephridia.  Far  forward,  in  front  of  the 
large  digestive  gland,  lie  two  small  white  glands  on  each  side 
of  the  gullet.  Their  ducts 
open  into  the  mouth,  and 
their  secretion  contains  a  dias- 
tatic  ferment.  At  the  other 
end  of  the  food  canal,  the  ink 
sac  full  of  black  pigment,  pro- 
bably of  the  nature  of  waste 
products,  opens  into  the  rec- 
tum close  to  the  anus.  This 
ink  sac  may  be  called  a  much 
enlarged  anal  gland,  for  while 
most  of  the  bag  is  made  of 
connective  tissue  and  some 
muscle  fibres,  a  distinct  gland 
is  constricted  off  at  the  closed 
end,  -and  the  neck  is  also 

glandular.  FIG.  121.— Diagram  of  the 

structure   of  Sepia.     (Mainly 
after  PELSENEER.  ) 


«.,  Eight  short  arms  around  mouth  ; 
/.#.,  one  of  the  two  long  arms ;  $.,  beak 
of  the  mouth  ;  e.g.,  cerebral  ganglia 
with  commissures  to  the  others;  £., 
eye  ;  g.,  gullet ;  d.g.,  digestive  gland  ; 
si.,  stomach;  a.,  anus;  sh.,  shell  sac 
with  sepiostaire  ;  £.,  kidney;  R., 
Reproductive  organ  ;  br.h.,  branchial 
heart;  g.,  a  gill;  /.£.,  ink  bag;  m.c., 
mantle  cavity  ;  f.,  funnel. 


Vascular  System. 

The  blood  of  Sepia  is 
bluish,  owing  to  the  presence 
of  haemocyanin  in  the  serum  \ 
the  blood  cells  are  colourless 
and  amoeboid.  The  median 
but  somewhat  oblique  ven- 
tricle of  the  heart  drives  the 
blood  forward  and  backward  to  all  parts  of  the  body. 
It  reaches  the  tissues  by  capillaries,  and  apparently  also 
by  lacunar  spaces.  The  venous  blood  of  the  head  region 
is  collected  in  an  annular  sinus  round  the  basis  of  the 
arms,  and  passes  towards  the  heart  by  a  large  vena  cava 
which  divides  into  two  branchial  veins,  covered  by  spongy 
outgrowths  of  the  nephridia.  Joined  by  other  vessels 
from  the  apical  region  of  the  viscera,  each  branchial  vein 
enters  a  "  branchial  heart "  at  the  base  of  each  gill.  The 


RESPIRATORY  AND  EXCRETORY  SYSTEMS.      381 


branchial  heart  is  contractile,  and  drives  the  venous  blood 
through  the  gills,  whence  purified  it  returns  by  two  contractile 
auricles  into  the  ventricle.  There  are  valves  preventing 
back  flow  from  the  ventricle  to  the  auricles,  or  from  the 
arteries  to  the  ventricle.  Beside  each  branchial  heart  lies 
an  enigmatical  glandular  structure  known  as  a  "pericardial 
gland,"  possibly  an  excretory  or  incipiently  excretory  organ. 
The  course  of  the  blood  differs  from  that  in  the  mussel  and 
snail  in  this,  that  none  returns  to  the  heart  except  from  the 
respiratory  organs.  In  the  nephridial  outgrowths  around 
the  branchial  veins  the  interesting  parasite  Dicyema  is  found. 

Respiratory  System. 

The  blood  is  purified  by  being  exposed  on  the  two 
feather-like  gills  which  are  attached  within  the  water- 
washed  mantle  cavity. 
The  water  penetrates 
them  very  thoroughly ; 
the  course  of  the  blood 
is  intricate.  At  the  base 
of  the  gills  there  is  some 
glandular  tissue,  which 
those  impatient  with  enig- 
mas have  credited  with 


blood  making  powers. 


Excretory  System. 

The  excretory  system  is  diffi- 
cult to  dissect  and  to  explain. 
On  each  side  of  the  anus  there 
is  a  little  papilla  through  which 
uric  acid  and  other  waste  pro- 
ducts ooze  out  into  the  mantle 
cavity,  and  so  into  the  water. 
A  bristle  inserted  into  either  of 
these  two  papillae  leads  into  a 
large  sac — the  nephridial  sac. 
But  the  two  sacs  are  united  by 

two  bridges,  and  they  give  off  an  unpaired  dorsal  elongation,  which 

extends  as  far  back  as  the  reproductive  organs. 

The  dorsal  wall  of  each  nephridial  sac  becomes  intimately  associated 

with  the  branchial  veins,  and  follows  their  outlines  faithfully.     It  is 

likely  that  waste  material  passes  from  the  blood  through  the  spongy 

appendages  into  the  nephridial  sacs. 


FlG.  122. — Diagram  of  circulatory 
and  excretory  systems  in  a  Decapod - 
like  Sepia.  (After  PELSENEER.) 

i,  Gill ;  2,  renal  sac  ;  3,  afferent  branchial 
vessel ;  4,  branchial  heart ;  5,  abdominal 
vein  ;  6,  heart ;  7,  pericardium ;  8,  genital 
organ;  9,  posterior  aorta;  TO,  "auricle"; 
n,  glandular  appendage  of  branchial  heart ; 
12,  renal  appendages  of  branchial  vein  ;  13, 
external  aperture  of  kidney  ;  14,  vena  cava  ; 
15,  anterior  aorta;  16,  bifurcation  of  vena 
cava  ;  17,  reno-pericardial  aperture. 


382  MOLLUSC  A. 

Into  the  terminal  portion  of  each  nephridial  sac,  a  little  below  its 
aperture  at  the  urinary  papilla,  there  opens  by  a  ciliated  funnel  another 
sac,  which  is  virtually  the  body  cavity.  It  surrounds  the  heart  and  other 
organs,  and  is  often  called  the  viscero-pericarclial  cavity.  Through  the 
kidneys  or  nephridial  sacs  it  is  in  communication  with  the  exterior. 

Reproductive  System. 

The  sexes  are  separate,  but  there  is  not  much  external 
difference  between  them,  though  the  males  are  usually 
smaller,  less  rounded  dorsally,  and  with  slightly  longer  arms. 
When  mature  the  male  is  easily  known  by  a  strange  modifi- 
cation on  his  fifth  left  arm.  The  essential  reproductive 
organs  are  unpaired,  and  lie  in  the  body  cavity  towards  the 
apex  of  the  visceral  mass. 

The  testis — an  oval  yellowish  organ — lies  freely  in  a  peritoneal  sac 
near  the  apex  of  the  visceral  mass.  From  this  sac,  the  spermatozoa  pass 
along  a  closely  twisted  duct — the  vas  deferens.  This  expands  into  a 
twofold  "seminal  vesicle,"  and  gives  off  two  blind  outgrowths,  of  which 
one  is  called  the  "prostate."  The  physiological  interest  of  these  parts 
is  that  within  them  the  spermatozoa  begin  to  be  arranged  in  packets. 
In  this  form  they  are  found  within  the  next  region — the  spermatophore 
sac  which  opens  to  the  exterior  to  the  left  of  the  anus.  Each  spermato- 
phore is  like  a  transparent  worm  of  complex  structure.  Think  of  a  little 
glass  tube,  closed  at  one  end,  drawn  out  and  somewhat  twisted  at  the 
other  ;  see  within  the  tube  at  the  closed  end  a  bag  of  dust  attached  to 
and  kept  in  its  place  by  a  sort  of  spiral  spring  ;  this  is  prevented  from 
expanding  by  the  fact  that  its  upper  end  is  fixed  by  cement  in  the  mouth 
of  the  tube.  Suppose  the  cement  be  soluble  in  water,  and  that  the  tiny 
machine  be  thrown  into  a  basin,  the  spring  will  expand  violently  as  the 
cement  is  dissolved,  and  the  bag  of  dust  will  be  torn  out  and  scattered. 
Somewhat  similar  but  more  complex  is  the  spermatophore — with  its 
clear  case,  its  contained  bag  of  spermatozoa,  its  spring-like  arrangement, 
and  its  explosiveness  in  water.  Even,  indeed,  on  your  scalpel,  or  on  a 
dry  slide,  these  strange  but  efficient  bombs  will  explode.  The  liberated 
spermatozoa  are  of  the  usual  sort. 

The  ovary — a  large  rounded  white  organ — lies  freely  in  a  peritoneal 
sac  near  the  apex  of  the  visceral  mass.  From  this  sac  the  eggs  pass 
along  a  short  direct  oviduct,  which  opens  into  the  mantle  cavity  to  the 
left  of  the  anus.  Associated  with  the  oviduct,  and  pouring  viscid 
secretion  into  it,  are  two  large  "  nidamental  glands,"  of  foliated  structure. 
Close  beside  these  are  accessory  glands,  of  a  reddish  or  yellowish  colour, 
with  a  median  and  two  lateral  lobes ;  while  at  the  very  end  of  the 
oviduct  are  two  more  glands.  All  seem  to  contribute  to  the  external 
equipment  of  the  egg. 

The  spermatophores  pass  from  the  genital  duct  of  the  male  to  the  fifth 
left  arm,  which  becomes  covered  with  them  and  quaintly  modified.  This 
is  usual  among  cuttlefish,  indeed  in  some,  e.g.>  Argonauta  and  Tr em- 
octopus^  the  modified  arm  with  its  load  of  spermatozoa  is  discharged 


CEPHALOPODA.  383 

bodily  into  the  mantle  cavity  of  the  female.  There  its  discoverers 
described  it  as  a  parasitic  worm  "  Hectocotylus"  The  lost  arm  is  after- 
wards regenerated.  In  Sepia,  however,  the  modified  arm  is  not  dis- 
charged, but  is  simply  thrust  into  the  mantle  cavity  of  the  female.  The 
spermatophores  probably  enter  the  oviduct  and  burst  there. 

The  eggs  when  laid  are  enclosed  within  separate  black  capsules  con- 
taining gelatinous  stuff,  but  the  stalks  of  the  capsule  are  united  so  that  a 
bunch  of  "  sea  grapes  "  results. 

Second  Type  of  CEPHALOPODA.     The  Pearly  Nautilus 
(Nautilus  pompilius). 

The  shells  of  the  pearly  nautilus  are  common  on  the 
shores  of  warm  seas,  but  the  animals  are  very  rare.  Natu- 
ralists do  not  seem  to  know  how  to  get  them,  though  the 
natives  of  Fiji  and  New  Hebrides,  who  appreciate  their 
flesh,  trap  them  successfully  in  lobster  pots  baited  with 
crustacean  or  sea  urchin.  The  animal  creeps  or  swims 
gently  along  the  bottom  at  no  great  depth,  and  its  appear- 
ance on  the  surface,  "floating  like  a  tortoiseshell  cat,"  is 
probably  the  result  of  storms.  It  is  called  "  pearly "  on 
account  of  the  appearance  of  the  innermost  layer  of  the 
shell.  This  is  exposed  after  the  soft  organic  stratum  and 
the  median  layer  which  bears  bands  of  colour  have  been 
worn  away,  or  dissolved  in  a  dolphin's  stomach,  or  artificially 
treated  with  acid. 

The  beautiful  shell  is  a  spiral  in  one  plane,  divided  into  a 
set  of  chambers,  in  the  last  of  which  the  animal  lives,  while 
the  others  contain  gas.  The  young  creature  inhabits  a  tiny 
shell  curved  like  a  horn ;  it  grows  too  big  for  this,  and 
proceeds  to  enlarge  its  dwelling,  meanwhile  drawing  itself 
forward  in  the  older  part,  and  forming  a  door  of  lime  behind 
it.  This  process  is  repeated  again  and  again  ;  as  an  addition 
is  made  in  front,  the  animal  draws  itself  forward  a  little,  and 
shuts  off  a  part  of  the  chamber  in  which  it  has  been  living. 
The  compartments  seen  on  a  divided  shell  are  not  exactly 
successive  chambers,  they  are  fractions  of  successive  cham- 
bers abandoned  and  partitioned  off  as  more  space  was  gained 
in  front.  Moreover,  all  the  compartments  are  in  communi- 
cation by  a  median  tube  of  skin — the  siphuncle — which  is 
in  part  calcareous. 

It  has  been  suggested,  that  "  each  septum  shutting  off  an 
air-containing  chamber  is  formed  during  a  period  of  quies- 


MOLLUSC  A. 


cence,  probably  after  the  reproductive  act,  when  the  visceral 
mass  of  the  Nautilus  may  be  slightly  shrunk,  and  gas  is 
secreted  from  the  dorsal  integument  so  as  to  fill  up  the 
space  previously  occupied  by  the  animal." 

The  only  other  living  Cephalopod  which  has  a  shell  like 
that  of  the  Nautilus  is  Spirula.  In  this  form  the  shell  is 
again  chambered  and  spirally  coiled  in  one  plane.  But  it  is 
without  a  siphuncle,  and  lies  enveloped  by  folds  of  the 
mantle. 

There  can  be  no  confusion  between  the  beautiful  shell  of 
the  cuttlefish  called  the  paper  Nautilus  (Argonauta  argo) 
and  that  of  our  type.  For  it 
is  only  the  female  Argonaut 
which  bears  a  shell,  it  is  not 
chambered,  and  is  a  shelter  for 
the  eggs — a  cradle,  not  a  house. 
It  is  usually  stated  to  be  formed 
by  two  of  the  arms,  but  it  seems 
doubtful  whether  it  is  not  in 
reality  due  to  the  activity  of  the 
mantle. 

It  is  instructive  also  to  com- 
pare the  Nautilus  shell  with 
that  of  some  Gasteropods,  for 
there  also  chambers  may  be 
formed.  But  these  arise  from 
secondary  alterations  of  an  ori- 
ginally continuous  spiral,  and 
the  resemblance  is  never  very 
striking.  The  fresh  water  snail 
Planorbis  has  an  unchambered  shell  spirally  coiled  in  one 
plane,  but  in  this  and  in  similar  Gasteropods,  the  foot  is 
turned  towards  the  internal  curve  of  the  coil,  while  that  of 
Nautilus  is  directed  externally. 

There  are  only  about  half  a  dozen  living  species  of  Nautilus, 
but  there  are  many  hundred  fossils  of  this  and  allied  genera. 
This  list  is  usually  swelled  by  the  addition  of  the  extinct 
Ammonites,  but  there  are  some  reasons  for  believing  that 
these  belong  to  the  cuttlefish  section  of  Cephalopods. 

The  following  table  states  the  chief  points  of  distinction 
between  Nautilus  and  the  other  series  of  Cephalopods. 


FIG.   123. — Section  of  shell 
of  Nautilus.     (After  LENDEN- 

FELD.) 


CLASSIFICATION  OF  CEPHALOPODA. 


385 


CEPHALOPODA. 


TETRABRANCHIATA  (Nautilus). 


DIBRANCHIATA  (Sepia,  Octopus,  &>c.). 


All  extinct  except  one  genus — 
Nautilus ;  the  extinct  forms  are 
usually  ranked  as  Nautiloid  and 
Ammonoid. 

Shell  external,  chambered,  straight 
or  bent  or  spirally  coiled.  That  in 
which  Nautilus  lives  has  been 
described,  with  its  siphuncle,  gas- 
containing  compartments,  &c. 


The  part  of  the  foot  surrounding  the 
mouth  bears  a  large  number  of 
lobes,  which  carry  tentacles  in 
little  sheaths,  but  no  suckers. 

The  two  mid  lobes  of  the  foot  form 
a  siphon,  but  they  are  not  fused 
into  a  tube. 

The  eye  is  without  a  lens,  and  is 
bathed  internally  by  sea  water 
which  enters  by  a  small  pinhole 
aperture.  There  are  two  ' '  os- 
phradia"  or  smelling  patches  at 
the  bases  of  the  gills. 

Two  pairs  of  gills ;  two  pairs  of 
nephridia;  two  genital  ducts  (the 
left  rudimentary). 


The  coelome  sac  opens  directly  to  the 
exterior  by  two  apertures. 

The  heart  has  two  pairs  of  auricles, 
and  there  are  no  branchial  hearts. 
No  ink  bag.     No  salivary  glands. 


Numerous  living  genera,  ranked  as 
Decapods  or  Octopods ;  along 
with  the  former  the  extinct  Bel- 
emnites  are  included. 

No  living  Dibranchiate  lives  in  a 
shell.  The  shell  is  internal  even 
in  the  extinct  Belemnites,  and  in 
modern  forms  it  occurs  in  various 
degrees  of  degeneration  (cf.  Spir- 
ula,  Sepia,  Loligo)  or  is  quite 
absent  (Octopoda). 

The  part  of  the  foot  surrounding  the 
mouth  is  divided  into  ten  or  eight 
arms,  which  carry  suckers,  stalked 
in  Decapods,  sessile  in  Octopods. 

The  two  mid  lobes  of  the  foot  fuse 
to  form  a  completely  closed  tubular 
siphon  or  funnel. 

The  covering  of  the  eye  may  be 
perforated,  but  the  mouth  of  the 
retinal  cup  is  closed  by  a  lens. 
There  are  no  osphradia,  though 
there  may  be  "olfactory  pits" 
behind  the  eyes. 

One  pair  of  gills  ;  one  pair  of  neph- 
ridial  sacs  ;  two  oviducts  in  Octo- 
poda and  Ommastrephes  ;  two  vasa 
deferentia  in  Eledone  moschata ; 
in  others  an  unpaired  genital  duct. 

The  coelome  opens  into  the  nephridia 
by  two  pores,  and  thus  to  the 
exterior. 

The  heart  has  two  auricles,  and 
there  are  branchial  hearts. 

An  ink  bag  and  salivary  glands. 


CLASSIFICATION  OF  CEPHALOPODA. 


Order  I.     Tetrabranchiata  (s> 
Family  I.     Nautilidae. 


25 


table). 

Nautilus  alone  alive ;  but  a  great  series 
of  fossil  forms,  Orthoceras — Trochoceras. 

Family  II.  Ammonitidse.  All  extinct,  but  with  shells  well 
preserved,  so  that  long  series  can  be  studied.  They 
furnish  striking  evidence  of  progressive  evolution  in 
definite  directions,  e.g.,  Bactrites,  Ceratites,  Baculites, 
Turrilites,  Heteroceras,  and  the  whole  series  of  genera 
formerly  classed  as  Ammonites. 


386  MOLLUSC  A. 

Order  II.     Dibranchiata  (see  table). 

Sub- Order  Decapoda.  Eight  shorter  and  two  longer  arms. 
Suckers  stalked  and  strengthened  by  a  strong  ring. 
Large  eyes  with  a  horizontal  lid.  Body  elongated,  with 
lateral  fins.  Mantle  margin  with  a  cartilaginous  ' '  hook 
and  eye"  arrangement.  Some  sort  of  internal  "shell," 
enclosed  by  upgrowths  of  the  mantley^ 
With  calcareous  internal  "shell."  Spirilla;  extinct  Bel- 

emnites  ;  Sepia.  ^ 
With  organic  internal  "shell." 

(a)  Eyes  with  closed  cornea,  e.g.,  Loligo. 

(b]  Eyes  with  open  cornea,  e.g.,  Ommastrephes. 

Sub- Order  Octopoda.  Eight  arms  only.  Suckers  sessile 
without  horny  ring.  Small  eyes  with  sphincter-like 
lid.  Body  short  and  rounded.  No  "hook  and  eye" 
arrangement.  No  "  shell,"  except  in  the  female 
Argonauta. 

e.g.,  Octopus,  JLledone,  Argonauta. 

The  classification  given  above  is  that  usually  adopted,  but  it  may  be 
noted  that  the  Ammonites  are  included  in  the  Tetrabranchiata  on 
insufficient  evidence. 

The  Cephalopods  are  the  most  specialised  of  the  Molluscs, 
and  present  much  variation  of  type.  Nautilus  appeared  very 
early  and  has  persisted,  apparently  unchanged,  until  the 
present,  while  the  Ammonites  and  Belemnites,  once  so 
abundant,  have  entirely  disappeared.  Among  recent  forms 
we  have  Squid,  Calamary,  Octopus,  Argonaut,  and  many 
others.  All  swim  freely  in  the  sea,  or  lurk  and  creep 
passively  among  the  rocks.  They  are  voracious  eaters,  and 
consume  very  diverse  kinds  of  animals,  their  parrot-like 
jaws  and  powerful  odontophore,  as  well  as  the  numerous 
suckers,  rendering  them  formidable  adversaries. 

A  chambered  external  shell,  serving  as  a  house,  is  present 
in  Nautilus  alone  among  living  Cephalopods.  In  Spirula, 
there  is  a  spiral'  chambered  shell,  but  it  is  very  small,  is 
enclosed  by  the  folds  of  the  mantle,  and  is  quite  useless  for 
purposes  of  protection.  Most  of  the  extinct  forms  were 
furnished  with  large  and  efficient  shells  of  very  variable 
shape,  some  straight  like  Orthoceras,  or  coiled,  with  cham- 
bers separated  by  complex  septa,  as  in  the  Ammonites,  and 
so  on.  Most  of  the  modern  forms  seem  to  be  more  active 
than  their  ancestors,  and  their  shells  have  degenerated^ 
While  the  fact  of  the  degeneration  is  perfectly  obvious,  the 
line  along  which  it  has  taken  place  is  difficult  and  still 


GENERAL  NOTES  ON  MOLLUSCS.  387 

debated.  In  Naiitilus,  although  the  animal  lives  within 
the  shell,  the  mantle  fold  is  for  some  distance  reflected  over 
it ;  in  the  other  series  of  Cephalopods,  this  process  has  gone 
further,  and,  where  a  shell  is  present,  it  is  entirely  enclosed 
within  the  mantle  fold,  and  is  at  the  same  time  much  reduced 
in  size.  In  the  extinct  Belemnites  the  internal  shell  was 
straight  and  chambered,  but  almost  concealed  by  secondary 
deposits  of  lime,  secreted  by  the  walls  of  the  shell  sac.  In 
Sepia ,  according  to  one  view,  the  central  laminated  region 
of  the  "  bone  "  represents  the  remains  of  the  chambered 
shell ;  the  remainder  corresponds  to  the  secondary  deposits 
of  lime  in  the  Belemnites.  In  Loligo,  there  is  no  deposit  of 
lime,  an  organic  chitinous  pen  only  being  left.  In  Octopus^, 
there  is  no  trace  of  shell  at  all.  According  to  some,  the 
shell  sac,  in  which  the  shell  or  pen  of  Cephalopods  is 
formed,  is  to  be  regarded  as  equivalent  to  the  embryonic 
shell  sac  plus  a  mantle  pocket.  According  to  Ray  Lankester, 
there  is  no  relation  between  the  secondary  shell  sac  and  the 
typical  primitive  Molluscan  shell  gland. 

The  sexes  in  Cephalopods  are  separate.  The  male 
elements  are  made  up  into  packets  or  spermatophores, 
which  usually  pass  out  on  to  one  of  the  "  arms,"  more  or 
less  modified  for  copulatory  purposes.  The  eggs  are  large, 
and  often  surrounded  by  capsules. 

The  eggs  are  always  furnished  with  a  large  amount  of 
yolk.  In  consequence,  development  is  much  modified  as 
compared  with  other  Molluscs,  trochosphere  and  veliger 
stages  not  being  recognisable. 

General  Notes  on  Molluscs. 

The  characters  of  the  widely  spread  trochosphere 
larva,  and  many  of  the  features  of  the  simple  Amphi- 
neura,  suggest  that  Molluscs  arose  from  some  worm 
type,  but  beyond  this  all  is  hypothesis.  Some  of  the 
Amphineura,  both  in  the  form  of  the  body  and  in  the 
arrangement  of  bristles  on  its  surface,  recall  Annelids,  but 
this  can  hardly  be  regarded  as  an  evidence  of  affinity,  for  it 
is  extremely  improbable  that  such  typically  unsegmented 
animals  can  have  arisen  from  segmented  worms.  It  is 
perhaps  more  likely  that  Molluscs  arose  from  a  Turbellarian- 


388  MOLLUSC  A. 

like  ancestor,  an  unsegmented  form  with  a  flat  creeping 
ventral  surface. 

It  is  certain,  however,  that  the  great  Mollusc  branch  must 
have  divided  at  a  very  early  stage  into  two.  One  branch 
bears  those  forms  which  live  sluggishly,  and  have  undeveloped 
heads  and  no  odontophore — the  bivalves  or  Lamellibranchs, 
The  other  branch  bears  more  active  forms,  in  which  the 
head  is  well  developed,  and  the  characteristic  radula  is 
present  in  the  mouth  —  the  primitive  Amphineura,  the 
Gasteropods  or  snails,  and  the  Cephalopods  or  cuttles. 

Phylum  Mollusca. 


Branch  GLOSSOPHORA 
(Syn.  Odontophora). 


Branch  LIPOCEPHALA 
(Syn.  Acephala). 


Cephatpodaj^^^7^  and  cuttle-fish. 

Class        /       Asymmetrical  snails, 
Gasteropoda  \  whelks,  &c. 

f  More  primitive,  bilater- 
Class  -c  ally  symmetrical  forms, 
phineura  V.  £•£••>  Chiton. 


Class  Lamellibranchiata  (Syn.  Pelecy- 
poda,  Conchifera  or  Bivalves) — cockle 
and  mussel,  clam  and  oyster. 


Amph 

The  position  of  the  small  class  Scaphopoda  is  uncertain.     Some  place  it  between 
Gasteropods  and  Cephalopods. 

Most  Molluscs  live  in  the  sea  from  the  shore  to  the  great 
depths,  but  there  are  many  fresh  water  Gasteropods  and 
bivalves,  and  the  terrestrial  snails  and  slugs  are  legion. 

The  bivalves  feed  on  microscopic  animals  and  organic 
debris  ;  the  Gasteropods  are  carnivorous  or  vegetarian  ;  the 
Cephalopods  are  voracious  flesh  eaters. 

The  alimentary  canal  and  its  associated  digestive  gland 
often  seem  as  if  they  were  too  big  for  the  body ;  in  bivalves, 
the  gut  tends  to  be  displaced  ventrally  and  coils  about  in 
the  foot ;  in  the  others,  it  tends  to  be  displaced  dorsally, 
often  protruding  on  the  back  as  a  visceral  hump. 

The  vascular  system  in  Molluscs  always  communicates 
freely  with  the  lacunar  spaces  which  constitute  the  apparent 
body  cavity.  These  are  never  lined  with  epithelium,  and 
are  of  secondary  origin.  A  true  epithelium,  however,  lines 
the  pericardium  and  the  cavity  of  the  reproductive  organs, 
which  are  both  ccelomic  in  origin. 


GENERAL  NOTES  ON  MOLLUSCS.  389 

The  shell  is  a  very  characteristic  molluscan  structure, 
but  in  spite  of  all  the  years  of  conchology,  we  cannot 
answer  the  fundamental  questions  about  shell  making. 
Mollusc  shells  are  very  beautiful  things  alike  in  form  and 
colour.  They  grow  larger  by  month  and  year,  and  mark 
their  progress  by  rings  of  growth  and  changing  tints.  That 
they  afford  their  bearers  efficient  protection,  is  shown  by 
the  appreciation  which  some  hermit  crabs  exhibit  for  stolen 
whelk  or  buckie  shells.  More  precise  observation  shows  us 
that  the  shell  consists  in  great  part  of  carbonate  of  lime ; 
that  it  has  a  thin  outer  "horny"  layer,  a  thick  median 
"  prismatic  "  stratum  of  lime,  and  an  internal  mother-of- 
pearl  layer.  This  last  exhibits  iridescence,  produced  by 
the  fine  lines  which  mark  successive  deposits.  The  mark- 
ings, fine  as  they  are,  were  repeated,  according  to  Brewster's 
famous  experiments,  on  a  piece  of  wax,  which  in  consequence 
became  iridescent.  On  the  dorsal  surface  of  almost  every 
mollusc  embryo,  there  is  a  little  shell  sac  in  which  an 
embryonic  shell  is  begun  ;  the  adult  shell,  however,  begins 
on  a  separate  area  on  the  skin,  and  it  is  always  lined  and 
increased  by  the  mantle.  If  the  increase  of  the  shell  be 
carefully  watched  on  young  Molluscs,  or  if  chemical  analysis 
be  made,  it  becomes  plain  that  the  shell  is  no  mere  deposi- 
tion of  carbonate  of  lime.  Like  other  cuticular  products,  it 
has  an  organic  basis  (called  conchiolin),  along  with  which, 
in  a  manner  that  we  do  not  clearly  understand,  the  lime 
is  associated. 

Mr.  Irvine's  experiments  at  Granton  Marine  Station  suggest 
that  the  lime  salt  originally  absorbed  is  not  the  carbonate 
(of  which  there  is  a  scant  supply  in  sea  water),  but  the 
sulphate  (which  is  abundant),  and  that  the  internal  transfor- 
mation from  sulphate  to  carbonate  is  perhaps  associated 
with  the  diffuse  decomposition  of  nitrogenous  waste  pro- 
ducts. Thus,  carbonate  of  ammonia,  which  seems  to  occur 
abundantly  in  the  mantle  of  perfectly  fresh  mussels,  would, 
with  calcium  sulphate,  yield  carbonate  of  lime  and  am- 
monium sulphate.  I  do  not  suppose  that  shell  making  is 
expressible  in  a  chemical  reaction  of  this  simplicity,  but  it 
is  time  that  we  ceased  to  think  that  Molluscs  simply  absorb 
carbonate  of  lime  from  the  sea  water,  and  sweat  it  out  on 
their  skins.  It  is  reasonable  to  inquire  how  far  shell  making 


390  MOLLUSC  A. 

may  express  a  primitive  mode  of  excretion  to  which  a 
secondary  significance  has  come  to  be  attached,  and  in 
what  way  carbonate  of  lime  shells  are  associated  with  pre- 
ponderant sluggishness  of  habit.  The  thickness  of  the  shell 
seems  often  to  bear  some  relation  to  the  external  and  internal 
activities  of  the  mollusc,  for  it  is  thin  in  the  active  scallop 
(Pecteri)  and  Lima,  thick  in  the  passive  oyster  and  Tridacna, 
slight  or  absent  in  the  pelagic  Pteropods  ("  sea  butterflies  "), 
and  in  the  more  or  less  active  cuttlefish,  but  heavy  in  most 
of  the  slowly  creeping  littoral  forms.  But  that  this  is  only 
one  condition  of  shell  development  is  evident  in  many  ways, 
— for  instance  when  we  compare  land  snails  with  slugs  ;  for 
the  latter,  though  not  more  active  than  the  former,  are 
practically  shell-less.  In  most  cases,  as  Lang  points  out, 
the  loss  of  the  shell  is  justified  by  increased  power  of 
locomotion,  by  increased  adaptation  to  peculiar  habits  of 
life,  and  so  forth. 

In  their  life  history  most  Molluscs  pass  through  two  larval 
stages.  The  first  of  these  is  a  pear  shaped  or  barrel  shaped 
form,  with  a  curved  gut,  and  with  a  ring  of  cilia  in  front  of 
the  mouth.  It  is  a  "  trochosphere,"  such  as  that  occurring 
in  the  development  of  many  "  worms."  So  far  there  is 
nothing  characteristic. 

Soon,  however,  the  trochosphere  grows  into  a  yet  more 
efficiently  locomotor  form — the  veliger.  Its  head  bears  a 
ciliated  area  or  "  velum,"  often  produced  into  retractile 
lobes  ;  its  body  already  shows  the  beginning  of  "  foot  "  and 
mantle ;  on  the  dorsal  surface  lies  the  little  embryonic  shell 
gland. 

But  although  trochosphere  and  veliger  occur  in  the  develop- 
ment of  most  forms,  they  do  not  in  any  of  the  three  types 
which  we  have  particularly  described, — not  in  Anodonta 
partly  because  it  is  a  fresh  water  animal,  with  a  peculiarly 
adhesive  larva  of  its  own,  not  in  Helix  partly  because  it  is 
terrestrial,  and  not  in  Sepia  partly  because  the  eggs  are  rich 
in  yolk. 

The  hard  shells  of  extinct  Molluscs  are  naturally  well 
preserved  in  the  rocks,  and  long  series  of  fossil  forms  have 
been  traced  with  much  success. 


CHAPTER    XVII. 

CLASS    HEMICHORDA   OR    ENTEROPNEUSTA. 
Type.     BALANOGLOSSUS. 

A  SPECIES  of  Balanoglossus  was  described  by  Delle  Chiaje 
at  the  end  of  the  eighteenth  century,  but  it  is  only  within 
the  last  few  years  that  the  researches  of  Spengel,  Bateson, 
and  others  have  led  to  an  appreciation  of  the  importance  of 
this  type,  and  to  a  recognition  of  its  peculiar  features. 

The  class  (Enteropneusta)  which  was  erected  for  the  re- 
ception of  Balanoglossus  is  now  known  to  include  a  few 
other  forms,  whose  more  or  less  distinct  affinities  with  Ver- 
tebrates are  suggested  by  the  alternative  title  Hemichorda. 
Taken  along  with  Tunicates  and  Amphioxus,  they  afford 
interesting  illustrations  of  the  gradualness  with  which  Ver- 
tebrate characters  have  appeared  in  history. 

GENERAL  CHARACTERS. — The  body  is  divisible  into  three 
regions — a  pre-oral  "proboscis"  a  "collar"  around  and  be- 
hind the  mouth,  and  a  trunk,  the  anterior  part  of  which  bears 
gill  slits.  A  dorsal  nerve  cord  arises  from  the  epiblast  along 
the  middle  line,  and  is  connected,  by  a  ring  round  the  pharynx, 
with  a  ventral  cord.  In  the  skin,  which  is  covered  with  cili- 
ated ectoderm,  there  is  also  a  nerve  plexus.  From  the  anterior 
region  of  the  gut  a  diverticulum  grows  forward  for  a  short 
distance,  becomes  a  solid  support  for  the  proboscis,  and  is  often 
called  the  "  notochord"  The  gill  slits  open  dorsally,  are 
very  numerous,  and  increase  in  number  during  life ;  in  some 
details  of  development  they  recall  those  of  Amphioxus.  The 
mesoblast  is  formed  by  the  outgrowth  of  pouches  from  the 
archenteron ;  i.e.,  the  body  cavity  is  enteroccelic.  An  un- 
paired anterior  pouch  forms  the  pre-oral  or  proboscis  cavity 


392 


HEMICHORDA    OR  ENTEROPNEUSTA. 


of  the  adult,  and  is  compared  to  the  anterior  unpaired  body 
cavity  ^Amphioxus. 

Spengel,  in  his  recent  monograph,  recognises  19  species 
and  4  genera  —  Balanoglossus,  Ptychodera,  Schizocardium, 
and  Glandiceps.  They  are  very  widely,  though  locally,  dis- 
tributed, except,  perhaps,  on  the  Pacific  coasts  of  America. 

Description  of  Balanoglossus. 

Habit 

The  species  which  form  this  genus  are  worm-like  marine 
animals,  found  in  sand  and  mud  in  the  English  Channel, 
the  Mediterranean, 
Chesapeake  Bay,  &c. 
They  vary  in  length 
from  about  an  inch  to 
over  six  inches,  and 
are  brightly  coloured 
and  of  a  peculiar 
odour.  The  sexes 
are  distinct,  and  are 
marked  externally  by 
slight  differences  in 
colour. 


Form. 

The  worm-like  body 
consists  of  a  promi- 
nent pre-oral  region 
or  "  proboscis,"  a  firm 
"  collar "  behind  the  mouth,  behind  this  a  region  with  gill 
slits,  and  finally,  a  long,  soft,  slightly  coiled  portion. 


FIG.    124. — Male   of    Balanoglossus 
Kowalevskii.     (After  BATESON.) 

Note  anterior  proboscis;  Mo.,  mouth;  0/., 
slight  operculum  behind  the  collar;  then  the 
region  with  gill  slits  ;  /s.,  testes  ;  a.,  anus. 


Skin. 

The  epidermis  is  ciliated,  and  exudes  abundant  mucus 
from  unicellular  glands.  In  B.  robinii  the  mucus  sets 
firmly,  and,  with  the  addition  of  grains  of  sand,  forms  a 
tube  round  the  body.  Some  species  are  phosphorescent. 

Muscular  System. 

The  muscular  system  is  best  developed  about  the  pro- 
boscis and  collar,  which  are  used  in  leisurely  locomotion 


DESCRIPTION  OF  BALANOGLOSSUS.  393 

through  the  soft  sand.  There  are  external  circular  and  in- 
ternal radial  and  longitudinal  muscles.  The  fibres  are 
unstriped. 

Nervous  System. 

The  dorsal  nerve  cord  is  most  developed  in  the  collar, 
but  is  continued  along  the  whole  length.  It  arises  as  a 
solid  cord  of  epiblast,  which  is  continued  both  forwards  and 
backwards  as  a  hollow  tube.  The  cavity  is  said  to  be  com- 
parable to  that  of  the  spinal  cord  in  Vertebrates.  But  the 
dorsal  nerve  cord  in  the  collar  is  connected  by  a  band  round 
the  pharynx  with  a  ventral  nerve.  There  is  also  a  nervous 
plexus  beneath  the  epidermis.  There  are  no  special  sense 
organs,  nor  should  we  expect  them  in  an  animal  which  spends 
most  of  its  life  immersed  in  muddy  sand.  In  the  larvae  of 
some  species  there  are  two  eye  spots. 

Alimentary  System. 

The  mouth  opens  ventrally  between  the  proboscis  and 
the  collar,  and  is  adapted  for  swallowing  the  sand  moved 
about  by  the  wriggling  proboscis  and  by  the  collar.  The 
pharynx  is  constricted  into  a  dorsal  and  ventral  region,  of 
which  the  former  is  respiratory  (Fig.  125,  £rl),  and  connected 
with  the  exterior  by  many  gill  slits,  while  the  latter  is  nutri- 
tive (Fig.  125,  g),  and  conveys  the  food  particles  onwards. 
This  ventral  region  may  be  compared  with  the  "ventral 
groove  "  in  Tunicates,  with  the  "  hypobranchial "  groove  in 
the  lancelet,  with  a  similar  region  in  the  lamprey,  and  even 
with  part  of  the  thyroid  gland  in  higher  Vertebrates.  Be- 
hind the  region  with  gill  slits,  the  gut  has  a  dorsal  and  a 
ventral  ciliated  groove,  and  bears,  throughout  the  anterior 
part  of  its  course,  numerous  glandular  sacculations,  which 
can  be  detected  through  the  skin.  The  animal  eats  its  way 
through  the  sand,  and  derives  its  food  from  the  nutritive 
particles  and  small  organisms  therein  contained. 

Skeletal  System. 

The  skeletal  system  is  represented  by  the  "notochord," 
which  lies  in  the  proboscis,  and  arises,  like  the  notochord  of 
indubitable  Vertebrates,  as  a  hypoblastic  structure  from  the 
dorsal  wall  of  the  gut.  Each  gill-slit  is  furnished  with  a 


394  HEMICHORDA    OR  ENTEROPNEUSTA. 

"  chitinous "  skeleton,  which  gives  the  slit  a  U  shape  on 
account  of  the  growth  downwards  of  a  "  tongue  bar,"  the 
whole  is  suggestive  of  Amphioxus.  Beneath  the  branchial 
skeleton  there  lies  a  "chitinous"  rod,  which  divides  into 
two  in  the  collar. 

The  Body  Cavity. 

The  body  cavity  is  somewhat  complex,  consisting  of  five 
distinct  parts,  all  of  which  are  lined  by  mesoderm,  and 
arise  as  pouches  from  the  primitive  gut  or  archenteron. 


d.  n 


R- 


FIG.  125. — Transverse  section  through  gill  slit  region  of 
Ptychodera  minuta.     (After  SPENGEL.) 

The  section,  somewhat  diagrammatic,  shows  a  gill  slit  (g-.s.)  to 
left,  and  a  septum  between  two  slits  to  the  right;  d.n.,  dorsal 
nerve  ;  d.v.,  dorsal  vessel;  z/.#.,  ventral  nerve  ;  z/.z>.,  ventral  vessel ; 
g-.,  nutritive  part  of  gut;  g-'.,  respiratory  part  of  gut;  c.,  lateral 
coelomic  spaces;  l.m.,  longitudinal  muscles;  R.,  reproductive 
organs.  As  the  gill  slits  are  oblique,  the  whole  of  one  could  not  be 
seen  on  a  single  cross  section. 

(a)  There  is  first  the  unpaired  cavity  of  the  proboscis,  which 
communicates  with  the  exterior  by  a  dorsal  pore  (or  some- 
times by  two)  at  the  base  of  the  proboscis  next  the  collar. 
It  is  possible  that  a  glandular  structure,  which  lies  in  front 
of  the  heart  in  the  proboscis,  may  have  excretory  signifi- 


DESCRIPTION  OF  BALANOGLOSSUS.  395 

cance,  but  it  seems  to  be  quite  enclosed,  (b)  In  the  collar 
region  there  are  two  small  paired  coelomic  cavities,  from 
which  two  funnels  open  to  the  the  exterior.  Both  these 
cavities  and  that  of  the  proboscis  tend  to  be  obliterated  by 
growth  of  connective  tissue,  (c)  Two  other  cavities  extend 
along  the  posterior  region  of  the  body,  to  some  extent  sep- 
arated by  the  dorsal  and  ventral  mesentery  which  moors  the 
intestine.  In  these  there  is  a  body  cavity  fluid  with  cells. 

Respiratory  System. 

The  respiratory  system  consists  of  many  pairs  of  ciliated 
gill  slits.  They  open  dorsally  by  small  pores  behind  the 
collar.  In  development  they  begin  as  a  pair,  increase  in 
number  from  in  front  backwards,  and  they  go  on  increasing 
long  after  the  adult  structure  has  been  attained.  Water 
passes  in  by  the  mouth  and  out  by  the  gill  slits,  where  it 
washes  branches  of  the  dorsal  blood  vessels.  There  are  no 
gill  lamellae  associated  with  the  slits. 

Vascular  System. 

The  vascular  system  includes  a  main  dorsal  blood  vessel, 
which,  at  its  anterior  end,  lies  above  the  notochord ;  an 
anterior  dilatation,  which  is  sometimes  called  the  "  heart ; " 
a  ventral  vessel  beneath  the  gut;  and  numerous  smaller 
vessels.  The  blood  flows  forwards  dorsally,  backwards 
ventrally.  This  system  should  be  contrasted  with  that  of 
Amphioxus. 

Excretory  System. 

The  excretory  system  is  slightly  developed.  No  nephridia 
are  known,  but  from  the  region  of  the  collar  two  ciliated 
funnels  open  to  the  exterior,  and  we  have  already  mentioned 
the  enigmatical  proboscis  gland. 

Reproductive  System. 

The  sexes  are  separate.  A  number  of  simple  paired 
genital  organs  lie  dorsally  in  a  series  on  each  side  of  the 
body  cavity  in  and  behind  the  region  with  gill  slits 
(Fig.  125,  R}.  They  open  by  minute  dorsal  pores  in  the 
skin,  or  in  the  American  species  by  rupture. 


396 


HEMICHORDA    OR  ENTEROPNEUSTA. 


Development. 

The  eggs  must  be  fertilised  outside  of  the  body.  Seg- 
mentation is  complete  and  approximately  equal ;  a  blasto- 
sphere  or  blastula  results ;  this  is  invaginated  in  the  normal 
fashion,  and  becomes  a  two-layered  gastrula. 

The  American  species  (B.  kowalevskii)  has  a  simpler 
development  than  the  others,  for  it  is  without  a  remarkable 
larval  form  (Tornaria)  which  occurs  in  them.  We  shall  take 
the  simpler  case  first,  though  it  is  probably  less  primitive. 

The  blastopore  or  mouth  of  the  gastrula  narrows  and 
closes;  the  external  surface  of  the  gastrula  becomes  ciliated; 


FIG.  126. — Development  of  Balanoglossus.     (After  BATESON.  ) 

The  mesoderm  is  represented  by  the  broken  dark  line. 
In  the  upper  row,  from  the  left — 

Section  of  blastula ;  beginning  of  gastrulation,  End,  endoderm  ; 
section   of  gastrula,  3/,    blastopore;    Ac,  Archenteron  ;    Sc, 
segmentation  cavity  ;   closure  of  blastopore,  outgrowth  of  five 
coelome  pouches  (M). 
In  the  lower  row,  from  the  left — 

Longitudinal  section,  showing  the  five  parts  of  the  body  cavity 

(&ri,  &T2,  ^3,)  or  coelome. 
Cross  section,  CNS,  central  nervous  system  ;  Nch,  notochord  ; 

bcz,  body  cavity  in  collar  region. 
Section  at  a  later  stage,  D.b.v.,  dorsal  blood  vessel. 

the  endoderm  lies  as  an  independent  closed  sac  within  the 
ectoderm.  Meanwhile  the  embryo  has  become  or  is  becom- 
ing free  from  the  thin  egg  envelope,  and  begins  to  move 
about  at  the  bottom  in  shallow  water.  It  elongates  and 
becomes  more  worm-like;  there  is  an  anterior  tuft  and  a 


DEVELOPMENT  OF  BALANOGLOSSUS. 


397 


posterior  ring  of  cilia ;  the  primitive  gut  forms  five  ccelomic 
pouches ;  a  mouth  and  an  anus  are  formed,  but  there  seem 
to  be  no  fore  gut  nor  hind  gut  invaginations.  The  regions 
of  the  body  are  defined  at  a  very  early  stage. 

The  Tornaria  larva  found  in  other  species  is  at  first  bell-shaped.  A 
ventral  mouth  opens  into  the  curved  gut,  which  is  furnished  with  a 
posterior  terminal  anus.  There  are  external  bands  of  cilia,  something 
like  those  of  an  Echinoderm  larva,  and  also  an  apical  sensory  plate  (like 
that  of  many  Annelid  trochospheres),  with  two  eye  spots.  The  Tornaria 
is  a  pelagic  form.  During  its  period  of  free  pelagic  life  it  gradually 

loses  its  distinctive  bands  of  cilia, 
becomes  diffusely  ciliated,  acquires  a 
proboscis  and  two  gill  slits,  and  thus 
approaches  the  form  of  the  larva  first 
described.  The  further  development  is 
the  same  in  both  cases.  The  Tornaria 
must  be  regarded  as  the  more  primitive 
larval  form ;  the  temporary  absence  of 
mouth  and  anus  in  the  other  type  is 
probably  an  adaptation  acquired  after 
the  pelagic  habit  was  lost. 

Johannes  Miiller  ranked  the  Tornaria 
larva,  whose  adult  form  was  not  then 
known,  beside  the  larvae  of  Echino- 
derms.  The  ciliated  bands  of  the  Tor- 
naria resemble  those  of  Echinoderm 
larvae,  but  this  is  only  a  superficial 
characteristic.  The  anterior  pouch, 
which  forms  the  cavity  of  the  proboscis 
and  communicates  with  the  exterior  has 
also  been  compared  with  the  beginning 
of  the  water  vascular  system  in  Echino- 
derms,  and  it  is  true  that  in  both  several 
independent  coelome  pouches  grow  out 
from  the  primitive  gut.  But  we  might 
with  as  much  force  compare  the  Tor- 
naria to  an  Annelid  trochosphere,  and  it  may  be  that  it  would  be  most 
profitable  to  compare  certain  features  in  the  development  of  Balano- 
glossus  with  that  of  the  lancelet. 

Affinities  with  Vertebrates  (especially  emphasised  by  Mr.  Bateson). 
(i)  " Notochord" — A  dorsal  outgrowth  from  the  anterior  region 
of  the  gut  grows  forward  for  a  short  distance  into  the  pro- 
boscis and  becomes  a  solid  supporting  rod  (Fig.  126,  Nch.}. 
It  may  be  compared  with  the  notochord  of  Vertebrates, 
which  also  arises  dorsally  from  the  gut.  But  it  lies  below 
the  main  dorsal  blood  vessel,  is  of  very  limited  extent, 
and  may  be  merely  an  analogue  of  the  notochord — a 
physiological  necessity  for  the  support  of  the  elongated 
proboscis. 


FIG.  127.  —  Tornaria 
larva,  from  the  side. 
(After  SPENGEL.) 

M,  mouth  ;  £-,  gut ;  a,  anus ;  k, 
heart ;  /,  pore  entering  proboscis 
cavity  ;  c.r,  chief  ciliated  ring  ; 
s.c.r,  secondary  ciliated  ring.  The 
dark  wavy  line  indicates  the  mar- 
gin of  the  lobes  of  the  larval  body. 


398  HEMICHORDA    OR  ENTEROPNEUSTA. 

(2)  "  Gill  slits" — Numerous  gill  slits  (Fig.  124,  gs. )  open  from  the 

anterior  region  of  the  gut  to  the  exterior,  and  are  separated 
from  one  another  by  skeletal  bars,  which  in  some  ways 
resemble  the  framework  of  the  respiratory  pharynx  in 
Amphioxus.  There  are,  however,  many  differences  in 
detail,  thus  the  slits  open  dorsally,  not  ventrally ;  the 
skeletal  bars  are  differently  disposed,  the  blood  supply  is 
different.  Nor  is  it  certain  that  the  gullet  of  Balano- 
glossus  is  endodermic  like  that  of  Vertebrates.  Still,  the 
possession  of  these  respiratory  slits  is  one  of  the  most 
satisfactory  of  the  alleged  Vertebrate-like  characters  of 
Balanoglossus. 

(3)  " Dorsal  nerve  cord '." — A  dorsal  median  insinking  (Fig.  125, 

d.n)  of  ectoderm,  especially  strong  in  the  region  of  the 
collar,  may  be  compared  with  the  medullary  canal  of  Verte- 
brates. But  it  must  be  noticed  that  there  is  also  a  ventral 
nerve  cord  (Fig.  125,  v.n)  which  cannot  be  ignored  as 
subsidiary  in  character. 

Mr.  Bateson  has  also  noted  that  the  mesoblast  arises,  as  in  Amphioxus ', 
&c. ,  in  the  form  of  coelome  pouches,  but  this  is  true  of  many  Inverte- 
brates. He  states  that  the  history  of  the  anterior  coelome  pocket,  which 
grows  forward  into  the  proboscis  of  Balanoglossus,  is  closely  like  that  in 
Amphioxus,  but  this  is  denied  by  Spengel.  He  compares  a  slight  fold, 
which  grows  backwards  from  in  front  of  the  gill  slits,  with  the  epipleural 
folds  of  Amphioxus  (Fig.  124,  op. ),  but  the  fold  appears  to  be  restricted  to 
one  species.  It  is  still  uncertain  what  weight  should  be  attached  to  the 
fact  that  Balanoglossus  is  unsegmented. 

Affinities  with  Annelids  (after  Prof.  Spengel). 

(1)  The  larva  (Tornaria)  (Fig.  127)  may  be  regarded  as  a  modi- 

fied Trochosphere,  but  this  points  at  most  to  a  far-off 
common  stock.  Moreover,  the  nephridia,  usually  present 
in  the  Trochosphere,  are  unrepresented  in  the  Tornaria. 

(2)  The    body   cavity    is    formed    from    segmentally    arranged 

coelome  pouches ;  but  there  is  a  pair  of  pre-oral  pouches 
absent  in  Annelids,  and  segmental  arrangement  in  the 
organs  of  the  body  in  Balanoglossus,  is,  to  say  the  least, 
very  vague. 

(3)  The  heart  lies,  as  in  some  Annelids,  dorsal  to  the  gut,  not 

ventral  as  in  Vertebrates ;  the  dorsal  vessel  carries  blood 
forwards,  the  ventral  backwards,  as  is  usual  in  Annelids. 
But  the  double  nervous  system  is  essentially  different  from 
that  of  Annelids  ;  and  the  gill  slits  are  also,  so  far  as  we 
know,  unrepresented.  If  there  be  a  relationship  between 
Enteropneusta  and  Annelids,  it  must  be  a  very  distant  one, 
perhaps  restricted  to  origin  from  some  common  stock. 

Besides  these  affinities,  others  have  been  ingeniously  detected.  Those 
alleged  to  exist  between  Enteropneusta  and  Nemerteans,  e.g. ,  the  exter- 
nal ciliation,  the  unsegmented  musculature,  the  correspondence  of  the 
"notochord"  and  the  Nemertean  proboscis,  are  even  more  unsatisfac- 


CEPHALODISCUS—RHABDOPLEURA.  399 

tory  than  those  above  cited.  Again,  the  resemblance  between  the 
Tornaria  larva  and  that  of  Echinoderms  is  unsatisfactory,  because  of  our 
relative  ignorance  of  the  development  of  the  Tornaria. 

Here,  then,  we  have  a  lesson  in  uncertainties,  for  all  that  we  can  say 
is  that  the  Enteropneusta  seem  to  be  synthetic,  possibly  transitional 
types,  exhibiting  affinities  with  various  others,  but  differing  markedly 
from  all. 

Appendix  (i)  to  ENTEROPNEUSTA— CEPHALODISCUS. 

A  single  species  (Cephalodiscus  dodecalophtis]  was  dredged  by  the 
"Challenger"  in  the  Magellan  Straits.  It  was  at  first  described  by 
M'Intosh  as  a  divergent  Polyzoon,  but  the  researches  of  Harmer  point 
to  relationship  with  Balanoglossus. 

The  minute  stalked  individuals  occur  associated  together  in  a  gelatin- 
ous investment,  the  colony  may  attain  a  size  of  9  inches  by  6  inches. 
The  gut  is  curved,  the  anus  being  beside  the  mouth,  beneath  which  are 
two  rows  of  ciliated  hollow  tentacles.  These  two  characters,  formerly 
supposed  to  indicate  Polyzoan  affinities,  may  perhaps  be  adaptations  to 
the  sedentary  life.  With  Balanoglossus  this  type  agrees  in  the  possession 
of  the  following  characters  : — (a)  The  body  is  divided  into  three  regions 
which  correspond  to  the  proboscis,  collar,  and  trunk  of  Balanoglossus ', 
this  is  especially  obvious  in  the  young  bud  ;  (b)  each  of  the  three  regions 
contains  a  ccelomic  cavity,  the  most  anterior  being  single,  while  the 
other  two  are  divided  by  a  median  partition ;  (<r)  the  anterior  pre-oral 
cavity  opens  to  the  exterior  by  two  pores  (cf.  proboscis  pores  of  Balano- 
glossus'] ;  (d]  the  collar  region  is  also  furnished  with  two  collar  pores, 
which  open  beneath  a  fold  or  operculum  developed  from  the  collar ;  (e) 
in  the  collar  region  the  dorsal  nervous  system  is  also  placed,  and  is  con- 
tinued to  some  extent  into  the  proboscis  ;  (/)  beneath  the  nervous  sys- 
tem lies  a  diverticulum  from  the  gut,  which  extends  towards  the  proboscis 
region  ("  notochord") ;  (£•)  the  anterior  region  of  the  gut  is  perforated 
by  a  pair  of  lateral  gill  slits. 

Appendix  (2)  to  ENTEROPNEUSTA— RHABDOPLEURA. 

This  genus  is  found  at  considerable  depths  in  the  North  Sea.  Like 
Cephalodiscus  the  individuals  are  minute  and  stalked,  and  occur  in  a 
colony  ;  in  this  case,  however,  they  remain  attached  to  one  another  by 
a  common  stalk  instead  of  being  united  only  by  an  investment.  In  the 
head  region  there  are  two  hollow  lateral  arms  bearing  numerous  ciliated 
tentacles,  which  have  a  skeletal  support.  The  gut,  as  in  Cephalodiscus ; 
has  a  U-shaped  curvature,  and  an  anterior  diverticulum  ("notochord"). 
There  are  five  coelomic  cavities,  of  which  the  unpaired  pre-oral  part  has 
two  pores.  There  are  no  gill  slits.  The  affinities  between  this  type  and 
Balanoglossus  must  still  be  held  doubtful. 


CHAPTER    XVIII. 

CLASS    UROCHORDA   OR   TUNICATA. 

(ASCIDIANS,  SEA  SQUIRTS,  &c.) 

THE  Tunicates  are  remarkable  animals,  which  seem  to 
stumble  on  the  border  line  between  Invertebrates  and 
Vertebrates.  They  were  classified  with  Polyzoa  and 
Brachiopoda  as  Molluscoidea,  until,  in  1866,  Kowalevsky 
described  for  the  first  time  the  development  of  a  simple 
Ascidian,  and  correlated  it,  step  by  step,  with  that  of 
Amphioxus.  He  showed  that  the  larval  Ascidian  possesses 
a  dorsal  nerve  cord,  a  notochord  in  the  tail  region,  gill  slits 
opening  from  the  pharynx  to  the  exterior,  and  an  eye 
developing  from  the  brain.  It  is  true  that  in  most  cases 
the  promise  of  youth  is  unfulfilled  ;  the  active  larva  settles 
down  to  a  sedentary  life,  loses  tail  and  notochord,  nerve 
cord  and  eye,  and  becomes  strangely  deformed.  Neverthe- 
less we  must  now  class  Tunicates  as  degenerate  Vertebrates. 
Of  their  possible  relations  to  simpler  forms  nothing  definite 
is  known, 

GENERAL  CHARACTERS. — The  Tunicates  are  marine 
Chordata,  but  the  chordate  characteristics — dorsal  nervous 
system,  notochord \  gill  slits,  and  brain  eye — are  in  most  cases 
discernible  only  in  the  free  swimming  larval  stages.  They 
usually  degenerate  in  adolescence,  and  the  adults,  which  are  in 
most  cases  sedentary,  tend  to  diverge  very  widely  from  the 
Vertebrate  type.  Thus  the  nervous  system  is  generally  reduced 
to  a  single  ganglion.  The  body  is  invested  by  a  thickened 
cuticular  tunic,  which  contains  cellulose.  The  pharynx  is  per- 
forated by  two  (Larvacea),  or  in  the  majority  by  numerous 
ciliated  gill  slits,  and  is  surrounded  to  a  greater  or  less  extent 


TUNIC  ATA.  401 

by  a  peribranchial  chamber,  which  communicates  with  the  ex- 
terior by  a  special  (atrial)  opening.  The  heart  is  simple  and 
tubular,  and  there  is  a  periodic  reversal  in  the  direction  of  the 
blood  current.  Nephridia  are  absent,  and  the  renal  organs 
are  always  devoid  of  ducts.  All  are  hermaphrodite.  There 
is  usually  a  metamorphosis  in  development.  Colonies  are  fre- 
quently formed. 

Though  typically  sedentary,  the  Tunicates  show  considerable  variation 
in  habit.  Many  grow  fixed  to  stones  or  shells,  or  to  the  muddy  bottom, 
and  are  common  on  or  near  the  coasts  of  all  seas.  They  live  on  minute 
organisms  carried  into  the  pharynx  by  the  water  of  respiration.  Through- 
out the  group  we  see  that  antithesis  between  sessile  vegetative  forms  and 
active  pelagic  forms,  which  is  so  vividly  exhibited  in  the  Ccelentera. 
Some  of  the  free  swimming  forms  are  indeed  as  typically  pelagic  in 
structure  and  habit  as  the  medusae  themselves.  Of  the  sessile  forms 
some  are  simple  (e.g.,  Ascidia]  ;  others,  in  which  the  vegetative  habit 
has  more  throughly  permeated  the  organism,  reproduce  themselves  freely 
by  budding.  The  clusters  so  formed  may  consist  of  individuals  united 
only  by  a  common  blood  system,  forming  the  so-called  social  Ascidians 
(e.g.,  Clavellind),  or  composite  organisms  may  be  formed  as  in  Botrylhis. 
Again  we  have  allied  colonial  forms,  such  as  Pyrosoma,  the  phosphor- 
escent fire  flame,  which  are  free  swimming  and  pelagic,  the  whole  colony, 
with  its  numerous  individuals,  moving  as  one  creature.  All  these 
individuals  are  formed  by  budding  from  a  rudimentary  larva  which  arises 
from  the  fertilised  egg. 

All  these  types  belong  to  the  Ascidian  series.  Different  from  them, 
but  connected  by  Pyrosoina,  are  the  free  swimming  genera  Salpa  and 
Doliolum,  together  with  the  strange  abyssal,  and  probably  sessile, 
"  Challenger  "  genus  Octacnemus.  Both  Salpa  and  Doliolum  exhibit  a 
complex  alternation  of  generations,  in  the  course  of  which  both  solitary 
and  aggregated  forms  occur,  the  latter,  like  the  floating  colonies  of 
Siphonophora,  often  showing  considerable  division  of  labour. 

Finally,  there  are  a  few  active  free  swimming  forms,  which  retain 
many  of  the  features  of  the  larval  Ascidian.  Of  these  Appendicularia 
is  the  simplest  type. 

Type  of  TUNICATA — a  simple  Ascidian  (Ascidia  mentuld). 

In  form,  an  adult  Ascidia  is  an  irregular  oval  of  three  to 
four  inches  in  length ;  one  end  is  attached  to  stones  or 
weed ;  the  other  is  more  tapering,  and  terminates  in  the 
mouth,  close  beside  which,  on  the  morphological  dorsal 
surface,  lies  another  opening,  the  exhalent  or  atrial  aperture. 
During  life,  water  is  constantly  being  drawn  in  by  the  mouth, 
and  passed  out  by  the  atrial  opening.  If  irritated,  the 
animal  frequently  drives  a  jet  of  water  with  considerable 
force  from  this  aperture,  whence  the  name  "  sea  squirt." 

26 


402  UROCHORDA    OR    TUNIC  ATA. 

Test  and  Skin. 

The  whole  body  is  clothed  in  a  thick  test  or  tunic,  which 
can  be  readily  peeled  off  from  the  underlying  body  wall. 
This  tunic  contains  a  carbohydrate  allied  to,  if  not  identical 
with,  the  cellulose  of  plants,  and  also  some  proteid  substance. 
The  whole  is  at  first  a  true  cuticle,  but  cells  soon  migrate 
into  it,  while  at  one  point  blood  vessels  also  enter  it  from 
the  body,  and  ramify  in  all  directions.  The  tunic  is  begun 
by  secretory  prolongations  of  ectoderm  cells,  and  some 
ectoderm  cells  pass  out  into  what  is  secreted.  But  the  tunic 
also  receives  important  contributions  from  mesenchyme  cells 
which  migrate  into  it.  Some  of  them  probably  act  as 
phagocytes  in  cases  of  injury  or  infection.  The  "  Cellulose  " 
or  "  tunicin  "  is  common  throughout  the  group,  and  it  is  in- 
teresting to  find  a  characteristically  vegetable  product  in  the 
very  passive  cuticle  of  these  passive  animals.  The  ectoderm 
which  secretes  the  tunic  is  a  single  layer  of  cells. 

The  Muscular  System. 

The  muscular  system  forms  beneath  the  epidermis  a 
netted  sheath  of  unstriped  fibres,  which  are  very  numerous 
on  the  right  side  of  the  body,  and  almost  absent  on  the  left. 
Special  sphincters  surround  the  apertures. 

The  Nervous  System. 

The  nervous  system,  which  in  the  larva  consists  of  a 
spinal  cord  with  a  slight  anterior  cerebral  swelling,  is  repre- 
sented in  the  degenerate  adult  only  by  a  ganglionic  mass, 
which  lies  between  the  two  apertures,  and  gives  off  a  few 
nerves. 

Sensory  Structures. 

Sensory  structures  in  the  adult  are  few  and  unspecialised. 
In  the  larva  there  is  a  well  developed  eye  and  an  auditory 
organ,  both  in  close  connection  with  the  brain.  These  do 
not  persist  in  the  adult. 

Beneath  the  ganglion  in  the  adult  there  lies  a  small  (sub-neural)  gland 
from  which  a  ciliated  duct  opens  into  the  pharynx.  According  to  some, 
this  corresponds  morphologically  to  the  pituitary  body  (see  page  436), 
and  so  partially  to  the  pre-oral  pit  oiAmphioxus  (q.  v.);  its  physiological 


SENSORY  STRUCTURES. 


403 


significance  to  the  individual  is  uncertain ;   perhaps  it  secretes  some  of 
the  mucus  which  entangles  the  food  particles. 

Further,  it  is  probable  that  all  the  following  structures  have  some 
sensory  function, — the  pigment  spots  between  the  lobes  of  the  apertures, 
the  tentacle-like  processes  beneath  the  mouth,  and  other  processes 
(languets)  on  the  dorsal  wall  of  the  pharynx  (absent  in  A.  mentula]. 


Ex.ap. 


FIG.  128.— Dissection  of  Ascidian.     (After  HERDM AN.) 

/«.#/.,  Inhalant  aperture;  71.,  tunic,  cut  away  below  to  show 
muscular  layer,  pharynx,  &c.  ;  En.t  endostyle  or  ventral  groove  of 
pharynx.  Note  removal  of  pharynx  to  show,  on  the  other — the  left — 
side,  stomach  (St.\  intestine  (with  fold  seen  at  incision),  and  repro- 
ductive organs  (G.)  ',  H.,  heart;  G.D.,  genital  duct;  A.,  anus; 
C7.,  cloacal  chamber ;  Ex.ap.,  exhalent  aperture;  Gn.,  lies  above 
the  ganglion  which  is  seen  between  the  two  apertures. 


404  UROCHORDA    OR    TUNIC  ATA. 

Alimentary  System. 

The  mouth  is  surrounded  by  eight  rounded  lobes,  between 
which  are  pigment  spots.  It  leads  into  a  large  pharynx,  or 
branchial  sac,  the  walls  of  which  are  perforated  by  numerous 
gill  slits.  At  the  beginning  of  this  pharynx  there  is  a  circle 
of  tentacles,  which  point  downwards  from  the  posterior 
edge  of  the  sphincter  muscle.  At  its  base  in  the  dorsal 
middle  line  the  pharynx  opens  into  a  short,  curved,  ciliated 
oesophagus,  which  leads  to  the  large  stomach,  placed,  like 
the  rest  of  the  gut,  on  the  left  side  of  the  body.  The 
intestine  describes  an  S-shaped  curve,  and  terminates  in  a 
cloacal  chamber  below  the  exhalent  opening.  Its  wall  is 
folded  inwards  as  in  the  earthworm.  With  the  stomach  a 
mass  of  ramifying  tubules  is  associated,  which  opens  by 
means  of  a  duct  into  the  cavity  of  the  stomach,  and  may  be 
a  digestive  gland. 

In  feeding,  water  is  drawn  into  the  mouth  by  the  action 
of  the  numerous  small  cilia  which  border  the  pharyngeal 
slits.  This  water  contains  small  algae  and  other  organisms. 
These  are  glued  together  by  a  mucus  secretion,  and  swept 
backwards  to  the  oesophagus  at  the  base  of  the  pharynx, 
while  the  water  passes  through  the  slits  into  the  peribranchial 
chamber.  This  lies  between  the  body  wall  and  the  outer 
wall  of  the  pharynx,  and  communicates  with  the  exterior  by 
the  exhalent  aperture. 

On  the  internal  ventral  surface  of  the  pharynx  (the  side 
on  which  the  ganglion  lies  being  morphologically  dorsal)  there 
is  a  longitudinal  groove — the  endostyle.  This  is  furnished 
with  very  long  cilia,  and  is  in  part  a  glandular  region.  It 
may  secrete  part  of  the  mucus  mentioned  above.  On  the 
dorsal  surface  of  the  pharynx  there  is  a  projecting  ciliated 
fold,  called  the  dorsal  lamina.  In  many  Ascidians  this  is 
broken  up  into  a  series  of  processes,  —  the  languets. 
Herdman  considers  that,  as  few  of  the  endostyle  cells  are 
glandular,  much  of  the  mucus  must  be  secreted  elsewhere, 
and  the  food  particles  probably  pass  backwards,  both  on 
the  ventral  and  dorsal  surfaces. 

The  ventral  endostyle  is  morphologically  comparable  to  the  ventral 
portion  of  the  respiratory  pharynx  in  Balanoglosstis,  and  to  the  "hypo- 
branchial  groove  "  of  Amphioxus.  It  may  even  be  compared  to  the 
thyroid  gland  of  Vertebrates,  for  that  organ  in  the  Ammocoete  larva  of 


RESPIRATORY  SYSTEM. 


405 


the  lamprey  arises  as  an  evagination  of  the  floor  of  the  pharynx,  and  for 
a  long  time,  i.e. ,  until  the  metamorphosis  into  the  lamprey,  has  a  glandular 
structure  opening  on  the  floor  of  the  pharynx,  remarkably  like  the  endo- 
style  of  the  Tunicate  Oikopleura. 

Respiratory  System. 

The  water  which  enters  by  the  mouth,  after  being  deprived 
of  some  of  its  food  particles,  passes  through  the  gill  slits  to 
the  peribranchial  chamber.  On  the  walls  of  the  pharynx 


FIG.  129. — Diagram  of  Ascidian.     (After  HERDM AN.) 

The  arrows  indicate  the  two  openings,  the  dark  border  the  tunic. 
Ph.,  pharynx,  with  gill  slits;  G.,  reproductive  organs;  H.,  heart, 
with  blood  vessels ;  G.D.,  genital  ducts  ;  /?.,  rectum  ending  in  cloacal 
chamber.  Surrounding  the  pharynx  the  peribranchial  cavity  is  shown. 


406  UROCHORDA    OR    TUNIC  ATA. 

the  blood  is  spread  out  in  vessels,  and  is  thus  aerated.  The 
peribranchial  chamber  is  lined  by  ectoderm,  for  it  is  formed 
in  development  by  the  union  of  two  ectodermic  invagina- 
tions,  which  grow  towards  each  other.  The  first  gill  slits  are 
formed  by  the  fusion  of  small  diverticula  of  the  pharynx 
with  two  separate  peribranchial  invaginations.  In  the  adult 
the  slits  are  very  numerous  and  of  secondary  origin ;  they 
are  formed  partly  by  the  division  of  primary  slits,  partly  by 
new  perforations  of  the  wall  of  the  pharynx.  With  regard 
to  the  development  of  all  these  structures,  however,  there  is 
as  yet  little  certainty. 

Vascular  System. 

The  simple  tubular  heart  (Fig.  129,  H.}  lies  in  a  peri- 
cardial  space  at  the  side  of  the  lower  end  of  the  pharynx. 
In  development,  two  diverticula  grow  out  from  the  pharynx; 
these  meet  and  fuse,  forming  the  pericardium.  The  heart 
arises  as  an  evagination  from  its  dorsal  wall.  According  to 
some  authorities,  the  cavities  of  the  heart  and  of  the  blood 
vessels  are  blastocoelic  in  origin,  i.e.,  they  are  said  to  be 
derived  from  the  segmentation  cavity  of  the  embryo.  A 
periodical  reversal  of  the  direction  of  the  waves  of  contrac- 
tion is  discernible  in  the  heart ;  for  a  certain  number  of  beats 
the  blood  is  driven  upwards,  and  then  the  direction  is 
reversed.  This  is  said  to  be,  at  any  rate  partially,  due  to 
the  differences  in  oxygenation  of  the  blood  at  the  two  ends 
of  the  heart.  This  same  reversal  also  occurs  in  Phoronis. 

According  to  Professor  Herdman,  the  ventro-dorsal  contractions 
occasion  the  following  circulation.  The  blood,  which  is  spread  out  on 
the  walls  of  the  pharynx  in  vessels  lying  between  the  slits,  collects  into 
one  large  vessel,  which,  after  receiving  a  vessel  from  the  tunic,  enters 
the  ventral  end  of  the  heart.  From  the  dorsal  end  it  is  poured  into  a 
great  trunk,  which  sends  one  branch  to  the  tunic  and  then  breaks  up 
among  the  viscera.  From  the  visceral  lacunae  the  blood  is  again  collected 
to  be  distributed  to  the  branchial  sac.  At  the  reversal  of  the  contrac- 
tions this  circulation  is  also  reversed.  The  blood  is  very  colourless,  but 
usually  contains  a  few  pigmented  corpuscles. 

Excretory  System. 

In  the  loop  of  the  intestine  there  lies  a  mass  of  clear 
vesicles  containing  uric  acid  and  other  waste  products. 
This,  therefore,  seems  to  be  a  renal  organ,  but  there  is  no 
duct.  Bacteria  are  usually  found  in  the  vesicles,  and  their 


REPRODUCTIVE  SYSTEM.  407 

activity  may  make  diffusion  easier.  It  is  interesting  to  find 
such  a  plant-like  method  of  storing  up,  instead  of  eliminat- 
ing, waste  products  in  these  very  passive  animals.  It  has 
been  suggested  that  the  sub-neural  gland  may  have  some 
renal  function. 

Reproductive  System. 

Tunicates  are  hermaphrodite.  The  reproductive  organs 
(Fig.  128,  G.)  are  very  simple,  and  lie  in  the  loop  of  the 
intestine.  The  ovary  is  the  larger,  and  contains  a  cavity 
into  which  the  ova  are  set  free,  and  from  which  they  pass 
outwards  along  an  oviduct  which  opens  into  the  cloacal 

chamber.       The     testis     sur- 

£•  »p  j     rounds     the     ovary,     and     is 

mature    at    a    different    time 

'A"""4lBP  "•  (dichogamy);  its  duct  runs  by 

fjEME^  the    side  of  the  oviduct.     In 

some  forms,  where  the  gonads 
are  near  the  cloaca,  there  are 
^_^     ^x  \  |     no  ducts.      The  ova  are  sur- 
^af"~\  rounded    by    follicular    cells, 

and  are  probably  fertilised  in 
the  cloaca. 


A  F™.-  I3°/~Y(Tg  xEmb!T;  >°f  Development.—  Most  of  the  As- 
Ascidian  (Clavelhna).  (After  ddians  exhibit  the  development  with 
VAN  BENEDEN  and  JULIN.)  metamorphosis  which  is  about  to  be 

«/.,  Neuropore  ;  nr.,  neural  canal  ;  described  ;  a  few  in  which  the  larvae 
ch.t  notx)chord;  ec.,  ectoderm;  en.,  are  retained  for  a  long  time  within 
endoderm.  the  kocjy  of  t^e  mother,  show  a  much 

abbreviated  life  history. 

The  fertilised  ovum  divides  completely  and  almost  equally.  The 
spherical  blastosphere  becomes  slightly  flattened,  and  ultimately  forms  a 
two-layered  gastrula. 

Along  the  dorsal  median  line  of  the  gastrula,  the  ectoderm  cells  form 
the  medullary  groove,  the  sides  of  which  arch  together  and  form  a 
canal  —  the  medullary  canal.  This  opens  anteriorly  to  the  exterior  by 
the  neuropore,  and  posteriorly  communicates  with  the  archenteron  by 
the  neurenteric  canal.  In  the  posterior  region  of  the  gut,  at  the  sides 
of  the  blastopore,  a  pair  of  diverticula,  according  to  Van  Beneden  and 
Julin,  grow  out.  These  form  the  mesoderm  ;  the  endoderm  cells 
between  them,  roofing  the  gut,  form  the  rudiment  of  the  notochord. 
The  mesoderm  masses  and  the  notochord  grow  forward  together  for  a 
time,  but  later  the  mesoderm  advances  much  further  into  the  anterior 
region,  the  notochord  being  limited  to  the  tail.  The  diverticula 
originally  contain  each  a  small  cavity  —  the  true  coelome,  but  this  is  soon 


408  UROCHORDA    OR    TUNIC  ATA. 

obliterated.  Two  ectodermal  invaginations  form  the  originally  double 
peribranchial  chamber,  and  the  primary  gill  slits  break  through  the  wall 
of  the  pharynx. 

For  some  hours  the  tadpole-like  larva  enjoys  a  free  swimming  life, 
using  its  tail  as  an  organ  of  locomotion.  Then  it  fixes  itself  by  a  papilla 
on  the  head,  and  begins  almost  immediately  to  degenerate.  The  tail 
shrinks  and  disappears,  the  notochord  being  consumed  by  phagocytes. 
The  nerve  cord  is  lost,  and  with  it  the  larval  sense  organs.  The 
pharynx  and  peribranchial  chamber  assume  their  adult  form,  and  the 
whole  animal  undergoes  a  metamorphosis,  which  one  of  the  most 
signal  instances  of  degeneration. 


FIG.   131. — Section  of  newly  fixed  larva  of  Clavellina. 
(After  SEELIGER.) 

•  z.,  Inhalent  aperture ;  ft. ,  ciliated  groove;  sb.,  sensory  vesicle ;  e., 
exhalent  aperture;  r.,  posterior  part  of  medullary  canal;  ck.,  noto- 
chord; ^.,  heart;  ks.,  gill  slits;/.,  peribranchial  space;  es.,  endo- 
style  ;  hp.,  attaching  papillae. 

Classification. 
Order  i.  LARVACEA  or  PERENNICHORDATA. 

Appendicularia,  Oikopleura,  Fritillaria,  Kowalevskia. 

This  order  includes  a  few  simple  pelagic  forms,  which  exhibit  many  of 
the  characters  of  the  larvae  of  the  Ascidians.  All  are  minute,  and 
furnished  with  a  large  locomotor  tail  which  is  bent  forwards  at  an  angle 
to  the  body.  Epidermic  cells  near  the  mouth  secrete  a  slimy  but 
consistent  test,  or  "  house,"  which  is  frequently  abandoned  and  formed 
anew.  The  tail  has  a  supporting  notochord,  and  very  large  muscle 
cells.  The  nervous  system  consists  of  a  lobed  ganglionic  mass  above 
the  mouth,  from  which  a  nerve  cord  is  continued  backwards  and  along 
the  tail  ;  this  is  furnished  with  other  ganglia  in  the  tail  region.  It  lies 
to  the  side  of  the  notochord,  and  like  the  ganglia  is  said  to  be  furnished 
with  an  axial  canal.  In  connection  with  the  cerebral  ganglion  there  is 
a  pigment  spot,  an  otocyst  (auditory  ? ),  and  a  tubular  process  com- 
municating with  the  pharynx,  and  corresponding  to  the  subneural  gland 
and  the  ciliated  duct  of  other  Tunicates.  There  are  two  gill  slits  which 
communicate  with  two  ectodermic  atrial  invaginations,  as  in  the  early 
larval  stages  of  Ascidia.  The  mouth  is  almost  at  the  anterior  end, 
the  anus  at  the  root  of  the  tail.  The  heart  is  very  simple,  and  there  are 
no  distinct  vessels.  The  hermaphrodite  reproductive  organs  lie 
posteriorly  and  are  ductless.  The  eggs  are  difficult  to  obtain,  and  in 
consequence  little  is  known  of  the  development. 


ASCIDIA  CEA—  THALIA  CEA .  409 

The  Larvacea  form  a  group  of  great  theoretical  interest.  As  to  their 
phylogenetic  position  two  views  have  been  maintained.  According  to 
one  recently  reasserted  by  Brooks,  they  are  the  slightly  modified 
descendants  of  the  primitive  Tunicates,  from  which  the  Ascidians  have 
diverged  in  the  direction  of  degeneration  ;  in  other  words,  they  are 
primitive  forms.  According  to  the  other  view,  they  have  been  derived 
from  an  Ascidian-like  form,  in  which  degeneration  had  already  taken 
place,  and  may  be  compared  to  prematurely  sexual  larvae. 

Order  2.  ASCIDIACEA. 

(a)  Ascidioe  simplices.     Ascidia,  Phallusia,  Ciona. 

(b)  Ascidke  Composites.     Botrylhis^  Polyclimim. 

(c)  Pyrosomidce.     Pyrosoma. 

The  characters  of  this  order  may  be  gathered  from  the  description  of 
Ascidia.  Most  are  sedentary,  with  the  notable  exception  of  Pyrosoma. 
In  several  there  is  an  alternation  of  sexual  and  asexual  forms  in  the  life 
history.  In  Pyrosoma  the  embryo  develops  within  the  body  of  the 
mother  ;  there  is  much  yolk,  and  development  is  shortened  and  direct. 
The  greater  number  of  the  Tunicates  are  included  in  this  order,  among 
them  several  well  known  British  genera. 

Order  3.  THALIACEA. 

(a)  Salpa,  Octacnemus. 
(b}  Doliolum^  Anchinia. 

This  order  contains  a  few  genera  which  show  considerable  modifica- 
tion from  the  Ascidian  type.  With  the  possible  exception  of  the  little 
known  Octacnemus •,  all  are  free  swimming,  and,  with  the  same  excep- 
tion, pelagic.  The  mouth  is  at  one  end  of  the  body,  the  atrial  aperture 
at  the  other  ;  the  animals  swim  by  forcing  the  water  out  of  the  peri- 
branchial  chamber  posteriorly.  The  cuticle  is  often  very  thin,  as  we 
should  expect  in  animals  so  active ;  in  some  species  it  seems  to  be 
absent.  In  Doliolum,  the  muscles  form  hoops  encircling  the  barrel 
shaped  body  ;  in  Salpa  the  body  is  fusiform,  and  the  muscle  bands  do 
not  form  complete  rings.  Both  are  devoid  of  reproductive  ducts,  and 
very  simple  in  structure. 

The  life  history  shows  an  alternation  of  generations.  In  Salpa,  a 
solitary  asexual  form,  or  "  nurse,"  gives  rise  to  a  complex  stolon,  which 
segments  to  form  a  chain  of  embryos.  This  chain  is  set  free,  its 
members  become  sexual,  and,  either  while  still  united,  or  after  separa- 
tion, produce  ova,  which  give  rise  to  the  nurse  form.  In  Doliolum,  a 
small  stolon  forms  a  number  of  primitive  buds,  which  creep  over  the 
parent  and  multiply.  They  form  a  lateral  series  of  individuals,  which 
furnish  the  parent  form  with  food,  and  a  median  series  which  are  set 
free  as  asexual  "  foster  mothers."  These  carry  with  them  some 
primitive  buds,  which  divide  into  secondary  buds,  and  these  finally 
become  the  solitary  sexual  forms,  producing  ova  which  develop  into 
"  nurses."  The  "  nurse  "  form  during  the  budding  process  degenerates 
greatly,  until,  like  the  swimming  bells  of  the  Siphonophora,  it  becomes 
a  mere  organ  of  locomotion,  nourished  by  the  lateral  buds.  There  is 
thus  considerable  division  of  labour  in  the  colonial  form. 


CHAPTER    XIX. 

CEPHALOCHORDA,    AMPHIOXUS. 
(SYNONYMS,  ACRANIA,  LEPTOCARDII,  PHARYNGOBRANCHII.) 

THERE  is  but  one  well-defined  genus  in  this  class, — the 
lancelet  (Amphioxus}.  Yet  it  deserves  a  class  for  itself,  so 
distinct  is  it  from  other  Chordata.  It  may  be  regarded  as 
an  offshoot  from  the  primitive  Vertebrate  stock,  lost,  it  is  to 
be  feared,  for  ever,  as  a  far-off  prophecy  of  a  fish,  or,  per- 
haps, as  a  degenerate  type,  "  a  weed  in  the  Vertebrate 
garden." 

GENERAL  CHARACTERS  OF  CEPHALOCHORDA. — A  class 
of  simple  Chordate  animals,  represented  by  one  distinct  type — 
Amphioxus.  The  nervous  system  has  no  well-defined  brain 
region.  The  notochord  is  persistent  and  unsegmented ;  it  is 
surrounded  by  a  continuous  sheath,  and  projects  in  a  unique 
manner  in  front  of  the  anterior  end  of  the  nerve  cord.  In  the 
adult,  the  gill  slits  are  very  numerous.  From  Fishes,  Amphi- 
oxus is  widely  removed  by  the  absence  of  limbs,  skull,  jaws, 
definite  brain,  sympathetic  nervous  system,  eye,  ear,  definite 
heart,  spleen,  and  genital  ducts.  The  species  have  a  wide 
marine  distribution  near  the  coasts  in  warm  and  temperate 
seas,  are  sluggish  in  habit,  and  feed  on  microscopic  organisms 
or  organic  particles. 

Description  of  Amphioxus  lanceolatus,  the  best  known 
species. 

Mode  of  Life. 

The  lancelets  are  fond  of  lying  in  the  sand  in  water  about 
two  fathoms  deep,  with  only  the  fringed  aperture  of  the 


V. 


FORM. 


411 


mouth  projecting.  Into  this  diatoms  and  other  small 
organisms  are  sucked.  At  times,  and  especially  in  the 
evening,  the  adults  sometimes  start  up  and  swim  about, 
but  they  are  always  less  active  than  the  larvae.  The  early 
larvae  are  indeed  pelagic. 

Form. 

The  body,  which  rarely  measures  as  much  as  two 
inches  in  length,  is  pointed  at  both  ends,  as  the  names 
suggest,  and  bears  a  dorsal  and  an  anal  cuticular  fin,  con- 
tinuous around  the  tail.  When  alive  the  animal  appears 
much  plumper  than  the  spirit  specimens,  and  is  translucent 
with  a  faint  flesh  colour.  The  muscles  are  arranged  in 
sixty-two  segments,  or  myotomes,  readily  visible  externally. 
There  are  three  unpaired  apertures — (a)  the  mouth  is 
median  and  ventral,  and  overarched  by  a  pre-oral  hood,  the 
edges  of  which  are  fringed  with  tentacle-like  cirri ;  (b)  the 
atriopore  opens  to  the  exterior  in  myotome  thirty-six,  and 


FIG.  132. — Lateral  view  of  Amphioxus. 
LANKESTER.) 

Note  the  notochord  running  from  tip  to  tip. 

/.,   Tentacular  cirri;   G.,   reproductive  organs;    a.p.,  atriopore; 
a.,  position  of  anus  ;  40  and  62,  indicate  number  of  myotomes. 

gives  exit  to  the  water  which  enters  by  the  mouth  ;  (c)  the 
anus  is  ventral  and  slightly  to  the  left  side,  behind  the 
atriopore,  but  some  distance  from  the  posterior  end  of  the 
body.  Along  the  back  there  is  &  median  fin,  which  is 
continued  around  the  tail,  and  along  the  ventral  surface  as  ' 
far  as  the  atriopore.  In  front  of  this  region  the  ventral 
surface  is  flattened,  the  flattened  area  being  fringed  on 
either  side  by  a  slight  fin-like  "  metapleural"  fold.  These 
are  continuations  downwards  of  the  walls  of  the  atrial  or 
branchial  chamber,  which, extends  from  behind  the  mouth 
to  the  atriopore,  and  into  which  the  gill  slits  of  the  pharynx 
open  in  the  adult. 


62 


4I2 


CEPHALOCHORDA,   AMPHIOXUS. 


Skin. 

The  epidermis  is  a  single  layer  of  columnar  cells.  Some 
of  them  project  slightly  from  the  surface,  and  are  connected 
at  the  base  with  nerve  fibres.  These  are  sensory  cells,  and 
may  be  analogous  with  the  cells  of  the  lateral  line  in  fishes 
and  tadpoles.  The  epidermis  lies  upon  a  thin  layer  of  clear 


•a.c.f. 


mp. 


FIG.  133. — Transverse  section  through  pharyngeal , region  of 
Amphioxus.     (After  RAY  LANK  ESTER.) 

sp.c.,  Spinal  cord  ;  nch.,  notochord,  beneath  which  the  two  dorsal 
aortae  ;  m.,  myotome  ;  a.c.f.,  atrio-coelomic  funnel;  C.,  caecum; 
G.,  a  genital  sac  with  ova;  nip.,  metapleural  fold;  atr.  ^atrial 
cavity  ;  Ph.,  pharynx,  with  dorsal  and  ventral  grooves,  and  bars 
between  gill  slits. 

cutis.  Beneath  this  there  is  a  layer  of  fine  tubes,  which 
unite  in  a  longitudinal  canal  running  along  each  metapleural 
fold.  These  metapleural  canals  may  be  derived  from  the 


SKELETON— MUSCULAR  AND  NERVOUS  SYSTEMS.  413 

body   cavity,  or,    according   to   another   view,    are   spaces 
secondarily  hollowed  out  in  the  tissues,  and  §Q pseudocalic. 

Skeleton. 

This  is  very  slightly  developed,  for  there  is  not  only  no 
bone,  but  the  supporting  material  is  not  even  definitely 
cartilaginous. 

(a)  The  notochord  runs  from  tip  to  tip.  It  consists  of 
vacuolated  cells,  and  probably  owes  its  supporting  power  to 
their  turgidity  (cf.  plants]. 

(&)  The  pharynx  is  supported  by  a  system  of  chitinoid 
bars,  which  border  the  numerous  gill  slits.  There  is  also  a 
paired  longitudinal  plate  along  the  mid  ventral  groove  of 
the  pharynx. 

(c)  The  mouth  is  embraced  by  two  curved  bars,  each 
segmented  into  about  a  dozen  pieces,  which  bear  filaments 
supporting  the  cirri. 

(d)  The  sheath  which  envelops   the   notochord   and   is 
continued  round  the  nerve  cord,  the  septa  of  connective 
tissue  which  divide  the  muscle  segments,  and  produce  the 
<-shaped  markings  ;  the  250  "  fin  rays  "  which  ^support  the 
dorsal  and  ventral  fins,  may  also  be  included  here. 

Muscular  System. 

The  swimming  movements  are  caused  by  lateral  wriggling 
of  the  body.  This  is  effected  by  the  segmented  lateral 
muscles,  in  which  the  muscle  fibres  run  longitudinally.  On 
the  ventral  surface  between  the  mouth  and  the  atriopore 
there  is  a  transverse  set  of  fibres  which  help  to  drive  out 
the  water  from  the  atrial  cavity.  Other  muscles  occur  in 
the  region  of  the  mouth  and  elsewhere.  Nearly  all  the 
fibres  are  striated. 

Nervous  System. 

The  dorsal  nerve  cord  is  shorter  than  the  notochord,  and 
has  no  anterior  swelling.  It  gives  origin  to  two  sets  of 
nerves,  dorsal  and  ventral.  The  dorsal  nerves  correspond 
to  the  segments,  except  in  the  anterior  region,  where  they 
are  more  numerous,  and  the  first  five  pairs  may  be  regarded 
as  cerebral ;  the  ventral  nerves  are  minute  and  numerous. 
The  two  sets  are  compared  to  the  single-rooted  sensory 


414  CEPHALOCHORDA,   AMPHIOXUS. 

dorsal  nerves,  and  the  many-rooted,  motor,  ventral  nerves 
of  higher  Vertebrates.  But  the  dorsal  nerves  of  Amphioxus 
supply  the  muscles  as  well  as  the  skin,  so  that  they  must  be 
partly  motor.  Furthermore,  there  is  no  connection  between 
the  two  sets,  and  the  dorsal  nerves  have  no  ganglia.  Nor 
are  there  any  sympathetic  ganglia. 

The  anterior  region  of  the  nerve  cord  is  said  to  exhibit  some  histolo- 
gical,  though  no  morphological  distinctiveness.  With  it  the  following 
structures  are  associated  : — 

(a)  Slightly  to  the  left  side  there  is  a  ciliated  pit,  often  called  olfactory. 
The  development  of  this  is  interesting.      The  cavity  of  the  medullary 
tube  opens  at  first  to  the  exterior  by  an  anterior  aperture,  the  neuropore. 
Later,  an  invagination  of  the  ectoderm  takes  place  at  this  point,  and 
carries  the  neuropore  in  with  it.     This  invagination  forms  the  olfactory 
pit ;  it  at  first  opens  into  the  neural  tube  by  the  persistent  neuropore  ; 
later  this  closes,  and  the  pit  becomes  a  mere  blind  sac.     This  invagina- 
tion may  perhaps  correspond  with  the  ciliated  duct  of  the  sub-neural  gland 
of  Tunicates,  and  so  with  part  of  the  hypophysis  of  other  Vertebrates. 

(b)  At  the  end  of  the  nerve  cord  there  is  a  pigment  spot,  sometimes 
called  an  eye  spot.     There  are  no  true  eyes. 

(c)  On  the  roof  of  the  mouth  there  opens  a  small  sac,  the  pre-oral  pit, 
which  may  have  a  tasting  or  smelling  function.     It  arises  in  development 
from  the  left  of  two  pouches  which  grow  out  anteriorly  from  the  gut  of 
the  embryo.     The  right  of  these  pouches  forms  the  head  cavity  of  the 
adult,  so  that  ontogenetically  the  pre-oral  pit  is  the  aborted  head  cavity 
of  the  left  side.     This  is,  however,  only  one  of  many  explanations  of  the 
organ. 

It  is  likely  that  the  most  important  sensory  structures  of  the  adult  are 
the  sensitive  cells  of  the  epidermis. 

We  may  connect  the  feeble  development  of  sense  organs  with  the 
almost  sedentary  habit. 

Alimentary  System. 

The  true  mouth  or  velum  lies  well  within  the  projecting 
pre-oral  hood  with  its  fringe  of  cirri.  In  the  larva  this  hood 
is  absent,  and  the  mouth  is  flush  with  the  surface. 

The  mouth  opens  into  the  pharynx,  which,  like  it,  is 
richly  ciliated.  The  pharynx,  like  that  of  Tunicates,  and 
indeed  of  Fishes  also,  is  modified  for  respiration  (Fig.  133, 
Ph.).  Its  walls  are  perforated  by  numerous  gill  slits  on  each 
side,  and  between  these  lie  supporting  bars  and  arches, 
alternately  split  and  unsplit. 

Along  the  mid-dorsal  and  mid-ventral  lines  there  are 
grooves  respectively  called  hyper-  and  hypobranchial.  The 
latter  is  comparable  to  the  endostyle  of  Ascidians. 


BODY  CAVITY. 


415 


The  pharynx  opens  into  the  intestinal  region  of  the  gut 
which  is  straight  and  simple.     Near  its  commencement  a 

pouch-like  "liver"  or  caecum 
(Fig.  133,  C.)  arises,  and  ex- 
tends forward  on  the  right  side 
of  the  pharynx.  The  anus  is 
some  distance  from  the  end  of 
the  body  (cf.  Fishes) ;  in  the 
larva  it  is  close  to  the  caudal 
fin. 

Body  Cavity. — This  can  only  be 
understood  when  its  development 
is  studied.  From  the  archenteron 
of  the  embryo  a  pouch  grows  out 
on  each  side,  and  becomes  almost 
at  once  segmented  into  a  series  of 
small  sacs.  These  lie  one  behind 
the  other,  and  soon  lose  all  con- 
nection with  the  gut.  Each  ulti- 
mately divides  into  two  portions, — 
an  upper,  the  true  primitive  seg- 
ment, and  a  lower,  corresponding 
to  the  lateral  plate  of  other  Verte- 
brates. The  primitive  segments 
form  the  body  musculature,  and 
retain  their  segmentation.  Their 
cavity,  the  myocoele,  persists  to 
some  extent  in  the  adult,  forming 
the  system  of  lymph  spaces  and 
canals  which  lie  below  the  cutis. 
In  the  region  of  the  lateral  plates 
the  septa  disappear,  and  the  en- 
closed spaces,  bounded  by  somato- 
pleure  and  splanchnopleure,  unite 
to  form  the  "splanchnocoele"  which 
surrounds  the  gut.  Posteriorly,  this 
space  exists  unconstricted  in  the 
adult  ;  anteriorly,  it  is  reduced  to 
small  spaces  and  ccelomic  canals  by 
the  development  of  the  atrial  cham- 
ber. This  pushes  the  somatopleure 
up  before  it  as  it  develops,  and  so 
is  hollowed  out  at  the  expense  of  the  true  ccelome.  The  ccelomic 
spaces  and  canals  contain  coagulable  fluid,  and  are  in  some  regions 
continuous  with  the  blood  vessels.  They  represent  the  lymphatic 
system  of  higher  forms. 


FIG.  134. — Cross  section  of 
Amphioxus  through  the  gill 
slit  region.  (After  BOVERI  and 
HATSCHEK.) 

The  spinal  cord,  nolochord,  pharynx, 
and  atrial  cavity  are  unlettered. 

«,  Sclera  layer ;  />,  fascia  ;  c,  muscle 
plate  ;  d,  cutis  ;  e,  nephridial  canal ; 
f,  traverses  the  sub-chordal  part  of  the 
coelome  on  left  side  ;  g;  glomerulus  of 
kidney  tube  ;  7z,  gonad  ;  ^metapleural 
cavity  ;  j\  transverse  muscle  ;  k,  cavi- 
ties in  junction  of  metapleural  folds  ; 
/,  cavity  in  dorsal  fin  ;  m,  aorta  ;  n, 
branchial  vessel ;  <?,  branchial  artery, 
the  line  traverses  first  the  metapleural 
cavity,  then  the  atrial  cavity,  and  finally 
a  minute  part  of  the  coelome  beneath 
the  pharynx. 


416  CEPHALOCHORDA,   AMPHIOXUS. 

Respiratory  System. 

The  water  which  enters  the  mouth  and  passes  down 
the  pharynx  leaves  this  by  the  numerous  gill  slits.  In 
the  embryo  these  open  directly  to  the  exterior;  in  the 
adult,  into  the  atrial  chamber,  which  opens  by  the  single 
atriopore. 

In  development  two  folds  appear  laterally  on  the  body  wall,  and  form 
the  hollow  metapleural  folds  of  the  adult.  On  their  inner  apposed,  but 
not  united,  surfaces,  two  ridges  appear.  These  grow  towards  one 
another  and  unite,  leaving  only  the  atriopore  open.  Thus  the  floor  of 
the  atrial  chamber  (Fig.  133,  atr. )  is  produced.  The  chamber,  as  first 
formed,  is  a  tube  with  a  very  small  lumen.  Secondarily,  it  becomes 
enlarged,  constricting  the  body  cavity,  as  we  noticed  above,  until  it 
comes  almost  to  surround  the  gut.  At  the  same  time,  the  metapleural 
folds  increase  in  size  until  they  assume  the  adult  appearance 
(Fig.  133,  mp.}.  The  water  currents  are  kept  up  by  the  action  of  cilia, 
and  by  the  movements  of  the  transverse  muscles. 

The  gill  slits  gradually  become  more  numerous  as  the  animal  grows 
older,  and  in  the  adult  there  are  more  than  a  hundred. 

The  original  number  of  gill  slits  is  doubled  by  the  growth  of  a 
secondary  bar  down  the  centre  of  each  slit,  thus  producing  two  gill  slits 
where  there  was  formerly  only  one.  The  primary  bars  are  distinguished 
from  the  secondary  in  being  split,  and  there  are  also  histological 
differences  between  them. 

Circulatory  System. 

The  blood  is  colourless,  with  a  few  amoeboid  cells. 
There  is  no  definite  heart,  but  the  vessels  are  said  to  be 
contractile  in  several  places. 

Vessels  from  the  body  and  from  the  caecum  unite  to  form  a  ventral 
vein,  the  cardiac  aorta,  which  runs  forward  beneath  the  pharynx.  From 
this  vessel  a  series  of  smaller  vessels  arise,  which  pass  up  the  primary 
branchial  rods,  and  are  termed  aortic  arches.  The  most  anterior  of  the 
right  side  is  larger  than  the  rest,  and  sends  branches  to  the  head  region. 
The  aortic  arches  apparently  open  into  two  dorsal  vessels,  the  right  and 
left  dorsal  aortoe,  which  unite  at  the  hinder  end  of  the  pharynx  to  form 
a  single  vessel  running  backward  above  the  intestine.  The  blood 
vessels  which  are  supposed  to  take  blood  from  the  intestine  to  the  liver 
are  termed  portal  veins  ;  those  passing  from  the  liver  to  the  ventral  vein 
are  termed  hepatic.  The  portal  vein  and  the  cardiac  aorta  are  said  to 
be  specially  contractile. 

Although  the  names  given  above  to  the  various  vessels  indicate  the 
views  generally  held  as  to  the  course  of  the  blood,  our  knowledge  of  this 
is  only  hypothetical. 

Excretory  System. — (a)  Quite  recently  Prof.  Boveri  has  discovered 
nephridial  tubes  in  the  adult.  "These  are  found  in  the  region  of  the 


.  REPRODUCTIVE  SYSTEM— DEVELOPMENT.       417 

pharynx,  and  are  short  tubes  which  place  the  sub-chordal  ccelome  in 
communication  with  the  atrial  chamber."  They  open  into  the  coelome 
by  three  or  four  funnel  shaped  openings,  and  around  them  the  vessels  of 
the  gill  slits  form  a  so-called  glomerulus.  They  occur  in  relation  to  the 
gill  slits,  and  open  on  the  secondary  branchial  bars.  Boveri  regards 
them  as  equivalent  to  the  pronephric  tubules  of  other  Vertebrates.  Of 
their  developmental  history  nothing  is  known. 

(b]  Professor  Hatschek  discovered  in  the  anterior  region  of  the  larva 
a  nephriclial  tube  which  is  absent  in  the  full  grown  adult ;  according  to 
Van  Wijhe,  this  is  the  residue  of  the  communication  between  the  left 
anterior  diverticulum  (or  pre-oral  pit)  and  the  gut. 

(<:)  Professor  Ray  Lankester  discovered  a  pair  of  short  pigmented 
funnel  tubes  (Fig.  133,  a.c.f.\  which  lie  in  the  twenty-seventh  segment, 
and  place  the  lymphatic  spaces  of  the  metapleural  folds  in  communica- 
tion with  the  atrial  cavity.  They  may  be  compared  with  the  pores 
which  open  from  the  collar  region  in  Balanoglossus,  and  with  the 
abdominal  pores  of  higher  Vertebrates.  "It  is  doubtful  whether  they 
represent  nephridia." 

Reproductive  System. 

The  sexes  are  separate  and  similar  to  one  another.  The 
organs  are  very  simple,  and  are  without  ducts.  They  form 
twenty-six  pairs  of  horse-shoe-shaped  sacs,  lying  along  the 
inner  wall  of  the  atrial  cavity  in  segments  ten  to  thirty-five 
on  each  side  (Fig.  133,  G.}.  Each  lies  in  a  "genital  chamber" 
formed  in  development  by  constriction  from  the  cavity  of 
the  lower  part  of  the  primitive  segment. 

In  the  mature  female  the  ovaries  are  large  and  con- 
spicuous ;  the  ova  burst  into  the  atrial  cavity,  whence  they 
pass  into  the  pharynx  by  the  gill  slits,  and  out  by  the  mouth, 
or  more  directly  by  the  atriopore. 

The  testes  are  like  the  ovaries ;  the  spermatozoa  burst 
into  the  atrial  cavity,  and  pass  out  by  the  atriopore.  The 
eggs  are  fertilised  in  the  surrounding  water. 

Development. 

The  fertilised  ovum  is  about  -jriir  inch  in  diameter.  The 
segmentation  is  complete  and  almost  equal.  The  first 
cleavage  is  vertical,  and  divides  the  ovum  into  two  equal 
parts ;  the  second  is  also  vertical,  along  a  meridional  plane 
at  right  angles  to  the  first,  and  the  result  is  four  equal  cells. 
The  third  cleavage  is  equatorial,  and  gives  rise  to  four  larger 
cells  (or  macromeres)  below  or  towards  the  vegetative  pole, 
and  to  four  smaller  cells  (or  micromeres)  above  or  towards 

27 


418  CEPHALOCHORDA,   AMPHIOXUS. 

the  animal  pole.  The  blastosphere,  which  is  the  final  result 
of  segmentation,  invaginates  to  form  a  gastrula. 

Along  the  mid-dorsal  line  of  the  gastrula  the  ectoderm 
cells  sink  in  slightly  so  as  to  form  a  groove.  This  is  the 
medullary  groove,  which  here  follows  an  unusual  course  of 
development.  Instead  of  immediately  closing  to  form  a 
canal,  the  groove  sinks  inwards,  and  the  lateral  ectoderm 
grows  over  it  before  closing  takes  place.  Later,  the  groove 
forms  the  medullary  tube,  which  opens  into  the  gut  by  the 
neurenteric  canal ;  to  the  exterior  by  the  anterior  neuropore. 

The  cavity  of  the  gastrula — the  archenteron — becomes 
the  gut  of  the  adult.  From  it  pouches  grow  out  as  was 
described  above. 

The  notochord  arises  along  the  mid-dorsal  line  of  the 
archenteron  ;  its  forward  extension  is  secondary. 

During  the  early  part  of  larval  life  the  ectodermal  cells 


FIG.  135. — Early  stages  in  the  development  of  Amphioxus. 
(After  HATSCHEK.  ) 

i.  Ovum  ;  2.  Four  cell  stage  ;  3.  Eight  cell  stage  ;  4.  External 
appearance  of  blastula  ;  5,  6.  Blastulae  in  section  (note  the  larger 
macromeres) ;  7.  Beginning  of  gastrula  stage  ;  8.  Section  of  com- 
pleted gastrula. 

including  those  forming  the  medullary  canal,  are  ciliated. 
At  this  stage  the  larva  is  much  more  active  than  the 
adult. 

The  later  larvae  are  more  sedentary,  lying  much  on  the 
right  side,  and  they  are  strongly  asymmetrical.  The  mouth 
is  placed  at  the  left  side ;  the  gill  slits  of  one  side  appear 
considerably  before  those  of  the  other ;  the  primitive  seg- 
ments of  one  side  are  not  opposite  those  of  the  other,  and 
so  on.  By  the  process  known  as  the  "  symmetrisation  "  of 
the  larva,  the  apparent  symmetry  of  the  adult  is  produced. 


DE  VEL  OPMENT.  4 1 9 

The  adult  position  of  the  anus  and  of  the  olfactory  pit,  both 
to  the  left  side,  and  the  position  of  the  unpaired  liver  diver- 
ticulum,  show  how  partial  this  process  is. 

Experimental  Embryology. — As  an  illustration  of  what  may  be  called 
experimental  embryology,  and  of  the  developmental  potentiality  of 
the  first  few  segmentation  cells,  reference  may  be  made  to  the  recent 
experiments  of  Prof.  E.  B.  Wilson. 

By  shaking  the  water  in  which  the  the  two-celled  stages  floated,  Mr. 
Wilson  separated  the  two  cells,  and  the  result  was  two  quite  separate 
and  independent  twins  of  half  the  normal  size.  Each  of  the  isolated 
cells  segments  like  a  normal  ovum,  and  gives  origin,  through  blastula 
and  gastrula  stages,  to  a  half-sized  metameric  larva. 

If  the  shaking  has  separated  the  two  first  segmentation  cells  incom- 
pletely, double  -embryos — like  Siamese  twins — result,  and  also  form 
short-lived  (twenty-four  hours)  segmented  larvae. 

Similar  experiments  with  the  four-celled  stages  succeeded,  though 
development  never  continued  long  after  the  first  appearance  of  meta- 
merism. Complete  isolation  of  the  four  cells  resulted  in  four  dwarf 
blastulae,  gastruloe,  and  even  larvae.  Separation  into  two  pairs  of  cells 


- 


FIG.    136. — Three  larval  stages  of  Amphioxus.      (After 
RAY  LANKESTER  and  WILLEY.) 

A,  The  metapleural  folds  still  separate;  ft,  united  posteriorly; 
C,  unked  altogether ;  ap,  atriopore  ;  gc,  gill  slits ;  If,  left  meta- 
pleural fold  ;  rf,  right  metapleural  fold  ;  m,  mouth  ;  cot  ciliated  pit. 

resulted  in  two  half-sized  embryos.  Incomplete  separation  resulted  in 
one  of  three  types — (a)  double  embryos,  (b)  triple  embryos — one  twice 
the  size  of  the  other  two — and  (c)  quadruple  embryos,  each  a  quarter 
size. 

The  eager  observer  proceeded  to  shake  up  the  eight-celled  stages,  but 
in  no  case  did  he  succeed  in  rearing  a  gastrula  from  an  isolated  unit  of 
the  eight-celled  stages.  Flat  plates,  curved  plates,  even  one-eighth  size 
blastulae  were  formed,  but  none  seemed  capable  of  full  development. 

Thus,  a  unit  from  the  four  cell  stage  may  form  an  embryo,  but  a  unit 
from  the  eight  cell  stage  does  not.  For  various  reasons  it  seems  likely 
that  this  is  due  to  qualitative  limitations,  not  merely  to  the  fact  that  the 
units  of  the  eight  cell  stage  are  smaller.  For  although  the  separated 
cells  of  the  eight  cell  stage  have  considerable  vitality,  and  swim  about 


420  CEPHALOCHORDA,    AMPHIOXUS. 

actively,  the  difference  between  macromeres  and  micromeres  has  by  this 
time  been  established  ;  in  fact  the  cells  have  begun  to  be  specialised, 
and  have  no  longer  the  primitive  indifference,  the  absence  of  differentia- 
tion, which  explains  the  developmental  potentiality  of  the  separated  units 
of  the  two-celled  or  four-celled  stages. 

Somewhat  similar  experiments  have  been  made  by  other  investigators 
on  the  developing  ova  of  ascidians,  sea  urchins,  &c.  Specialisation  of 
segmentation  cells  appears  to  occur  at  different  times  in  different  animals, 
but  it  is  illogical  to  infer  the  absence  of  specialisation  from  the  fact  that 
any  of  the  first  four  blastomeres,  let  us  say,  can  produce  an  entire  embryo. 
For  specialised  cells  may  retain  a  power  of  regeneration. 


CHAPTER  XX. 

STRUCTURE  AND  DEVELOPMENT  OF  VERTEBRATA. 

SINCE  the  time  of  Aristotle — over  two  thousand  years  ago 
—  the  distinction  between  backboned  and  backboneless 
animals  must  have  been  more  or  less  evident  to  all  who, 
with  any  precision,  thought  of  the  forms  of  animal  life. 

Yet  it  was  not  till  about  a  century  ago  that  the  line  of 
separation  was  drawn  with  adequate  firmness.  This  Lamarck 
did  in  1797. 

But  the  doctrine  of  descent — the  idea  of  organic  evolu- 
tion— with  which  Darwin  impressed  the  thoughtful  in  1859, 
suggested  inquiry  into  the  apparently  abrupt  apartness  of 
the  group  of  Vertebrates. 

The  inquiry  bore  fruit  in  1866,  when  the  Russian  naturalist, 
Kowalevsky,  worked  out  the  development  of  the  Vertebrate 
characteristics  of  Amphioxus,  correlated  this  with  the 
development  of  Ascidians,  and  discovered  the  pharyngeal 
gill  slits  of  Balanoglossus. 

From  what  has  been  said  in  regard  to  these  three  types, 
it  will  be  plain  that  the  apparent  apartness  of  the  Vertebrata 
was  thus  annulled. 

GENERAL  CHARACTERS.  —  Vertebrates  are  ccelomate 
Metazoa,  with  a  segmental  arrangement  of^  parts.  The 
central  nervous  system  lies  in  the  dorsal  median  line,  and  is 
tubular  in  its  origin.  A  skeletal  rod  or  notochord  is  formed 
as  an  outgrowth  along  the  dorsal  median  line  of  the  primitive 
gut,  but  though  this  is  always  present  in  the  embryo  at  least, 
it  tends  to  be  replaced  by  a  mesodermic  axial  skeleton — the 
backbone.  Pharyngeal  gill  slits,  which  may  or  may  not  per- 
sist in  adult  life,  are  always  developed,  but  gill-lamellce  do  not 
occur  above  Amphibians.  The  heart  is  ventral  The  eye 
begins  to  develop  as  an  outgrowth  from  the  brain. 


422 


STRUCTURE   OF    VERTEBRATA. 


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GENERAL    CLASSIFICATION. 


423 


General  Classification. 


f  Carinatag  (flying). 
BIRDS  K  Ratitae  (running). 

MAMMALS. 

I^Saururas  (extinct). 

3.   Eutheria,  Placentalia,  Mono- 

/f^Crocodilia   (croco- 

delphia  :  the  higher  placen- 

diles,  &c.). 

tal  mammals. 

Ophidia  (snakes). 

Lacertilia  (lizards, 

'2.  Metatheria,  Marsupialia,  Di- 

&c.). 

delphia  :    Kangaroos,    &c.  ; 

REPTILES 

Rhynchocephalia  — 

young    born    precociously, 

Sphenodon. 

usually  nurtured  in  pouches. 

w 

Chelonia  (  tortoises  , 

.^ 

&c.). 

? 

Extinct  Reptiles  — 

J>-  i.   Prototheria,  Monotremata, 

42 

5 

(many  classes). 

Ornithodelphia  :  oviparous, 

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Sauropsida.                                        Mammalia. 

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Amniota,  embryos  with  amnion  and  allantois. 

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O 

FISHES.  —  Dipnoi  (double  breath- 
ing mud  fishes).    - 

?                i  ? 

^-  AMPHIBIANS. 

U 

Teleostei     (modern 

Anura  (tailless  frogs,  &c.  ). 

bony  fishes). 

Urodela(  tailed  newts,  &c.). 

, 

Ganoidei    (sturgeon, 

Gymnophiona    (worm-like 

&c.). 

CcBcilia,  &c.). 

Elasmobranchii  (incl. 

Extinct  Stegocephala  (La- 

Holocephali),  skate, 

byrinthodon,  &c.). 

shark,  &c. 

Ichthyopsida  (fishes  and  amphibians). 

CYCLOSTOMATA  (Round  Mouths),  without  true  jaws. 
Myxine,  hag  fish.          Petromyzon,  lamprey. 


CEPHALOCHORDA. — Amphiuxus,  or 
Lancelet. 


UROCHORDA  fSalpa  type. 

Ascidian     type     (sea 

-]      squirts). 
TUNICATA.    I  Appendiciilaria    (lar- 

^     val  type  persistent). 


Surviving  offshoots  of  ancestral  Vertebrates. 
HEMICHORDA,  or  ENTEROPNEUSTA  (offshoots  of  incipient  Vertebrates?) 

Balanoglossus ,  &c. ;  probably  Cephalodiscus  ;  possibly  Rhabdopleura. 
Nemertean  affinities  (?)       Chaetopod  affinities  (?)       Arthropod  affinities  (?) 


424  STRUCTURE   OF    VERTEBRATA. 

Ancestry  of  Vertebrates. 

It  is  not  at  present  possible  to  trace  the  path  along  which  Vertebrates 
have  evolved,  though  our  faith  in  the  doctrine  of  evolution — as  a  modal 
theory  of  origins — leads  us  to  believe  that  Vertebrates  arose  from  forms 
which  were  not  Vertebrates. 

But,  even  when  we  recognise  that  Amphioxus  is  a  Vertebrate  very 
simple  in  its  general  features,  and  that  the  Tunicata,  especially  in  their 
youth,  are  Vertebrates,  we  have  to  remember  that  degeneration  seems 
to  have  been  by  no  means  uncommon  in  the  history  of  animals,  and 
that  the  types  above  mentioned  may  be  less  primitive  than  they  seem. 

The  Enteropneusta  carry  us  a  little  further  back.  For,  while  many 
of  their  alleged  Vertebrate  characteristics  are  debateable,  one  cannot 
gainsay,  for  instance,  the  possession  of  pharyngeal  gill  slits.  But  the 
affinities  of  the  Enteropneusta  with  Invertebrate  types  are  entirely 
obscure. 

We  have,  in  fact,  to  acknowledge  frankly  that  the  pedigree  of  Verte- 
brates remains  unknown.  At  the  same  time,  it  is  useful  to  enquire  into 
certain  convergences  towards  Vertebrate  structure  which  are  exhibited 
among  various  sets  of  Invertebrates. 

In  regard  to  these,  speculation  has  been  abundant.  Alleged  affinities 
have  been  discovered  among  Annelids,  Nemerteans,  Arachnids,  Crus- 
taceans, &c.  Indeed,  there  is  almost  no  great  class  of  Invertebrate 
Metazoa  whose  characters  have  not  been  ingeniously  interpreted,  or 
wrested,  so  as  to  reveal  affinities  with  Vertebrates.  It  will  be  enough 
to  select  two  illustrations. 

Annelid  Affinities. — Dohrn,  Semper,  Beard,  and  others,  maintain  that 
Annelids  have  affinities  with  Vertebrates. 

1 i )  Both  Annelids  and  Vertebrates  are  segmented  animals. 

(2)  The  segmental  nephridia  of  Annelids  correspond  to  the  primi- 

tive kidney  tubes  of  a  Vertebrate  embryo. 

(3)  The  ventral   nerve   cord  of  Annelids   may  be  compared  (in 

altered  position)  to  the  dorsal  nerve  cord  of  Vertebrates. 
Both  cords  are  bilateral,  and  it  is  likely  enough  that  the 
tubular  character  of  the  spinal  cord  and  brain  is  the  neces- 
sary result  of  its  mode  of  development,  and  without  much 
morphological  importance. 

(4)  Segmentally  arranged  ganglia  about  the  appendages  of  some 

Chsetopod  worms  may  correspond  to  the  branchial  and  lateral 
sense  organs  of  Ichthyopsida,  and  the  ganglia  associated  with 
some  of  the  nerves  from  the  brain. 

(5)  The   formation    of  the   oral   part  of  the   pituitary  body  (see 

page  436)  is  suggestive  of  the  way  in  which  the  mouth  of 
Annelids  is  sometimes  formed.  Perhaps  the  pituitary  body 
represents  an  old  lost  mouth  and  its  ancient  innervation. 

To  minor  points,  such  as  the  red  blood,  well  developed  body  cavity, 
and  slight  internal  skeleton  of  some  Chaetopods,  little  importance  can  be 
attached. 

The  absence  of  anything  like  gill  slits  in  Annelids  remains  as  a  diffi- 


THE  SKIN.  425 

culty,  even  if  we  grant  that  no  emphasis  is  to  be  laid  on  the  tubular 
nerve  cord  of  Vertebrates,  and  admit  the  possibility  of  an  inversion 
bringing  the  ventral  nerve  cord  to  the  dorsal  surface. 

Neinertean  Affinities. — Hubrecht  and  others  have  emphasised  the 
affinities  between  Nemerteans  and  Vertebrates. 

In  Nemerteans  :  — 

(1)  The   lateral   nerve    cords    sometimes    approach   one   another 

ventrally,  and  in  rare  cases  dorsally.  An  approximation 
dorsalwards,  and  union  on  that  surface,  would  result  in  a 
double  dorsal  nerve  cord. 

(2)  The  firm  dorsal  sheath  of  the  proboscis  may  correspond  to  a 

notochord. 

(3)  The   proboscis   itself  may  correspond   to   the  hypophysis   or 

pituitary  process  characteristic  of  Vertebrate  brains. 

(4)  Two  ciliated  slits  on  the  head  may  correspond  to  a  pair  of  gill 

clefts. 

(5)  There  is  no  segmentation,  but  the  branches  given  oft*  from 

the  nerve  cords  are  sometimes  serially  arranged. 

It  must  be  noted,  that  those  who  support  these  theories  do  not  assert 
that  any  Nemertean  or  Annelid  is  in  the  direct  line  of  Vertebrate  ascent. 
They  simply  emphasise  the  demonstrable  affinities.  When  these  are 
thoroughly  worked  out,  it  will  be  possible  to  say  what  Invertebrate  types 
are  most  nearly  related  to  Vertebrates. 


STRUCTURE  AND  DEVELOPMENT  OF  VERTEBRATES. 

„  Having  separately  discussed  the  Hemichorda,  Urochorda, 
and  Cephalochorda,  we  propose  in  this  chapter  to  discuss 
the  general  structure  of  Craniata  and  the  development  of 
some  of  the  important  organs. 

The  Skin. 

This  forms  a  continuous  covering  over  the  surface  of  the 
body,  serves  as  a  protection  to  the  underlying  tissues,  in 
some  instances  retains  its  primitive  respiratory  significance, 
and  is  frequently  concerned  in  the  excretion  of  waste  and 
the  regulation  of  the  body  temperature.  As  one  or  other  of 
its  many  functions  predominates,  there  are  corresponding 
structural  modifications.  One  function  which  we  find 
oftenest  emphasised  at  the  expense  of  the  others  is  that  of 
protection,  and  yet  the  fossil  Glyptodon,  the  sluggish 
Chelonia,  the  decadent  Ganoids,  seem  to  indicate  that  this, 
in  itself  or  in  its  correlated  variations,  is  not  conducive  to 
the  continuance  of  the  species.  Indeed,  the  great  develop- 


426 


STRUCTURE   OF    VERTEBRATA. 


ment  of  exoskeleton  may  perhaps  be  regarded  as  the 
outcrop  of  a  constitution  which  tends  to  extreme  and 
unprofitable  passivity. 

The  skin  includes : — 

(a)  The  epidermis,   usually  in  several  layers,  the  outer   "horny" 

stratum  corneum,  the  inner  u  mucous  "  stratum  malpighii,  or 
mucosum  ;  both  derived  from  the  ectoderm  or  epiblast  of  the 
embryo. 

(b]  The  dermis,   cutis,    corium,    or   under-skin,    derived  from  the 

mesoderm  or  mesoblast  of  the  embryo. 

From  the  epidermis  are  derived  feathers,  hairs,  and  some  kinds  of 
scales.  The  dermis,  as  is  natural  when  we  consider  its  origin  from 
the  mesoblast  (mesenchyme)  or  vascular 
layer,  assists  in  nourishing  these  epider- 
mic structures.  From  the  dermis  are 
derived  the  bony  shields  of  armadillos 
and  a  few  related  mammals,  the  bony 
scutes  of  crocodiles  and  some  other  rep- 
tiles, and  the  scales  of  most  bony  (Teleo- 
stean)  fishes.  This  again  is  readily  ex- 
plained by  the  fact  that  the  mesenchyme 
is  also  the  skeletal  layer  of  the  embryo. 
The  ordinary  teeth  of  Vertebrates,  as 
well  as  the  superficial  or  skin  teeth  of 
gristly  fishes,  are  largely  formed  from 
the  dermis,  but  are  usually  covered  by  a 
thin  coating  of  ectodermic  enamel.  It 
should  be  noted,  however,  that  Klaatsch 
has  recently  maintained  the  ectodermic 
origin  of  the  skeleton  forming  cells 

(scleroblasts)  which  form  the  scales  of    section  through  an  Elasmo- 
Elasmobranchs    and    Teleosteans,    and     branch    Embryo     (diagram- 
that  there  are  hints  in  higher  forms  that     matic).      (After  ZIEGLER.  ) 
the  ectoderm  has  more  to  do  with  the 
skeleton  than  is  usually  allowed.     There 
is,  indeed,  a  growing  tendency  among 
morphologists  to  strip  the  mesoderm  of  its 
importance.     It  may  be  noted  also  that 
Klaatsch  ventures  to  suggest  that  the  be- 


Ec.,  Ectoderm ;  S.c.,  spinal 
cord  ;  N.,  notochord  ;  ao.,  aorta  ; 
s.d.,  segmental  duct ;  R.,  repro- 
ductive cells  ;  s.c.,  secondary 
coelome ',  p.c.,  primary  coelome 
filled  up  with  connective  tissue  ; 

~~  s.i.v.,  sub-intestinal  vein  ;  g-.,  gut ; 

ginning  of  skeleton  in  the  ectoderm  may  c.v.,  cardinal  vein ;  s.d.,  segmental 
have  something  to  do  with  excretion.  duct ;  mt.t  myotome. 

Muscular  System. — In  all  Vertebrates 

the  muscles  of  the  trunk  arise  from  the  primitive  segments,  or  muscle 
plates,  found  in  the  embryo  at  the  sides  of  the  nerve  cord.  In  Amphi- 
oxus  and  Fishes  the  primitive  segmented  condition  of  the  muscles  is 
retained,  as  is  seen  in  the  myotomes  visible  externally  in  the  lancelet. 
Above  Fishes  little  trace  of  the  segmented  condition  persists  in  the 
adult,  except  in  the  tail  region.  The  muscles  of  the  head  arise  from 
the  primitive  segments  of  that  region. 


FIG.    137.  —  Transverse 


SKELETAL   SYSTEM— SKULL.  427 

The  muscles  of  the  limbs  arise  in  Elasmobranchs  as  buds  from  the 
primitive  segments ;  buds  from  several  contiguous  segments  grow  into 
each  fin.  In  most  other  Vertebrates  the  formation  of  the  limb  muscles 
is  more  complicated  ;  they  seem  in  some  cases  to  arise  independently  of 
the  primitive  segments. 

Most  of  the  visceral  muscles  consist  of  unstriped  fibres,  but  those  of 
the  trunk,  head,  and  limbs,  as  well  as  of  the  heart,  show  the  usual  striped 
structure. 

Skeletal  System. 

Apart  from  the  exoskeleton  of  skin-teeth,  scutes,  shields, 
&c.,  the  skeleton  consists  of  the  following  parts  : — 

{The  skull  and  its  associated  "  arches." 
The  backbone  and  associated  ribs. 
(The  notochord  is  transitory  except  in 
the  simplest  Vertebrates). 

(b]  Appendicular/Fore  limbs,  and  pectoral  girdle. 
Skeleton         \Hind  limbs,  and  pelvic  girdle. 

Skull. 

The  notochord  grows  forward  anteriorly  as  far  as  that 
region  of  the  brain  known  as  the  optic  thalami.  Around 
notochord  and  brain  the  mesenchyme  forms  a  continuous 
sheath  which  is  the  foundation  of  the  skull. 

As  in  the  case  of  the  notochordal  sheath  of  the  trunk 
region,  so  also  here  cartilage  is  formed  in  the  primitive 
membranous  cranium.  The  first  cartilages  to  appear  are 
the  two  parachordals,  which  lie  on  the  lower  surface  of  the 
head  at  the  sides  of  the  notochord,  and  the  two  trabeculae 
lying  in  front.  The  parachordals  grow  round  and  above 
the  notochord,  producing  the  basilar  plate,  while  the 
trabeculee  unite  in  front  to  form  the  ethmoid  plate. 
The  continuance  of  the  process  of  cartilage  formation, 
together  with  the  addition  of  cartilaginous  nasal  capsules  in 
front  and  auditory  capsules  behind,  completes  the  forma- 
tion of  the  primitive  cartilaginous  brain  box  or  chondro- 
cranium  of  the  lower  Vertebrates. 

Also  connected  with  the  head  region,  and  of  great  import- 
ance are  the  visceral  or  gill  arches  which  loop  around  the 
pharynx  on  either  side,  and  separate  the  primitive  gill  clefts. 
At  the  time  when  cartilage  begins  to  be  formed  in  the  mem- 


428  STRUCTURE   OF    VERTEBRATA. 

branous  cranium,  the  arches  also  become  chondrified,  and 
at  the  same  time  divided  into  successive  segments. 

Of  these  arches,  there  are  never  more  than  eight.  The 
most  anterior  is  the  mandibular  arch  which  bounds  the 
mouth,  the  second  the  hyoid  •  these  two  are  of  great 
importance  in  the  development  of  the  skull.  The  others, 
in  Fishes  and  at  least  young  Amphibians,  bound  open  gill 
slits  and  support  the  pharynx  ;  above  Amphibians,  they 
are  less  completely  developed. 

In  the  Elasmobranch  fishes,  the  mandibular  and  hyoid  arches  do  not 
form  any  direct  part  of  the  cartilaginous  brain  case,  but  in  the  Teleo- 
steans  and  thence  onwards,  the  cartilages,  or  bones,  arising  in  connection 
with  the  mandibular  and  upper  part  of  the  hyoid  arches,  contribute 
directly  to  the  formation  of  the  skull.  The  hyoid  proper,  or  lower 
part  of  the  hyoid  arch,  forms  the  skeleton  supporting  the  tongue. 
Cartilages  arising  in  the  lower  part  of  the  third  visceral  arch  assist  in 
the  formation  of  the  hyoid  bones  of  the  higher  Vertebrates,  and  parts  of 
two  other  arches  appear  to  help  in  forming  the  laryngeal  skeleton  of 
Mammals. 

The  mandibular  arch  in  Elasmobranchs  and  frogs  divides  into  a  lower 
portion — Meckel's  cartilage — which  forms  the  lower  jaw  or  its  basis, 
while  from  the  upper  portion  a  bud  grows  forward — the  palato-pterygo- 
quadrate  cartilage,  which  forms  the  upper  jaw  in  shark  and  skate,  and 
has  a  closer  union  with  the  skull  in  the  frog.  In  higher  Vertebrates,  the 
lower  portion  of  the  mandibular  always  forms  the  basis  of  the  lower 
jaw,  a  quadrate  element  is  segmented  off  from  the  upper  part, 
but  the  palato-pterygoid  part  seems  to  arise  more  independently.  The 
hyoid  arch  also  divides  into  a  lower  portion,  the  hyoid  proper,  and  an 
upper  portion,  the  hyo-mandibular,  which  may  connect  the  jaws  with 
the  skull,  or  from  Amphibians  onwards  may  be  more  remarkably  dis- 
placed and  modified  as  a  columella  or  stapes  connected  with  the  ear. 
We  adhere  to  the  old  interpretation,  according  to  which  the  mandibular 
and  hyoid  form  two  arches  ;  even  if  Dohrn's  theory  that  they  are 
equivalent  to  four  be  accepted,  the  general  fact  remains  that  certain 
arches  aid  in  the  development  of  the  skull. 

Returning  now  to  the  brain  box  itself,  we  must  notice 
another  complication, — the  development  of  "  membrane  " 
bones.  If  we  examine  the  skull  of  the  skate,  we  find  that 
the  brain  lies  within  a  cartilaginous  capsule,  but  this  is  not 
entirely  closed,  spaces  (the  fontanelles)  being  left  in  the 
roof,  which  during  life  are  covered  only  by  the  tough  skin 
with  its  numerous  "  dermal  denticles."  In  the  sturgeon, 
again,  the  small  skin  teeth  are  replaced  by  stout  bony  plates 
covering  over  the  cartilaginous  capsule.  From  such  super- 


THEORY  OF   THE  SKULL.  429 

ficial  bony  plates  it  is  supposed  that  the  "  membrane " 
bones  or  ossifications  in  membrane,  which  form  so  im- 
portant an  element  in  the  skull  of  the  higher  Vertebrate, 
have  originated. 

In  some  bony  fishes,  notably  the  salmon,  we  find  the  brain  enclosed 
in  a  double  capsule.  Inside  there  is  a  cartilaginous  brain  case  in  which 
what  are  called  centres  of  ossification  have  appeared,  and  upon  this  a 
layer  of  membrane  bones  is  placed,  which  can  be  readily  removed  without 
injury  to  the  cartilage  beneath.  In  general,  however,  we  must  recognise 
that,  with  the  appearance  of  membrane  bones,  two  changes  tend  to 
occur, — first,  the  cartilaginous  cranium  tends  to  be  reduced  and  to 
exhibit  considerable  openings  ;  second,  in  the  remaining  cartilage 
centres  of  ossification  appear,  and  we  thus  have  "cartilage"  bones 
formed.  Further,  in  spite  of  the  developmental  differences,  the  mem- 
brane and  cartilage  bones  become  closely  united  to  one  another,  or 
even  fused,  and  there  is  thus  formed  "a  firm,  closed,  bony  receptacle 
of  mixed  origin,"  as  exemplified  by  the  skull  of  any  of  the  higher 
Vertebrates. 

We  may  thus  say  that  in  the  evolution  of  the  skull  we 
have  first  a  cartilaginous  capsule,  that  this  becomes  invested 
to  a  greater  or  less  extent  by  dermal  ossifications,  and  that 
finally  the  dermal  bones  lose  their  superficial  position,  and, 
fusing  with  the  ossified  remainder  of  the  cartilaginous 
cranium,  form  a  complete  bony  capsule.  In  Cyclostomes 
and  Elasmobranchs  the  brain  box  is  wholly  cartilaginous  ; 
above  Elasmobranchs,  the  cartilage  is  more  or  less  thoroughly- 
replaced  or  covered  by  bones.  In  the  individual  develop- 
ment there  is  a  parallel  progress. 

Although  one  is  safe  in  saying  that  skeletal  structures  in 
Vertebrates  are  mostly  mesodermic  in  origin,  it  should  be 
noted  (i)  that  the  notochord  is  endodermic,  and  (2)  that  in 
the  head  certain  ectodermic  proliferations  may  give  rise  to 
skeletal  rudiments  of  a  connective  tissue  nature  which  sub- 
sequently become  differentiated  into  cartilage  (Goronowitsch, 
Platt).  But  there  is  still  doubt  as  to  this  last  point. 

Theory  of  the  Skull. 

Near  the  beginning  of  this  century,  Oken  and  Goethe  independently 
propounded  what  is  known  as  the  vertebral  theory  of  the  skull.  Regard- 
ing the  skull  as  an  anterior  portion  of  the  vertebral  column,  composed 
of  three  or  four  vertebrae,  they  compared  the  bones  of  the  different 
regions  to  the  parts  of  a  vertebra.  Thus  in  the  hindmost  region  of  the 
skull,  the  basi-occipital,  the  two  ex-occipitals,  and  the  supra-occipital 


430  STRUCTURE    OF    VERTEBRATA. 

were  held  to  correspond  to  the  centrum,  the  neural  arches,  and  the 
neural  spine  of  a  vertebral  body. 

This  undoubtedly  suggestive  theory,  modified  in  various  details,  per- 
sisted for  a  long  period,  but  ultimately  gave  way  before  the  advances  in 
comparative  anatomy  and  embryology.  Huxley  gave  it  its  death  blow, 
and  Gegenbaur  replaced  it  by  what  may  be  called  the  segmental  theory 
of  the  skull. 

To  realise  this  theory,  we  must  go  back  in  development  to  the  period 
before  the  mesoblast  has  ensheathed  the  notochord.  At  this  time  the 
segmentation  of  the  body  is  expressed,  not  in  the  skeleton  (notochord), 
but  in  the  primitive  segments.  These  segments,  though  less  obvious 
than  in  the  trunk,  are  represented  in  the  head  region.  Formerly  nine 
were  enumerated,  but  it  appears  that  in  Elasmobranchs  they  are  more 
numerous.  Subsequently,  brain  and  spinal  cord  become  alike  enveloped 
in  the  mesoblastic  sheath,  which  gives  rise  to  the  skeleton  of  both  head 
and  trunk. 

The  great  development  of  the  muscle  segments  of  the  trunk  region 
induces  a  secondary  segmentation  of  the  mesoblastic  skeleton  (vertebral 
column),  while  the  slight  development  of  the  muscles  of  the  head  region 
exercises  no  such  influence  upon  its  skeleton,  this  is  therefore  always 
quite  devoid  of  segmentation.  The  segmentation  of  the  head,  in  contra- 
distinction to  the  skull,  is  expressed,  although  indistinctly,  by  the 
muscle  segments  and  by  the  nerves  supplying  these,  perhaps  also  by 
the  lateral  sense  organs,  the  ganglia,  and  the  arches.  While  it  is  quite 
certain  that  it  is  the  head  that  is  segmented  and  not  the  skull,  the 
details  of  the  segmentation  are  still  much  debated. 


[TABLE. 


COMPOSITION  OF    THE    SKULL. 


431 


ELEMENTS. 

ORIGIN. 

RESULTS. 

I.   Parachordals 

Their      precise 

Occipital  region,  with  four  bones 

and       trabeculae, 

relations,  e.g.,  to 

—  basi-occipital,    two    ex-occipitals, 

aided      in     some 

the  notochord  are 

and  a  supra-occipital  (in  part).    The 

cases  by  the  end 

unknown. 

basi-occipital    is  .  distinct     only    in 

of  the  notochord. 

Reptiles,  Birds,  and  Mammals. 

Sphenoidal  and  ethmoidal  region, 

withbasi-sphenoid  and  pre-sphenoid, 

paired  alisphenoids  and  orbitosphen- 
oids,   the   inter-orbital  septum,    the 

lateral   or   ectoethmoids,   the   inter- 

nasal  septum. 

II.  Sense-capsules. 

From  cartilage 

(a]  Nasal. 

surrounding    the 

(a)  Unite  with  ethmoidal  region. 

\t)  Auditory. 

ectodermic      pits 

(b}  May  give  origin  to  five  bones  — 

which    form    the 

pro-,  sphen-,  pter-,  epi-,  and  opisth- 

foundation         of 

otics,  or  to  the  single  periotic  of  Mam- 

nose and  ear. 

mals. 

III.  Arches. 

(a)  Mandibular. 

These    arches, 
like   those  which 

(a)  Upper    part  —  palato-pterygo- 
quadrate  cartilage  of  Elasmobranchs, 

follow  them,   are 

palatine,    pterygoid,    and    quadrate 

supports    of    the 

bones  in  the  higher  Vertebrates,  but 

pharynx,       lying 

in  Mammals  the  quadrate  is  believed 

between  primitive 

by  many  to  become  the  incus  of  the 

or  persistent   gill 

inner  ear. 

slits.          Perhaps 

Lower  part  =  Meckel's  cartilage  — 

they  may  be  com- 

the basis   of  the   lower  jaw   in   all 

pared  to  ribs. 

animals  ;   the  part  next  the  quadrate 

becomes  the  articular  bone,  which  in 

Mammals   is   believed   by  many  to 

become  the  malleus  of  the  inner  ear. 

(b)  Hyoid  arch. 

(b}  Upper  part  or  hyo-mandibular— 

the    "  suspensorium  "    cartilage     of 

Elasmobranchs,  the  hyo-mandibular 

and  symplectic  of  Teleosteans,  the 

columella  auris  of  Amphibians,  Rep- 

tiles,   and  Birds,  the  stapes  of  the 

Mammal's  ear. 

Lower    part  =  the    hyoid    proper 

(cartilage  or  bone). 

IV.   Investing 

Originally      of 

membrane  bones. 

the  nature  of  ex- 

(a)  From       the 

ternal            bony 

(a)  Parietals,  frontals,  nasals,  &c. 

roof  of  the  skull. 

plates,           tooth 

(6)  On  the  floor 

structures,       and 

(b}  Vomer,  parasphenoid,  &c. 

of  the  skull,  i.e., 

the  like. 

from   the   roof  of 

the  mouth. 

(c)    About      the 

(c)  Lachrymal,  squamosal,  orbitals, 

sides  of  the  skull. 

&c. 

(d)    About     the 

(d)  Premaxilla,  maxilla,  jugal,  and 

upper  jaw. 

quadrato-jugal  (in  part). 

(e)    About      the 

(e)  Dentary,      splenial,      angular, 

lower  jaw. 

supra-angular,  coronoid. 

432  STRUCTURE   OF    VERTEBRATA. 

The  Vertebral  Column. 

A  dorsal  skeletal  axis  is  characteristic  of  Vertebrata,  and 
its  usefulness  is  evident.  It  gives  coherent  strength  to  the 
body ;  it  is  usually  associated  very  closely  with  a  skull,  with 
limb  girdles,  and  with  ribs ;  it  affords  stable  insertion  to 
muscles ;  its  dorsal  parts  usually  form  a  protective  arch 
around  the  spinal  cord. 

To  understand  this  skeletal  axis  we  must  distinguish 
clearly  between  the  notochord  and  the  backbone. 

The  notochord  is  the  first  skeletal  structure  to  appear  in 
the  embryo.  It  arises  as  an  axial  differentiation  of  endo- 
derm  along  the  dorsal  wall  of  the  embryonic  gut  or 
archenteron.  The  backbone,  which  in  most  Vertebrates 
replaces  the  notochord,  has  a  mesoblastic  origin;  it  develops 
as  the  substitute  of  the  notochord,  but  not  from  it. 

In  Balanoglossus,  what  is  sometimes  dignified  with  the  name  of 
notochord,  is  restricted  to  the  most  anterior  part  of  the  body ;  in  the 
Tunicata  the  notochord  is  confined  to  the  tail,  in  Amphioxus  it  runs 
from  tip  to  tip  of  the  body,  in  Cyclostomata  and  Dipnoi  it  persists  as  an 
unsegmented  gristly  rod,  in  other  Vertebrates  it  is  more  or  less  com- 
pletely replaced  by  its  better  substitute — the  backbone. 

In  Cyclostomata  the  notochord  forms  and  is  ensheathed  by  a  cuticula 
chordce  (or  membrana  limitans  interna) ;  outside  this  there  is  a  meso- 
blastic or  skeletogenous  sheath  ;  and  outside  this  again  lies  a  cuticula 
sceleti  (or  membrana  limitans  externd}.  It  is  likely  that  this  represents 
a  primitive  condition.  What  happens  in  most  Vertebrates  is  that  the 
skeletogenous  or  mesoblastic  sheath  forms  the  backbone,  and  more  or 
less  completely  obliterates  the  notochord.  The  formation  of  cartilage 
takes  place  at  regular  intervals  in  the  notochordal  sheath,  and  the 
vertebral  bodies  thus  formed  alternate  regularly  with  the  primitive 
muscle  segments.  This  arrangement  is  necessary  for  the  proper 
attachment  of  the  muscles  to  the  future  vertebrce,  and  makes  it  pro- 
bable, as  we  noticed  above,  that  the  segmentation  of  the  backbone  is 
secondary,  and  was  only  acquired,  as  a  mechanical  necessity,  when  the 
notochordal  sheath  became  chondrified,  and  so  rigid.  Thus  we  reach 
the  conclusion  that  the  primitive  segmentation  of  the  Vertebrates,  alike 
in  head  and  trunk,  finds  its  expression  in  the  arrangements  of  the 
primitive  segments  and  the  nerves  supplying  these,,  and  not  in  the 
skeleton. 

In  the  higher  Vertebrates,  soon  after  the  formation  of  the  bodies  of 
the  vertebrae,  the  rudiments  of  the  neural  arches  appear  in  the  mem- 
brane surrounding  the  spinal  cord.  Finally,  centres  of  ossification  may 
occur,  and  so  produce  the  segmented  backbone. 

In  Amphioxus,  in  Myxine..  and  in  young  lampreys  (known  as  Ammo- 
ccetes\  the  notochord  persists,  unsegmented  and  with  a  simple  sheath. 
In  the  adult  lamprey,  there  are  rudimentary  arches  of  cartilage  forming  a 


APPENDICULAR  SKELETON.  433 

trough  in  which  the  spinal  cord  lies.  In  the  cartilaginous  Ganoid  fishes, 
in  the  Chimczra  type,  and  in  the  Dipnoi,  arches  appear  both  above  and 
below,  but  yet  there  are  no  vertebral  bodies.  These  begin  in  the 
Elasmobranchs,  in  which  the  notochord  is  constricted  by  its  encroaching 
sheath.  In  the  bony  Ganoids  the  vertebrae  are  ossified,  and  so  they  are 
in  all  the  higher  Vertebrates.  Moreover,  the  notochord  is  more  and 
more  completely  obliterated  as  the  backbone  grows. 

It  will  be  remembered  (see  p.  34)  that  according  to 
Kleinenberg  the  notochord  supplies  the  necessary  growth- 
stimulus  for  the  rise  of  its  substitute,  the  backbone. 

A  vertebra  generally  consists  of  several  more  or  less 
independent  parts :  the  substantial  centrum,  the  neural 
arches  which  form  a  tube  for  the  spinal  cord,  and  are 
crowned  by  a  neural  spine,  the  transverse  processes  which 
project  laterally,  and  are,  perhaps,  homologous  with  the 
inferior  haemal  processes  in  the  posterior  region  of  Fishes 
and  some  Amphibians. 

The  ribs  which  support  the  body  wall  and  usually  arti- 
culate with  the  transverse  processes,  or  with  the  transverse 
processes  and  centra,  perhaps  bear  the  same  relation  to  the 
vertebrae  that  the  visceral  arches  do  to  the  skull. 

In  Amphibians  for  the  first  time  a  breast  bone  or  sternum 
is  developed.  It  arises  from  two  cartilaginous  rods  in  a 
tendinous  region  on  the  ventral  wall  of  the  thorax,  and 
seems  to  be  different  from  that  of  higher  animals.  For  the 
sternum  which  is  present  in  some  Reptiles,  and  in  all  Birds 
and  Mammals,  arises  from  a  cartilaginous  tract  uniting  the 
ventral  ends  of  a  number  of  ribs. 

Appendicular  Skeleton. 

No  secure  conclusion  has  yet  been  reached  as  to  the 
origin  of  the  paired  limbs.  According  to  Gegenbaur,  the 
pectoral  and  pelvic  girdles  are  homologous  with  branchial 
arches,  while  the  primitive  limbs  are  made  up  of  modified 
fin  rays,  originally  like  those  of  the  unpaired  fins. 
According  to  Dohrn,  the  limbs  are  residues  of  a  longi- 
tudinal series  of  segmentally  arranged  outgrowths,  perhaps 
comparable  to  the  parapodia  of  an  Annelid.  According  to 
Wiedersheim,  the  girdle  portion  is  primarily  due  to  the 
centripetal  growth  of  the  fin  skeleton,  which  arose  from 
a  localisation  of  the  supports  of  continuous  lateral  folds. 

The  pectoral  or  shoulder  girdle  consists  of  a  dorsal 
28 


434  STRUCTURE   OF    VERTEBRATA. 

scapular  portion  or  shoulder  blade,  a  ventral  coracoid 
portion,  with  the  articulation  for  the  limb  between  them, 
and  of  a  forward  growing  clavicle  or  collar  bone. 

The  pelvic  or  hip  girdle  consists  of  a  dorsal  iliac  portion, 
a  ventral  ischiac  portion,  with  the  articulation  for  the  limb 
between  them,  and  of  a  pubic  region  possibly  homologous 
with  the  clavicle. 

The  fore  limb — from  Amphibians  onwards — consists  of  a 
humerus  articulating  with  the  girdle,  a  lower  arm  composed 
of  radius  and  ulna  lying  side  by  side,  a  wrist  or  carpus  of 
several  elements,  a  "hand"  with  metacarpal  bones  in  the 
"palm,"  and  with  fingers  composed  of  several  phalanges. 

The  hind  limb — from  Amphibians  onwards — consists  of 
a  femur  articulating  with  the  girdle,  a  lower  leg  composed  of 
a  tibia  and  fibula  lying  side  by  side,  an  "  ankle  "  region  or 
tarsus  of  several  elements,  a  foot  with  metatarsal  bones  in 
the  "  sole,"  and  with  toes  composed  of  several  phalanges. 

The  fin-like  limbs  peculiar  to  Fishes,  are  discussed  along 
with  the  other  characteristics  of  that  class. 

Distinct  from  the  other  bones  are  a  few  little  sesamoids  of 
occasional  occurrence,  e.g.,  the  knee  pan  or  patella.  They 
develop  in  the  tendons  of  muscles. 

Nervous  System. 

This  includes  (a)  the  central  nervous  system,  consisting 
of  brain  and  spinal  cord ;  (b)  the  peripheral,  consisting  of 
spinal  and  cranial  nerves ;  and  (c)  the  sympathetic  nervous 
system. 

The  central  nervous  system  first  appears  as  a  superficial 
groove  along  the  mid-dorsal  line  of  the  embryo.  The  sides 
of  this  ectodermic  groove  meet,  and  uniting,  convert  the 
'  medullary  groove  into  the  medullary  canal.  The  greater 
part  of  this  canal  forms  the  spinal  cord;  the  anterior  portion 
of  it  is  specialised  as  the  brain.  There  is  at  first  a  posterior 
connection  between  the  neural  canal  and  the  primitive  gut 
of  the  embryo ;  when  this  is  lost  the  cavity  still  persists  as  a 
little  ciliated  canal  in  the  centre  of  the  cord,  and  as  the  in- 
ternal cavity  of  the  brain.  In  Cyclostomes  and  Bony  Fishes 
the  central  nervous  system  arises  as  a  solid  cord  of  cells, 
the  cavities  not  appearing  until  a  later  stage ;  this  condition 
does  not  seem  to  be  primitive. 


THE   BRAIN.  435 


The  Brain. 

At  an  early  stage,  even  before  the  closing-in  process  is 
completed,  certain  portions  of  the  anterior  region  of  the 
medullary  canal  grow  more  rapidly  than  others,  and  form 
the  three  primary  brain  vesicles.  By  further  processes  of 
growth  and  constriction,  these  three  form  the  five  regions  of 
the  adult  brain. 

When  first  formed  the  brain  vesicles  lie  in  a  straight  line,  but  as  a 
consequence,  probably,  of  their  rapid  and  unequal  growth,  this  condition 
is  soon  lost,  and  a  marked  cranial  flexure  is  produced.  In  the  lower 
forms,  e.g.,  Cyclostomata,  the  flexure  is  slight,  and  is  corrected  later,  but 
in  the  higher  types  it  is  very  distinct,  and  causes  the  marked  overlapping 
of  parts  so  obvious  in  the  adult. 

We  must  now  follow  the  metamorphoses  of  the  primary 
brain  vesicles. 

The  first  vesicle  gives  rise  anteriorly  to  the  cerebral  hemi- 


FIG.  138. — Longitudinal  section  of  brain  of  young  Dogfish 
(diagrammatic).     (After  GASKELL.) 

CJt.,  Cerebral  hemispheres;  o.tk.,  optic  thalami ;  3-F.,  third  ven- 
tricle ;  /«.,  infundibulum ;  pt.b.,  pituitary  body;  o.L,  optic  lobes; 
cb.,  cerebellum;  M.O.,  medulla  oblongata  ;  4. K.,  fourth  ventricle; 
S.C.,  spinal  cord. 

spheres,  while  the  remainder  forms  the  region  of  the  optic 
thalami  or  thalamencephalon. 

The  cerebral  hemispheres,  also  called  prosencephalon,  or 
fore  brain,  are  exceedingly  important.  They  predominate 
more  and  more  as  we  ascend  in  the  scale  of  Vertebrates, 
and  become  more  and  more  the  home  of  thought.  Except 
in  a  few  instances,  the  prosencephalon  is  divided  into  two 
parts — the  cerebral  hemispheres — which  contain  cavities 
known  as  the  lateral  ventricles.  The  two  hemispheres  are 


436 


STRUCTURE   OF    VERTEBRATA. 


united  by  bridges  or  commissures,  which  have  considerable 
classificatory  importance.  With  the  anterior  region  of  the 
hemispheres  olfactory  lobes  are  associated. 

In  Cyclostomata,  Ganoids,  and  Teleosteans,  the  fore  brain  has  no 
nervous  roof,  but  is  covered  by  a  epithelial  pallium  homologous  with 
what  is  called  the  choroid  plexus  of  the  third  ventricle  in  higher  Verte- 
brates. This  choroid  plexus  is  a  thin  epithelium,  with  blood  vessels  in 
it.  But  in  Elasmobranchs,  Dipnoi,  and 
Amphibians  the  basal  parts  of  the  fore 
brain  have  grown  upwards  to  form  a  ner- 
vous roof,  and  this  persists  in  higher 
Vertebrates. 

The  optic  thalami  (thalamen- 
cephalon,  or  tween-brain)  form  the 
second  region  of  the  adult  brain. 
Hence  arise  the  optic  outgrowths, 
which  form  the  optic  nerves  and 
some  of  the  most  essential  parts  of 
the  eyes.  The  original  cavity  per- 
sists as  the  third  ventricle  of  the 
brain ;  the  thin  roof  gives  off  the 
dorsal  pineal  outgrowth  or  epiphysis, 
and  uniting  with  the  vascular  pia 
mater,  or  brain  membrane,  forms 
a  choroid  plexus;  the  lateral  walls 
become  much  thickened  (optic 
thalami) ;  the  thin  floor  gives  off 
a  slight  ventral  evagination,  or  in- 
fundibulum,  which  bears  the  enig- 
matical pituitary  body  or  hypo- 
physis. 


FIG.  139. — Origin  of 
Pineal  Body.  "( After 
BEARD.) 


The  Pituitary  Body. — This  is  derived 
in  part  from  the  brain  and  in  part  from 
the  mouth,  and  is  extremely  difficult  to 
understand.  It  is  apparently  equivalent 
in  part  to  the  sub-neural  gland  of  Tuni- 
cates,  but  this  does  not  carry  us  much 
further.  Dohrn  connected  it  with  two 
abortive  gill  slits,  but  the  evidence  seems 
insufficient.  Beard  has  interpreted  it  as  a 
residuum  of  the  original  mouth  which  Vertebrates  are  supposed  to  have 
possessed  before  the  persistent  one  with  which  we  are  familiar  was 
evolved,  and  of  the  innervation  of  that  hypothetical  structure,  but 
again  confirmation  seems  wanting.  Of  its  physiological  nature  we 


Lowest  figure — a  section 
through  the  first  embryonic 
vesicle,  while  the  medullary 
groove  (g)  is  still  open ;  o, 
optic  outgrowths.  Middle 
figure  shows  beginning  of 
pineal  upgrowth  (/).  Top- 
most figure  shows  a  later 
stage. 


THE  BRAIN.  437 

know  almost  nothing,  beyond  that  a  pathological  state  of  this  organ 
is  associated  in  man  with  certain  diseases,  e.g.,  acromegaly. 

The  Pineal  Body. — The  dorsal  upgrowth  (or  epiphysis)  from  the  roof 
of  the  thalamencephalon  is  represented,  though  to  a  varying  extent,  in 
all  Vertebrates.  It  is  terminally  differentiated  into  a  little  body  known 
as  the  pineal  body.  This  was  entirely  an  enigma  until  De  Graaf  dis- 
covered its  eye-like  structure  in  Anguist  and  Baldwin  Spencer  securely 
confirmed  this  in  the  New  Zealand  "lizard"  (Sphenodon}  where  the 
pineal  body  shows  distinct  traces  of  a  retina. 

In  Elasmobranchs  the  pineal  process  is  very  long,  and,  perforating 
the  skull,  terminates  below  the  skin  in  a  closed  vesicle.  In  the  young 
frog  it  also  comes  to  the  surface  above  the  skull,  but  degenerates  in 
adolescence.  In  Sphenodon  the  stalk  passes  through  the  skull  by  the 
"  parietal  foramen,"  so  that  the  "  eye"  itself  lies  close  beneath  the  skin, 
the  scales  of  which  in  this  region  are  specialised  and  transparent. 

In  Iguana,  Anguis^  Lacerta^  &c.,  the 

_—•  epiphysis    loses    connection    with    the 

^tfflfe  SjJ-  "eye"  portion;    and  it  is  also  to  be 

noticed  that  in  Angttis  and  Igttana  the 
pineal   body  receives  a   nerve  from   a 
"parietal   centre"   near   the    base   of, 
'.'CJjkfkl  but  independent  of,  the  epiphysis ;  this 

nerve  is  transitory  in  Anguis,  more  or 
I  'SSnoF  IGSS  Persistent  in  Iguana.     Above  Rep- 

tiles the  pineal  stalk  is  always  relatively 
short,  and  its  terminal  portion  forms  a 
U  glandular  structure.     In  fact,  the   de- 

velopment of  the  pineal  body  is  much 
more  complicated  than  at  first  appeared ; 
thus  according  to  Locy's  researches  on 

FIG.  140.— Diagram  of  the       Acanthias  embryos,    it   represents   the 
parts  of  the  brain  m  Verte-       fusion  of  an  extra  pair  of  eyes, 
brates.     (After  GASKELL.  )  The  fan   significance  of  the   pineal 

c.h.,  Cerebral  hemispheres ;  body  is  thus  uncertain.  According  to 
c.pl.,  choroid  plexus ;  o.th.,  optic  one  view,  its  primitive  function  is  that 
thalami;  oL,  optic  lobes;  cb.,  of  an  unpaired,  median,  upward-looking 

St^Sfob^u,^.;       eye-a   faction   retained   only  in   the 
spinal  cord.  Reptiles   mentioned  above,   the   organ 

having  elsewhere  undergone  (inde- 
pendent) degeneration.  It  has  also  been  interpreted  as  an  "organ  for 
the  perception  of  warmth."  It  may  be,  however,  that  the  function  dis- 
cernible in  some  Reptiles  is  not  primitive,  but  the  result  of  a  secondary 
modification  of  the  structure  in  question.  Thus,  one  of  first  inter- 
pretations (Dohrn's)  connected  the  pineal  and  the  pituitary  outgrowths 
with  a  supposed  passage  of  the  original  hypothetical  mouth  through  the 
nerve  cord. 

The  second  primary  vesicle  of  the  brain  undergoes  little 
alteration,  and  forms  the  third  region,  that  of  the  optic  lobes 
(mesencephalon  or  mid  brain)  in  the  adult  brain.  The  floor 


438  STRUCTURE   OF    VERTEBRATA. 

and  lateral  walls  form  the  thickened  crura  cerebri,  the  roof 
becomes  the  two  optic  lobes,  which  are  hollow  in  almost  all 
Vertebrates.  In  Mammals  a  transverse  furrow  divides  each 
optic  lobe  into  two  (corpora  quadrigemina).  The  cavity  of 
the  vesicle  becomes  much  contracted,  and  forms  the  narrow 
iter  or  aqueduct  of  Sylvius,  a  canal  connecting  the  third 
ventricle  with  the  fourth. 

The  third  primary  vesicle  gives  rise  to  the  metencephalon, 
or  hind  brain,  or  region  of  the  cerebellum,  and  to  the 
myelencephalon,  or  after  brain,  or  region  of  the  -medulla 
oblongata. 

In  the  metencephalon  the  roof  develops  greatly,  and 
gives  rise  to  the  cerebellum,  which  often  has  lateral  lobes, 
and  overlaps  the  next  region.  In  the  higher  forms  the 
floor  forms  a  strong  band  of  transverse  fibres — the  Pons 
Varolii. 

From  the  region  of  the  medulla  oblongata  most  of  the 
cranial  nerves  are  given  off.  Here  the  roof,  partly  over- 
lapped by  the  cerebellum,  degenerates,  becoming  thin  and 
epithelial,  the  cavity — called  the  fourth  ventricle — is  con- 
tinuous with  the  canal  of  the  spinal  cord. 

Summary. 

(i)  Cerebral  hemispheres,  prosencephalon,  or 
fore  brain.  Note  commissures,  olfactory 
lobes  and  nerves,  and  first  and  second 


First  Embryonic 
Vesicle. 


ventricles. 
(2)  Optic  thalami,  thalamencephalon,  or  tween- 


brain.  Note  (a)  optic,  (b]  pineal,  (c] 
pituitary  outgrowths,  and  the  third  ven- 
tricle. 

Median  Fmhrvonic    f   (3)  °ptic  lobeS'  mesencePhalon>  or   mid  brain- 
Median  fcmbryon  N  cerebri,  and  the  aqueduct  of 

Veslcle"  I  Sylvius. 

1(4)  Cerebellum,  metencephalon,  or  hind  brain. 
Note  Pons  Varolii. 
(5)  Medulla  oblongata,  myelencephalon,  or 
after  brain.  Note  rudimentary  roof, 
fourth  ventricle,  and  origin  of  most  of 
the  cranial  nerves. 

Enswathing  the  brain,  and  following  its  irregularities  is  a  delicate 
membrane — the  pia  mater — rich  in  blood  vessels  which  supply  the  ner- 
vous system.  Outside  this,  in  higher  Vertebrates,  there  is  another 
membrane — the  arachnoid — which  does  not  follow  the  minor  irregulari- 


THE  SPINAL    CORD. 


439 


ties  of  the  brain  so  carefully  as  does  the  pia  mater.  Thirdly,  a  firm 
membrane — the  dura  mater— lines  the  brain  case,  and  is  continued  down 
the  spinal  canal.  In  lower  Vertebrates  the  dura  mater  is  double,  in 
higher  Vertebrates  it  is  so  in  the  region  of  the  spinal  cord,  where  the 
outer  part  lines  the  bony  tunnel,  while  the  inner  ensheathes  the  cord 
itself.  In  Fishes  the  brain  case  is  much  larger  than  the  brain,  and  a 
large  lymph  space  lies  between  the  dura  and  the  pia  mater. 

An  understanding  of  the  relations  of  the  different  regions  will  be 
facilitated  by  a  study  of  the  following  table  which  Dr.  Gadow  gives  in 
his  great  work  on  Birds  in  Bronn's  Thierreich  : — 


REGION. 

FLOOR. 

SIDES. 

ROOF. 

CAVITY. 

Spinal  cord. 

Anterior  grey 
and  white  com- 
missure. 

White    and   grey 
substance. 

Posterior  commis- 
sure. 

Central  canal. 

Myelen- 
cephalon. 

Medulla  oblongata. 

Epithelium         of 
choroid  plexus. 

Posterior  part  of 
fourth  ventricle. 

Meten- 
cephalon. 

Commissural 
part. 

Pedunculi  or  crura 
cerebri. 

Cerebellum. 

Anterior  part  of 
fourth  ventricle. 

Mesencephalon. 

Pedunculi 
cerebri. 

Cortex     of    optic 
lobes. 

Anterior  commis- 
sure, velum  of  Syl- 
vius. 

Aqueduct  of  Syl- 
vius and  lateral 
extensions. 

Thalamen- 
cephalon. 

Infundibulum, 
hypophysis, 
chiasma. 

Inner  part  of  optic 
lobes  and  optic  thai- 
ami. 

Epiphysis        and 
epithelium    of   cho- 
roid plexus. 
Corpus  callosum. 
Anterior  commis- 
sure. 

Third  ventricle. 

Prosen- 
cephalon. 

Corpus   stria- 
turn. 
Lamina    ter- 
minalis. 
Olfactory 
lobes. 

Cerebral  hemisphere. 

Lateral       ven- 
tricles. 

The  Spinal  Cord. 

After  the  formation  of  the  brain  vesicles,  the  remainder  of 
the  medullary  canal  forms  the  spinal  cord. 
vThe  canal  is  for  a  time  continuous  posteriorly  with  the 
food  canal  beneath,  so  that  a  =3  -shaped  tube  results.  The 
connection  between  them  is  called  the  neurenteric  canal, 
and  though  it  is  only  temporary  its  frequent  occurrence  is  of 
much  interest. 

The  wall   of   the  medullary  canal  becomes  very  much 


440  STRUCTURE   OF    VERTEBRATA. 

thickened,  the  roof  and  floor  grow  less  rapidly,  and  thus  the 
cord  is  marked  by  ventral  and  dorsal  longitudinal  furrows. 
At  the  same  time,  the  canal  itself  is  constricted,  and  persists 
in  the  fully  formed  structure  only  as  a  minute  canal  lined  by 
ciliated  epithelium,  and  continuous  with  the  cavity  of  the 
brain.  It  can  hardly  be  said  to  have  any  function  ;  it  may 
be  simply  the  result  of  a  developmental  necessity.  But 
Sutton  and  Gaskell  have  independently  suggested  that  the 
central  canal  of  the  nervous  system  represents  a  disused 
alimentary  passage,  which  has  been  replaced  by  surrounding 
nervous  material,  and  which  ceased  to  be  functional  when 
the  permanent  gut  became  a  tube  open  at  each  end.  This 
suggestion,  however,  if  indeed  it  was  serious,  has  not  been 
accepted  by  any  morphologist. 

In  the  cord  it  is  usually  easy  to  distinguish  an  external  region  of  white 
matter,  composed  of  medullated  nerve  fibres,  and  an  internal  region  of 
grey  matter,  containing  ganglionic  cells,  and  non-medullated  fibres. 

The  arrangement  of  the  grey  matter,  together  with  the  longitudinal 
fissures,  give  the  cord  a  distinct  bilateral  symmetry,  which  is  sometimes 
obvious  at  a  very  early  stage. 

The  brain  substance  is  also  composed  of  grey  and  white  matter, 
but  there,  at  any  rate  in  higher  forms,  the  arrangement  is  very 
complicated. 

Concerning  the  development  of  the  peripheral  nervous 
system  there  is  far  less  certainty  than  with  regard  to  the 
central. 

The  motor  nerves,  even  the  motor  parts  of  the  cranial 
nerves,  appear  to  arise  as  fibrillar  or  as  cellular  outgrowths 
of  the  central  nervous  system.  All  the  sensory  nerves  take 
their  origin  from  peripheral  ganglia,  and  the  root  fibres 
grow  thence  into  the  central  system.  According  to  some 
embryologists,  certain  sensory  peripheral  nerves,  e.g.,  the 
lateralis  of  the  tenth,  arise  in  situ  from  the  sensory 
epithelium. 


[TABLE. 


CRANIAL   NERVES. 


441 


Cranial  Nerves. 

The  origin  and  distribution  of  the  cranial  nerves  may  be 
summarised  as  follows  : — 


NAME. 

ORIGIN. 

DISTRIBUTION. 

NOTES. 

i.  Olfactory.        s.* 

Front    of   fore 

Olfactory  organ. 

brain. 

2.  Optic.               s. 

Optic  thalami. 

Eye. 

Rather  parts  of  the 

brain     than    nerves. 

They     cross     before 

they  enter  the  brain, 

and   generally  unite 

at  their  intersection. 

3.  Oculomotor    or 

Floor    of  mid- 

All  the  muscles  of 

A  ciliary  ganglion 

ciliary.         ;//.  * 

brain. 

the  eye  but  two. 

at  roots. 

4.  Pathetic          or 
trochlear.     m. 

Front    of   cere- 
bellum. 

Superior    oblique 
muscle  of  the  eye. 

Perhaps  belongs  to 
5,  as  a  ventral  root. 

5.  Trigeminal. 

Medulla  oblon- 

(i)  Ophthalmic  to 

Gasserian  ganglion 

s.  and  in. 

gata. 

snout.                      .?. 

at  roots. 

(2)  Maxillary    to 

The  nature  of  the 

the  upper  jaw,&c.  s. 
(3)  Mandibular  to 

ophthalmicus       pro- 
fundus,  often  includ- 

lower jaw,  lips,  &c.m. 

ed  with  5,  sometimes 

with  3,  is  doubtful. 

6.  Abducens.     m. 

External  rectus  of 

Perhaps  belongs  to 

eye. 

7,as  a  ventral  branch. 

7.  Facial, 

(i)  Hyoidean. 

chiefly        ;;?. 

(2)  Palatine. 

partly           s. 

(3)  Buccal  to  space  be- 

tween jaws  and  snout. 

8.  Auditory.        s. 

Ear. 

Ganglion    at     the 

roots  of  7  and  8. 

9.  Glossopharyn- 

M 

First     gill     arch 

geal. 

and  cleft. 

s.  and  ?//. 

10.  Vagus  or  Pneu- 
mogastric. 

" 

Posterior  gills  and 
arches,  lungs,  heart, 

Apparently  a  com- 
plex,  including    the 

s.  and  m. 

gut        and       body 

elements   of  four  or 

generally. 

five  nerves. 

The  fourth  or  pathetic  nerve  is  peculiar  among  motor  nerves  in  that  it  arises  from 
the  extreme  dorsal  summit  of  the  brain,  between  the  mid  and  hind  brain,  from  the 
region  known  as  the  "valve  of  Vieussens."  In  Fishes,  the  seventh  nerve  is  mainly 
a  nerve  of  special  sense  ;  in  higher  Vertebrates  it  has  lost  most  of  its  sensory  branches, 
and  become  chiefly  motor. 

*  The  letters,  is  a  contraction  for  sensory  or  afferent,  i.e.,  transmitting  impulses 
from  a  sensitive  area  to  the  centre;  and  111.  is  a  contraction  for  motor  or  efferent,  i.e., 
transmitting  impulses  from  the  centre  to  the  body. 

There  is  much  uncertainty  in  regard  to  the  morphological  value  of 
the  various  cranial  nerves,  but  the  following  conclusions  are  impor- 
tant :— 

(i)  The  nerves  arise  either  as  outgrowths  of  the  central  system,  or  as 
specialisations  of  peripheral  cells.  Each  spinal  nerve  has  two  roots — 
a  dorsal  and  a  ventral,  but  in  most  cases  at  least  a  cranial  nerve 
has  primitively  a  single  dorsal  root  developing  from  a  neural  ridge 
of  the  dorsal  surface  of  the  brain.  In  many  cases  this  root  divides 


442  STRUCTURE   OF    VERTEBRATA. 

into  "dorsal,"  "ventral,"  and  other  branches.  As  these  typically 
innervate  a  gill  arch  and  slit,  as  may  be  well  studied  in  9,  the  branches 
may  be  called  (as  Beard  proposes)  supra-branchial  (dorsal),  post- 
branchial,  prae-branchial,  &c.  In  the  course  of  growth  the  nerve  often 
shifts  from  the  position  whence  its  root  originated. 

(2)  Some  of  the  cranial  nerves  mark  distinct  segments  of  the  head, 
while  others  are  secondary  derivatives.     It  is  likely  that  I,  3,  5,  7,  8,  9, 
and  several  parts  of  10  mark  segments.     It  is  possible  that  the  oculo- 
motor is  a  ventral  root  associated  with  the  third  or  ciliary  nerve,  that 
the  trochlear  is  a  ventral  root  of  the  trigeminal,  that  the  abducens  is  a 
ventral  root  of  the  facial. 

(3)  It  is  possible  that  each  truly  segmental  nerve  supplied  a  primitive 
gill  slit,  as  7  supplies  the  spiracle,  9  the  first  branchial,  10  the  second, 
third,  fourth,  and  fifth  branchials. 

(4)  It  is  likely  that  each   segmental  nerve  was   associated   with   a 
branchial  sense  organ  (Beard  and  Froriep).     These  organs  arise  above 
the  gills,  and  grow  thence  into  various  parts  of  the  head,  and  along  the 
trunk  as  the  "  lateral  line."     It  is  possible  that  a  branchial  sense  organ 


P,C  Pi 


FIG.  141. — Diagrammatic  section  of  Spinal  Cord. 

p.f,.  Posterior  fissure;  p.c.^  posterior  column  of  white  matter; 
d.p.s.,  dorsal,  posterior,  sensory  or  afferent,  root;  g.,  ganglion; 
v.a.m.,  ventral,  anterior,  motor  or  efferent,  root;  c.n.,  compound 
spinal  nerve  with  branches;  s.g.,  sympathetic  ganglion;  a.c., 
anterior  column ;  the  anterior  fissure  is  exaggerated  ;  g.c.,  a 
ganglion  cell;  g.m.)  grey  matter  ;  iv.m.,  white  matter. 

lay  over  each  primitive  gill  cleft,  and  had  an  associated  ganglion.  The 
ganglia  known  as  ciliary,  gasserian,  &c.,  may  be  the  ganglia  of  branchial 
sense  organs,  and  it  seems  that  parts  of  them  arise  in  development 
independently  of  the  brain.  It  may  be  that  nose  and  ear  were  originally 
branchial  sense  organs. 

Spinal  Nerves. 

Each  spinal  nerve  has  two  roots — a  dorsal,  posterior,  or 
sensory,  and  a  ventral,  anterior,  or  motor.  These  arise 
separately  and  independently,  but  combine  in  the  vicinity 
of  the  cord  to  form  a  single  nerve.  The  dorsal  root  exhibits 
at  an  early  period  a  large  ganglionic  swelling — the  spinal 


SENSE   ORGANS.  443 

ganglion ;  the  ventral  root  is  apparently  non-ganglionated. 
Moreover,  the  dorsal  root  has  typically  a  single  origin  (as 
in  the  cranial  nerves)  while  that  of  the  ventral  root  is  often 
multiple. 

The  dorsal  roots  are  outgrowths  of  a  continuous  ridge  or  crest  along 
the  median  dorsal  line  of  the  cord.  As  the  cord  grows  the  nerve  roots 
of  each  side  become  separated.  They  shift  sidewards  and  downwards 
to  the  sides  of  the  cord.  The  ventral  roots  are  later  in  arising  ;  they 
spring  as  outgrowths  from  the  latero-ventral  angle  of  the  cord. 

Beard  maintains  that  the  spinal  ganglia  do  not  arise  from  the  spinal 
cord,  but  have  an  independent  origin  from  the  deeper  layers  of  the 
epiblast. 

According  to  most  authorities,  the  sympathetic  ganglia  are  offshoots 
from  the  same  rudiment  as  that  from  which  the  dorsal  ganglia  arise, 
and  it  is  possible  that  they  are  the  more  or  less  vagrant  ganglia  of  the 
ventral  roots,  with  which  they  are  connected  by  small  fibres.  On  this 
view  (Gaskell's)  both  roots  may  be  said  to  be  ganglionated.  But  the 
ganglion  of  the  dorsal  root  is  stationary  in  position,  and  the  nerve  fibres 
which  pass  through  it  come  both  from  the  visceral  (splanchnic)  and 
from  the  peripheral  somatic  parts,  separating  from  one  another  within 
the  cord.  On  the  other  hand,  the  supposed  ganglion  (sympathetic)  of 
the  ventral  root  is  more  or  less  vagrant,  and  off  the  main  line  of  the 
root,  from  which  it  receives  small  fibres  passing  to  splanchnic  or  visceral 
structures. 

Sense  Organs. 

The  central  nervous  system  has  doubtless  arisen  in  the 
course  of  history  from  the  insinking  of  external  nerve  cells  : 
it  does  arise  in  development  as  an  involution  of  ectoderm 
or  epiblast.  The  same  layer  gives  origin  to  the  essential 
parts  of  the  sense  organs.  The  Vertebrate  eye  is  formed  in 
great  part  as  an  outgrowth  from  the  brain,  but  as  the  brain 
is  itself  an  involution  of  epiblast,  the  eye  may  be  also 
referred  to  external  nerve  cells. 

Branchial  Sense  Organs. — In  many  Fishes  and  Amphib- 
ians there  are  lateral  sense  organs  which  form  the  "  lateral 
lines,"  while  others  lie  in  the  head  and  were,  in  all  likelihood, 
primitively  connected  with  gill  clefts.  In  Sauropsida  and 
Mammals  these  branchial  sense  organs  are  no  longer  distinct 
as  such. 

The  Nose. — It  is  possible  that  the  sensory  pits  of  skin 
which  form  the  nasal  sacs  are  two  branchial  sense  organs. 
They  are  lined  by  epithelium  in  great  part  sensory,  and  are 
connected  posteriorly  with  the  olfactory  nerves.  In  all 
Fishes,  except  Dipnoi,  the  nasal  sacs  remain  blind ;  in 


444  STRUCTURE    OF    VERTEBRATA. 

Amphibians  and  in  all  the  higher  Vertebrates,  they  open 
posteriorly  into  the  cavity  of  the  mouth,  and  serve  for  the 
entrance  of  air.  The  peculiar  nostril  of  hagfish  and 
lamprey  is  referred  to  in  the  chapter  on  Cyclostomata. 

The  Ear  in  Invertebrates  is  formed  by  a  simple  invagina- 
tion  of  the  ectoderm  forming  a  little  sac,  which  may  become 
entirely  detached  from  the  epidermis,  or  may  retain  its 
primitive  connection  :  so  in  Vertebrates,  at  an  early  stage 
an  insinking  forms  the  auditory  pit.  This  sinks  further  in, 
and  the  originally  wide  opening  to  the  exterior  becomes  a 
long  narrow  tube.  In  Elasmobranchs,  which  exhibit  many 
primitive  features,  this  condition  is  retained  in  the  adult,  in 
other  Vertebrates  the  tube  loses  its  connection  with  the 
exterior,  and  becomes  a  blind  prolongation  of  the  inner 
ear — the  aqueductus  vestibuli,  or  ductus  endolymphaticus. 

The  auditory  vesicle,  at  first  merely  a  simple  sac,  soon 
becomes  very  complicated.  It  divides  into  two  chambers, 
the  larger  utriculus  and  the  smaller  sacculus.  From  the 
utriculus  three  semicircular  canals  are  given  off,  except  in 
the  lamprey  and  hag,  which  have  two  and  one  respectively. 
From  the  sacculus  an  outgrowth  called  the  cochlea  or 
lagena  originates  ;  it  is  little  more  than  a  small  hollow  knob 
in  Fishes  and  Amphibians,  but  becomes  large  and  important 
in  Sauropsida  and  Mammals. 

As  this  differentiation  of  the  parts  of  the  internal  ear  takes  place,  the 
lining  epithelium  also  becomes  differentiated  into  flattened  covering  cells 
and  sensory  auditory  cells.  The  auditory  cells  are  arranged  in  patches 
to  which  branches  of  the  auditory  nerve  are  distributed.  With  these 
sensory  patches  calcareous  concretions  (otoliths)  are  associated,  except 
in  the  cochlea  of  Mammals. 

The  fact  that  lime  salts  are  often  deposited  in  the  skin,  and  that  the 
ear  sac  arises  as  an  insinking  of  epiblast,  may  perhaps  shed  some  light 
on  the  origin  of  otoliths. 

The  parts  which  we  have  so  far  considered  constitute  together  the 
membranous  labyrinth  of  the  ear.  Round  about  them  the  mesoblast 
(mesenchyme)  forms  a  two-layered  envelope.  Its  inner  layer  disin- 
tegrates to  produce  a  fluid,  the  perilymph,  which  bathes  the  whole  outer 
surface  of  the  membranous  labyrinth.  Its  outer  layer  forms  a  firm  case, 
the  cartilaginous  or  bony  labyrinth,  surrounding  the  internal  ear.  The 
membranous  labyrinth  itself  contains  another  fluid,  the  endolymph. 

Certain  facts  of  development  suggest  that  the  ear,  like  the  olfactory 
organ,  may  be  a  branchial  sense  organ. 

With  regard  to  the  function  of  the  parts  of  the  ear,  the  semicircular 
canals  are  believed  by  many  to  be  concerned  with  the  appreciation  of  a 


SENSE   ORGANS.  445 

change  in  the  direction  or  velocity  of  movement.  How  far  the  ears  of 
lower  Vertebrates  (e.g.,  Crustacea  and  Molluscs),  are  adapted  for  any 
function  except  this .  is  still  doubtful,  and  we  can  hardly  see  that  any 
other  would  be  of  much  use  to  purely  aquatic  animals.  It  seems 
likely  at  any  rate  that  the  primitive  function  of  the  ear  was  the  percep- 
tion of  vibrations,  and  that  from  this  both  the  sense  of  hearing,  and  the 
sense  of  equilibration  have  been  differentiated. 

It  is  in  accordance  with  the  facts  mentioned  above,  that  we  rarely 
find  in  Fishes  any  special  path  by  which  impressions  of  sound  may 
travel  from  the  external  world  to  the  ear.  In  Amphibians  and  higher 
Vertebrates,  however,  the  ear  has  sunk  further  into  the  recesses  of  the 
skull,  and  a  special  path  for  the  sound  is  present.  In  Elasmobranchs 
the  spiracle,  or  first  gill  cleft,  is  situated  immediately  behind  the  ear ;  in 
higher  forms,  according  to  many  authors,  this  first  gill  cleft  is  metamor- 
phosed into  the  conducting  apparatus  of  the  ear.  In  development  a 
depression  beneath  the  closed  gill  cleft  unites  with  an  outgrowth  from 
the  pharynx,  and  thus  forms  the  tympanic  cavity,  which  communicates 
with  the  back  of  the  mouth  by  the  Eustachian  tube.  The  tympanic 
cavity  is  closed  externally  by  the  drum  or  tympanum,  which  may  be 
flush  with  the  surface,  as  in  the  Frog,  or  may  lie  at  the  end  of  a  narrow 
passage  which  in  many  Mammals  is  furnished  externally  with  a  projec- 
tion or  pinna.  In  Amphibia  and  Sauropsida  the  tympanic  cavity  is 
traversed  by  a  bony  rod — the  columella,  which  extends  from  the  drum 
to  the  fenestra  ovalis,  a  little  aperture  in  the  wall  of  the  bony  labyrinth. 
In  Mammals  this  is  replaced  by  a  chain  of  three  ossicles,  an  outermost 
malleus,  a  median  incus,  an  internal  stapes. 

The  homologies  of  these  ossicles  are  still  uncertain.  The  following 
is  Hertwig's  interpretation  : — 

Malleus  =  Articular  +  angular  elements  of  Meckel's  cartilage. 

Incus  =  Palato-quadrate  of  lower  Vertebrates. 

Stapes  of  Mammals  has  double  origin,  being  formed  from  the 
upper  part  of  hyoid  arch  +  an  ossification  from  the  wall  of 
the  ear  capsule  =  (wholly  ?)  columella  of  Birds,  Reptiles,  and 
Amphibians. 

The  Eye. — There  is  no  eye  in  Amphioxus,  it  is  rarely 
more  than  larval  in  Tunicates,  it  is  degenerate  in  Myxine, 
and  in  the  young  lamprey.  In  higher  forms  the  eye  is  always 
present,  though  occasionally  degenerate,  e.g.,  in  fishes  from 
caves  or  from  the  deep  sea.  It  is  hidden  under  the  skin  in 
Proteus,  an  amphibian  cave  dweller,  and  in  the  subterranean 
amphibians  like  Ccecilia,  very  small  in  a  few  snakes  and 
lizards,  and  abortive  as  to  its  nerves  in  the  mole. 

The  adult  eye  is  more  or  less  globular,  and  its  walls  con- 
sist of  several  distinct  layers.  The  innermost  layer  bounding 
the  posterior  part  of  the  globe  is  the  sensitive  retina,  inner- 
vated by  fine  branches  from  the  optic  nerve.  It  may  be 
compared  to  the  nervous  matter  of  the  brain,  from  which, 


446  STRUCTURE   OF    VERTEBRATA. 

indeed,  it  arises.  Outside  of  the  retina  is  a  pigmented 
epithelium,  and  outside  of  this  a  vascular  membrane ; 
together  these  are  often  called  the  choroid.  The  vascular 
part  may  be  compared  to  the  pia  matter  covering  the  brain, 
and  like  it  is  derived  from  mesoblast.  Outside  of  the 
choroid  is  a  protective  layer  or  sclerotic,  comparable  to,  and 
continuous  with,  the  dura  mater  covering  the  brain,  and 
also  mesoblastic  in  origin.  Occupying  the  front  of  the 
globe  is  the  crystalline  lens,  a  clear  ball  derived  directly 
from  the  skin.  It  is  fringed  in  front  by  a  pigmented  and 
muscular  ring — the  iris,  which  is  for  the  most  part  a  con- 


FlG.  142. — Diagram  of  the  Eye.     (Compiled). 

C.i  Cornea;  a.h.,  aqueous  humour;  c.b.,  ciliary  body;  /.,  lens; 
/.,  Iris;  Sc.,  sclerotic;  Ch.,  choroid;  R.,  retina;  v.h.,  vitreous 
humour  ;  y.sp.,  yellow  spot  ;  «.,  optic  nerve. 

tinuation  of  the  choroid.  The  space  enclosed  by  the  iris 
in  front  of  the  lens  is  called  the  pupil.  Protecting  and 
closing  the  front  of  the  eye  is  the  firm  cornea  continuous 
with  the  sclerotic,  and  covered  externally  by  the  conjunctiva 
— a  delicate  epithelium  continuous  with  the  epidermis. 
Between  the  cornea  and  the  iris  is  a  lymph  space  containing 
aqueous  humour,  while  the  inner  chamber  behind  the  lens 
contains  a  clear  jelly — the  vitreous  humour.  The  lens  is 


SENSE    ORGANS. 


447 


moored  by  "  ciliary  processes  "  of  the  choroid,  and  its  shape 
is  alterable  by  the  action  of  accommodating  muscles 
arranged  in  a  circle  at  the  junction  of  iris  and  sclerotic.  In 
many  Reptiles,  and  in  Birds,  a  vascular  fold  called  the 
pecten  projects  from  the  back  of  the  eye  into  the  vitreous 
humour.  A  similar  fold  in  Fishes  (processus  falciformis) 
ends  in  a  knot-like  structure  in  the  lens.  The  retina  is  a 
very  complex  structure  with  several  layers  of  cells,  partly 
supporting  and  partly  nervous ;  the  layer  next  the 


FIG.  143. — Developmental  of  the  eye.     (After  BALFOUR 
and  HERTWIG.) 

i.  Section  through  first  embryonic  vesicle,  showing  outgrowth  of 
optic  vesicles  (pp.v.)  to  meet  the  skin  ;  f.b.,  fore  brain  ;  G.,  the  gut. 

2-4.  The  formation  of  the  lens  (/)  from  the  skin,  and  the  modifica- 
tion of  the  optic  vesicle  into  an  optic  cup  ;  ^.,  retina  ;  ?/./?.,  vitreous 
humour. 

5.  External  aspect  of  embryonic  eye  ;  /. ,  lens. 

vitreous  humour  consists  of  nerve  fibres,  while  that 
furthest  from  the  rays  of  light  and  next  the  pigment 
epithelium  consists  of  sensitive  rods  and  cones.  The 
region  where  the  optic  nerve  enters,  and  whence 
the  fibres  spread,  is  called  the  blind  spot,  and  near 


448  STRUCTURE   OF   VERTEBRATA. 

this  there  lies  the  most  sensitive  region — the  yellow  spot, 
with  its  fovea  centralis,  where  all  the  layers  of  the  retina 
have  thinned  off  except  the  cones. 

Among  the  extrinsic  structures,  must  be  noted  the  six  muscles  which 
move  the  eyeball,  the  upper  and  lower  eyelids  which  are  often  very 
slightly  developed,  and  the  third  eyelid  or  nictitating  membrane. 
Above  Fishes  there  is  a  lachrymal  gland  associated  with  the  upper  lid, 
and  a  Harderian  gland  associated  with  the  nictitating  membrane.  •  In 
Mammals  there  are  also  Meibomian  glands.  The  secretions  of  all  these 
glands  keep  the  surface  of  the  eye  moist. 

While  the  medullary  groove  is  still  open,  the  eyes  arise 
from  the  first  vesicle  of  the  brain  as  hollow  outgrowths  or 
primary  optic  vesicles.  Each  grows  till  it  reaches  the  skin, 
which  forms  a  thickened  involution  in  front  of  it.  This 
afterwards  becomes  the  compact  lens.  Meantime  it  sinks 
inwards,  and  the  optic  vesicle  becomes  invaginated  to  form 
a  double  walled  optic  cup.  The  two  walls  fuse,  and  the  one 
next  the  cavity  of  the  cup  becomes  the  retina,  while  the 
outer  forms  the  pigmented  epithelium.  Meanwhile  sur- 
rounding mesoblast  has  insinuated  itself  past  the  lens  into 
the  cavity  of  the  optic  cup,  there  forming  the  vitreous 
humour,  while  externally  the  mesoblast  also  forms  the 
vascular  choroid,  the  firm  often  cartilaginous  sclerotic,  the 
inner  layer  of  the  cornea,  &c.  Along  the  thinned  stalk  of 
the  optic  cup  the  optic  nerve  is  developed.  Its  protective 
sheath  is  continuous  with  the  sclerotic  of  the  eye  and  the 
dura  mater  of  the  brain.  As  the  nerves  enter  the  optic 
thalami,  they  always  cross  one  another  in  a  chiasma,  and 
their  fibres  usually  interlace  as  they  cross. 

The  Alimentary  System  and  Associated  Structures. 

The  alimentary  tract  exhibits  much  division  of  labour, 
for  not  only  are  there  parts  suited  for  the  passage,  digestion, 
and  absorption  of  the  food,  but  there  are  numerous  out- 
growths, e.g.,  lungs  and  allantois,  which  have  nothing  to  do 
with  the  main  function  of  the  food  canal. 

By  far  the  greater  part  of  the  food  canal  is  lined  by 
endoderm  or  hypoblast,  and  is  derived  from  the  original 
cavity  of  the  gastrula — the  primitive  gut  or  archenteron. 
This  is  the  mid  gut  or  mesenteron.  But  the  mouth  cavity 
is  lined  by  ectoderm,  invaginated  from  in  front  to  meet  the 


THE  ALIMENTARY  SYSTEM.  449 

mid  gut.  This  region  is  the  fore  gut  or  stomatodaeum. 
Finally,  there  is  usually  a  slight  posterior  invagination  of 
ectoderm,  forming  the  anus.  This  is  the  hind  gut  or  proc- 
todaeum. 

Associated  with  the  mouth  cavity  or  stomatodaeum  are  (a)  teeth 
(ectodermic  rudiments  of  enamel  combined  with  a  mesodermic  papilla 
which  forms  dentine  or  ivory) ;  (b]  from  Amphibians  onwards  special 
salivary  glands  ;  (c)  a  tongue  or  muscular  and  sensitive  outgrowth  from 
the  floor.  The  tongue  develops  as  a  fold  of  mucous  membrane  in  front 
of  the  hyoid,  and  afterwards  becomes  increased  by  growth  of  connective 
tissue,  &c.  In  larval  Amphibians  muscle  strands  find  their  way  into  it, 
and  it  seems  likely,  as  Gegenbaur  has  recently  indicated,  that  their 
original  function  was  to  compress  the  glands.  As  they  gained  strength 
they  became  able  for  a  new  function,  that  of  moving  the  tongue.  In 
Myxine,  Dipnoi,  and  higher  animals,  the  nasal  sac  opens  posteriorly  into 
the  mouth  ;  in  some  Reptiles  and  Birds,  and  in  all  Mammals,  the  cavity 
of  the  mouth  is  divided  by  a  palate  into  an  upper  nasal  and  lower 
buccal  portion. 

The  origin  of  the  oral  aperture  is  not  quite  certain.  In  Tunicates  it 
is  formed  by  an  ectodermic  insinking  which  meets  the  archenteron  ;  in 
Amphioxus  it  seems  to  be  formed  as  a  pore  in  an  ectodermic  disc ;  in 
other  cases  it  is  a  simple  ectodermic  invagination,  or  it  may  owe  its  origin 
to  the  coalescence  of  an  anterior  pair  of  gill  clefts  innervated  by  the 
fifth  nerve.  If  the  last  interpretation  be  true,  its  origin  illustrates 
that  change  of  function  which  has  been  a  frequent  occurrence  in 
evolution.  But  if  the  mouth  arose  from  a  pair  of  gill  clefts,  and  in  some 
cases  it  actually  has  a  paired  origin,  then  there  must  have  been  an  older 
mouth  to  start  with.  Thus  Beard  in  his  brilliant  morphological  studies, 
distinguishes  between  "  the  old  mouth  and  the  new."  The  new  mouth 
is  supposed  to  have  resulted,  as  Dohrn  suggested,  from  a  pair  of  gill 
clefts ;  the  old  mouth  was  an  antecedent  stbmatodseum,  of  which  the 
so-called  nose  of  Myxine  and  the  oral  hypophysis  of  higher  forms  may  be 
vestiges.  This  theory  harmonises  with  the  observations  of  Kleinenberg, 
on  the  development  of  the  mouth  in  some  Annelids  (Lopadorhynchus], 
in  which  the  larval  stomatodgeum  is  replaced  by  a  paired  ectodermic 
invagination. 

The  mouth  cavity  leads  into  the  pharynx,  on  whose  walls 
there  are  the  gill  clefts.  Of  these  the  maximum  number 
is  eight,  for  the  hundred  slits  in  Amphioxus  cannot  be 
directly  compared  with  the  ordinary  clefts.  If  we  exclude 
the  hypothetical  clefts,  such  as  those  possibly  represented 
in  the  mouth,  the  first  pair  form  the  spiracles — well  seen  in 
skates.  In  the  position  of  the  spiracles  the  Eustachian 
tubes  of  higher  Vertebrates  develop.  In  front  of  the 
spiracle  there  is  sometimes  a  spiracular  cartilage,  which 
Dohrn  dignifies  as  a  distinct  arch.  The  other  gill  clefts  are 
associated  with  gills  in  Fishes  and  Amphibians  while  in 

29 


450  STRUCTURE   OF    VERTEBRATA. 

Sauropsida  and  Mammals,  in  which  there  are  no  gills,  four 
"visceral"  clefts  persist  as  practically  functionless  residual 
structures.  In  some  cases  their  openings  are  very  evane- 
scent. The  clefts  are  bordered  by  the  branchial  arches,  and 
supplied  by  blood  vessels  and  nerves. 

With  the  anterior  part  of  the  alimentary  canal,  two  strange 
structures  are  associated — the  thyroid  and  the  thymus. 

The  thyroid  gland  arises  as  a  diverticulum  from  the  ventral 
wall  of  the  pharynx.  It  may  be  single  (as  in  some  Mam- 
mals), or  bilobed  (as  in  Birds),  or  double  (as  in  some 
Mammals  and  Amphibians),  or  diffuse  (as  in  bony  fishes). 
Only  in  the  larval  lamprey  does  it  retain  its  original  con- 
nection with  the  pharynx,  and  is  then  a  true  gland. 

As  to  its  morphological  nature,  its  mode  of  origin  suggests 
comparison  with  the  hypobranchial  groove  in  Amphioxus 
and  the  endostyle  of  Ascidians.  According  to  Dohrn  it  is  a 
residue  of  the  gill  cleft  between  the  hyo-mandibular  and  the 
hyoid. 

Almost  the  only  light  which  has  been  cast  on  the  physiological  nature 
of  the  thyroid  is  from  the  pathological  side.  Goitre  and  Derbyshire 
neck  are  associated  with  an  enlargement  and  diseased  state  of  this 
organ,  and  myxcedema  with  its  degeneration  or  absence.  As  injection 
of  extract  of  sheep's  thyroid,  or  even  eating  this  organ,  alleviates  myxce- 
dema, it  is  concluded  that  the  thyroid  must  naturally  have  some  specific 
effect  on  the  large  quantity  of  blood  which  flows  through  it.  It  is 
probably  safe  to  say  that  the  thyroid  aids  in  keeping  the  blood  at  a 
certain  standard  of  health,  perhaps  through  some  specific  ferment. 

The  thymus  arises  as  a  dorsal  endodermic  thickening  where  the 
outgrowths  which  form  the  gill  clefts  meet  the  ectoderm.  It  may  be 
associated  with  a  variable  number  of  clefts — five  in  the  skate,  four  in  Tele- 
osteans,  three  in  the  lizard,  one  in  the  chick  ;  in  mammals  it  often  seems 
to  degenerate  after  youth.  As  it  has  from  its  first  origin  a  distinct 
lymphoid  nature,  and  apparently  forms  leucocytes,  it  has  been  inter- 
preted (Beard)  as  a  structure  adapted  for  the  phagocytic  protection  of 
the  gills  from  Bacteria,  parasites,  and  the  effects  of  injury.  If  this  be 
so,  we  can  understand  its  diminishing  importance  in  Sauropsida  and 
Mammalia,  where  its  place  may  be  to  some  extent  taken  by  the 
palatal  and  pharyngeal  tonsils,  which  are  believed  by  some  (Stohr, 
Killian,  Gulland)  to  have  a  similar  phagocytic  function. 

The  pharynx  leads  into  the  gullet  or  oesophagus,  which  is 
a  conducting  tube,  and  this  into  the  digestive  stomach, 
which  is  followed  by  the  digestive,  absorptive,  conducting 
intestine,  ending  in  the  rectum  and  anus. 

From  the  oesophagus,  the  air-  or  swim-bladder  of  most 


THE  ALIMENTARY  SYSTEM. 


451 


Fishes,  and  the  lungs  of  higher  Vertebrates,  grow  out.  The 
air  bladder  usually  lies  dorsally  and  is  almost  always  single ; 
the  lungs  lie  ventrally  and  are  double,  though  connected 
with  the  gullet  by  a  single  tube.  It  is  not  certain  that  these 
outgrowths  are  homologous,  though  the  air  bladder  of 
Dipnoi  acquires  the  functions  of  a  lung. 

The  beginning  of  the  intestine  gives  origin  to  the  liver 
which  regulates  the  composition  of  the  blood  and  secretes 
bile,  and  to  the  pancreas  which  secretes  digestive  juices. 

The    pancreas    has    often    a 
multiple  rudiment. 

From  the  hindmost  region 
of  the  gut,  the  allantois  grows 
out  in  all  animals  from  Am- 
phibians onwards.  In  Am- 
phibians it  is  represented  by 
an  unimportant  bladder;  in 
the  higher  Vertebrates  it  is  a 
vascular  foetal  membrane  con- 
cerned with  the  respiration 
or  nutrition  of  the  embryo, 
or  both. 

Cilia  are  very  common  on 
the  lining  of  the  intestine  in 
Invertebrates,  but  they  are 
much  rarer  in  Vertebrates. 
Yet  as  they  occur  in  Amphi- 
oxus,  lampreys,  many  fishes, 
Protopterus,  some  Amphi- 
bians, and  in  embryonic 
Mammals,  it  seems  not  unlikely  that  the  alimentary  tract 
was  originally  a  ciliated  tube. 

Speculative. — The  primitive  gut  was  probably  a  smooth  straight  tube, 
but  the  rapid  multiplication  of  well  nourished  cells  would  tend  to  its 
increase  in  diameter  and  in  length.  But  on  increase  in  both  directions 
the  slower  growth  of  the  general  body  would  impose  limitations,  and  in 
this  we  may  find  the  immediate  growth-condition  determining  the  origin 
of  folds,  crypts,  caeca,  and  coils,  which  would  be  justified  by  the  increase 
of  absorptive  and  digestive  surface.  There  are  regular  longitudinal 
folds  in  MyocinC)  cross  folds  traversing  these  would  form  crypts,  which 
may  be  exaggerated  into  the  pyloric  caeca  of  Teleosteans  and  Ganoids, 
while  other  modifications  would  give  rise  to  "spiral  valves  "and  the 
like.  In  the  same  way  it  may  be  suggested  that  the  numerous  important 


FIG.  144. — Origin  of  lungs, 
liver,  and  pancreas  in  the  chick. 
(After  GCETTE.) 

The  mesoderm  is  shaded  ;  the  endo- 
derm  dark. 

lg:,  One  of  the  lungs ;  st.,  stomach  ; 
/.,  liver ;  /.,  pancreas. 


452 


STRUCTURE   OF   VERTEBRATA. 


outgrowths  of  the  mid  gut,  such  as  lungs,  liver,  pancreas,  and  allantois, 
so  thoroughly  justified  by  their  usefulness,  may  at  first  have  been  due  to 
necessary  conditions  of  growth — to  the  high  nutrition,  rapid  growth,  and 
rapid  multiplication  of  the  endoderm.  It  may  be  noted  that  in  the 
development  of  the  Amphibian  Necturus*  there  are  hints  of  more 
numerous  endodermic  diverticula  (Platt).  Even  the  notochord,  which 
arises  as  a  median  dorsal  fold,  may  be  speculatively  compared  to  a 
typhlosole — folded  outwards  instead  of  inwards.  The  future  elaboration 
of  the  organs,  which  arise  as  outgrowths  of  the  gut,  would,  however, 
depend  on  many  factors,  such  as  their  correlation  with  other  parts  of  the 
body,  and  would  at  each  step  be  affected  as  usual  by  natural  selection. 

It  is  often  said  that,  in  some  cases  at  least,  as  in  lamprey,  frog,  and 
newt,  the  blastopore  or  opening  of  the  primitive  gastrula  cavity  persists 
as  the  anus  of  the  adult,  but  it  seems  doubtful  whether  the  anus  is 
not  always  a  new  formation.  In  many  cases,  at  least,  an  ectodermic 
invagination  or  proctodseum  meets  the  closed  archenteron,  and  at  the  junc- 
tion the  two  epithelial  layers  give  way,  so  that  an  open  tube  is  formed. 

The  formation  of  the  anus  does  not  take  place  close  to  the  posterior 
end  of  the  primitive  gut,  but  at  a  point  some  short  distance  in  front 
of  this.  In  consequence  the  so-called  post-anal  gut  is  formed.  This 
is  continuous  with  the  neurenteric  canal,  and  so  communicates  with 
the  neural  canal.  The  post-anal  gut  attains  in  Elasmobranchs  a  re- 
latively considerable  length.  It  has  been  very  frequently  found  in 
Vertebrates,  and  is  probably  of  universal  occurrence.  After  a  longer  or 
shorter  period  it  becomes  completely  atrophied,  and  with  it  the  com- 
munication between  neural  and  alimentary  canals  is  completely  destroyed. 

ALIMENTARY  SYSTEM. — SUMMARY. 


REGION  OF  THE  GUT. 

OUTGROWTHS. 

ASSOCIATED  STRUCTURES. 

Mouth  cavity, 
or  Stomatodaeum, 
or  Fore  gut, 
originating  as  an  ectodermic 
invagination,   or  from  a  co- 
alescence of  two  gill  clefts. 

Teeth. 
Salivary  glands. 
Tongue. 
(Note  relation  between  the 
mouth  and   the  oral   part  of 
the  hypophysis.) 

Pharynx,    gullet    or   oeso- 
phagus,  stomach,    small    in- 
testine,  large   intestine,  and 
rectum  ;  =  the  mesenteron  or 
mid  gut,  originating  from  the 
cavity  of    the  gastrula,   the 
archenteron  or  primitive  gut  ; 
lined  by  endoderm. 

Thyroid  \and  the  gill 
Thymus  /       clefts. 
Air  bladder  ;  lungs. 
Liver. 
Pancreas. 
Allantois. 

With     the     several      out- 
growths     the      surrounding 
mesoderm  becomes  associated 
often  to  a  great  extent. 
(Note  also  the  origin  of  the 
notochord  as  an  axial  differ- 
entiation of  cells  along  the 
mid  dorsal  line  of  the  embry- 
onic gut.) 

Anal  Region, 
or  Proctodaetim, 
or  Hind  gut. 
Where    the   mouth   of  the 
gastrula  persists,  it  forms  the 
terminal  aperture  of  the  gut, 
in  other  cases  there  is  an  ecto- 
dermic invagination  or  procto- 
daeum. 

In  some  Fishes,  all  Amphi- 
bians, all  Sauropsida,  and  the 
Prototherian  Mammals,  the 
terminal  part  of  the  gut  is  a 
cloaca  or  common  chamber 
into  which  the  rectum,  the 
urinary  and  the  genital  ducts 
open. 

BODY  CAVITY— VASCULAR  SYSTEM. 


453 


Body  Cavity. 

In  Amphioxus  a  paired  pouch  grows  out  from  the  archen- 
teron.  Almost  at  once  this  becomes  divided  up  on  either 
side  to  form  a  series  of  small  sacs,  the  cavities  of  which 
form  ultimately  the  true  body  cavity  or  coelome.  According 
to  Hertwig,  this  is  in  type,  the  method  of  formation  of  the 
coelome  throughout  the  Vertebrata.  In  the  other  Verte- 
brates, owing  to  modified  processes  of  development,  probably 
first  arising  from  the  presence  of  much  yolk,  we  have  solid 
cell  masses  growing  out  in  place  of  hollow  sacs,  but  the 

cavities  which  appear 
later,  apparently  by  split- 
ting of  the  cell  mass,  are 
in  reality  the  retarded 
cavities  of  true  gut 
nil  |  ^  pouches. 

Vascular  System. 

From  Cyclostomata  on- 
wards the  blood  fluid 
contains  red  corpuscles, 
i.e.,  cells  coloured  with 
haemoglobin — a  pigment 
which  readily  forms  a 
loose  union  with  oxygen, 
and  bears  this  from  the 
exterior  (gills  or  lungs) 
to  the  tissues.  These 
pigmented  cells  are  usu- 
ally oval  and  nucleated. 
In  all  Mammals  except 
Camelidae  they  are  cir- 
cular. Moreover,  the  full 
grown  red  corpuscles  of 
Mammals  have  no  visible 
nuclei.  The  blood  fluid 
also  contains  uncoloured 
nucleated  amoeboid  cells,  the  white  corpuscles  or  leuco- 
cytes, of  much  physiological  importance,  e.g.,  by  bearing 
food  particles  from  one  part  of  the  body  to  another, 


FIG.  145. — Transverse  section 
through  a  Teleostean  Embryo 
(diagrammatic).  (After  ZIEGLER.) 

s.c. ,  Spinal  cord;  IV.,  notochord;  ao.,  aorta; 
c.v.,  cardinal  veins  (united);  s.d.,  segmental 
duct;  c.,  coelome  or  pleuro-peritoneal  cavity; 
v.v.,  position  of  median  vitelline  vein  ;  y., 
yolk  ;  £n.,  Endoderm  of  gut ;  s.d.,  segmental 
duct;  int.,  myotome.  The  dots  represent 
mesenchyme  cells,  the  little  circles  blood 
corpuscles. 


454  STRUCTURE   OF   VERTEBRATA. 

and  by  attacking  and  destroying  micro-organisms  within 
the  body. 

The  heart  receives  blood  from  veins,  and  drives  it  forth 
through  arteries.  Its  contractions  in  great  part  cause  the 
inequality  of  pressure  which  makes  the  blood  flow.  It  lies 
in  a  special  part  of  the  body  cavity  known  as  the  pericardium, 
and  develops  from  a  single  blood  vessel  in  Cyclostomata, 
Fishes,  and  Amphibians,  from  a  pair  in  Reptiles,  Birds,  and 
Mammals. 

The  receiving  region  of  the  heart  is  formed  by  an  auricle 
or  by  two  auricles,  thence  the  blood  passes  into  the  muscular 
ventricle  or  ventricles,  and  is  driven  outwards.  Except  in 
adult  Birds  and  Mammals  the  veins  from  the  body  enter  the 
auricle  (or  the  right  auricle  if  there  are  two)  by  a  porch 
known  as  the  sinus  venosus.  In  Fishes  (except  Teleosteans) 
and  in  Amphibians,  the  blood  passes  from  the  ventricle  into 
a  valved  conus  arteriosus  which  seems  to  be  a  continuation 
of  the  ventricle.  In  Teleosteans  there  is  a  superficially 
similar  structure,  but  without  valves  and  non-contractile, 
and  apparently  developed  from  the  aorta,  not  from  the 
ventricle ;  it  is  called  the  bulbus  arteriosus,  and  may  occur 
along  with  the  conus  arteriosus  in  other  Fishes.  In  Verte- 
brates higher  than  Amphibians  the  conus  is,  to  say  the 
least,  less  distinct. 

In  Cyclostomata,  and  in  all  Fishes  except  Dipnoi,  the  heart  has  one 
auricle  and  one  ventricle,  and  contains  only  impure  blood,  which  it 
receives  from  the  body  and  drives  to  the  gills,  whence  purified  it  flows 
to  the  body. 

In  Dipnoi,  the  heart  is  incipiently  three  chambered. 

In  Amphibians,  the  heart  has  two  auricles  and  a  ventricle.  The  right 
auricle  alwTays  receives  venous  or  impure  blood  from  the  body,  the  left 
always  receives  arterial  or  pure  blood  from  the  lungs.  The  single 
ventricle  of  the  Amphibian  heart  drives  the  blood  to  the  body  and  to 
the  lungs. 

In  all  Reptiles,  except  Crocodilia,  the  heart  has  two  auricles  and  an 
incompletely  divided  ventricle.  By  means  of  the  partition  in  the 
ventricle  much  of  the  venous  blood  is  sent  to  the  lungs ;  indeed  the 
heart,  though  possessing  only  three  chambers,  works  almost  as  if  it 
had  four. 

In  Crocodilia,  there  are  two  auricles  and  two  ventricles.  But  the 
dorsal  aorta,  which  supplies  the  posterior  parts  of  the  body,  is  formed 
from  the  union  of  two  aortic  arches,  one  from  each  ventricle.  Therefore 
it  contains  mixed  blood. 

In  Birds  and  Mammals,  the  heart  has  two  auricles  and  two  ventricles, 


VASCULAR  SYSTEM. 


455 


and  one  aortic  arch  supplies  the  body  with  wholly  pure  blood.  This 
aortic  arch  always  arises  from  the  left  ventricle,  but  in  Birds  it  curves 
over  the  right  bronchus,  i.e.,  is  a  right  aortic  arch,  and  in  Mammals 
over  the  left,  i.e,  is  a  left  aortic  arch.  Impure  blood  from  the  body 
enters  the  right  auricle,  passes  into  the  right  ventricle,  is  driven  to  the 
lungs,  returns  purified  to  the  left  auricle,  enters  the  left  ventricle,  and  is 
driven  to  the  body. 

The  arterial  system  of  a  fish  consists  of  a  ventral  aorta  continued 
forwards  from  the  heart,  of  a  number  of  arching  vessels  diffusing  the 
impure  blood  on  the  gills,  and  of  efferent  vessels  collecting  the  purified 
blood  into  a  dorsal  aorta  which  runs  along  under  the  backbone  and 
supplies  the  body. 

So  in  the  embryo  of  higher  Vertebrates  the  same  arrangement  persists, 
though  there  are  no  gills  beyond  Amphibians.  From  a  ventral  arterial 
stem  arches  arise,  which  are  connected  so  as  to  form  the  roots  of  the 
dorsal  aorta.  This  aorta  gives  off  vessels  to  the  body,  while  in  embry- 
onic life  it  sends  important  vitelline  arteries  to  the  yolk,  and  (in 
Reptiles,  Birds,  and  Mammals)  equally  important  allantoic  arteries  to 
the  allantois. 

Returning  to  the  arterial  system  of  a  fish,  we  must  consider  the  arches 
more  carefully,  and  compare  them  with  those  of  Sauropsida  and  Mam- 
mals, where  they  are  no  longer  connected  with  functional  gill  clefts,  and 
also  with  those  of  Amphibians,  where  the  complications  due  to  lungs, 
&c.,  begin. 


FISHES. 


AMPHIBIANS. 


SAUROPSIDA  AND  MAMMALS. 


(#)Mandibular  aortic  arch 
usually  aborts  ;  has 
a  persistent  trace  in 
Elasmobranchs  (spir- 
acular  artery). 

(b]  Hyoid      aortic      arch 

aborts,  or  is  rudi- 
mentary, persists  in 
Elasmobranchs  and 
some  Ganoids. 

(c]  ist  branchial. 

(d]  2nd  branchial. 


(e)  3rd  branchial. 


(/)  4th  branchial  (gives 
off  artery  to  "  lung" 
of  Dipnoi). 


Aborts,  or  is  not  de- 
veloped. 


Aborts. 


Carotid. 

Systemic  arches, 
unite  to  form 
dorsal  aorta. 

Rudimentary  or  dis- 
appears. 

Pulmonary. 


At      most     merely     em- 
bryonic. 


At      most      merely     em- 
bryonic. 


Carotid. 

Systemic.  Only  the  right 
persists  in  Birds  ;  only 
the  left  in  Mammals. 

Possibly  the  pulmonary 
(unless  that  be/!). 

The  pulmonary  (unless 
that  be  e. ). 


456 


STRUCTURE   OF   VERTEBRATA. 


The  important  features  in  the  development  of  the  venous  system  are 
as  follows  : — 

(a)  In  the  embryo  the  vitelline  veins  bring  back  blood  from  the  yolk 
sac,  at  first  directly  to  the  heart,  and  later  to  the  liver.  Into 
these  veins,  blood  returned  from  the  intestine  is  poured  in 
increasing  quantity  by  other  veins.  In  the  adult  these  persist  to 
form  the  hepatic  portal  system  by  means  of  which  blood  from  the 


FIG.  146. — Diagram  of  Circulation.     (After  LEUNIS.) 

r.a.,  Right  auricle  receiving  superior  vena  cava  (s.v.c.)  and  inferior 
vena  cava  (i.v.c.')  ;  r.v.,  right  ventricle  ;  p. a.,  pulmonary  artery  to 
lungs  (Z,)  ;  p.v.,  right  pulmonary  vein  ;  /.#.,  left  auricle  ;  /.z/.,  left 
ventricle  ;  ao.,  aortic  arch  ;  d.ao.,  dorsal  aorta  giving  off  arteries  to 
liver  (//.),  to  gut  (£-.),  to  body  (B.)  ;  #o.v.,  portal  veins  ;  k.v.,  hepatic 
vein. 


stomach  and  intestines  is  carried  to  the  liver,  and  not  directly  to 
the  heart. 

(b)  At  an  early  stage  in  development  the  blood  is  brought  back  from 
the  anterior  region  by  the  superior  cardinal  veins,  from  the 
posterior  region  by  the  inferior  cardinals.  The  two  cardinals  on 


VASCULAR  SYSTEM.  457 

each  side  unite  to  form  the  short  transverse  ductus  Cuvieri,  the 
two  ducts  entering  the  sinus  venosus  of  the  heart.  In  Fishes  the 
superior  cardinals  persist,  the  inferior  cardinals  bring  back  blood 
from  the  kidneys,  and  also  to  some  extent,  by  means  of  their 
union  with  the  caudal  vein,  from  the  posterior  region  of  the  body. 
In  some  cases  this  union  with  the  caudal  is  only  indirect,  through 
the  medium  of  the  kidney  (Elasmobranchs),  in  this  way  the  renal 
portal  system  is  constituted.  In  higher  Vertebrates  before 
development  is  completed,  the  superior  cardinals  are  replaced  by 
the  superior  vense  cavse  (into  which  the  superior  cardinals  open 
as  external  jugulars).  The  inferior  cardinals  at  first  return  blood 
from  the  Wolffian  bodies  and  the  posterior  region,  later  they 
atrophy,  and  are  replaced  by  an  unpaired  inferior  vena  cava 
which  brings  back  blood  from  the  kidney  (efferent  renals),  from 
the  liver  (hepatics),  and  from  the  hind  limbs  except  when  there 
is  a  renal  portal  system.  In  Mammals  the  azygos  vein  persists 
as  a  remnant  of  inferior  cardinals. 

(c)  In  Amphibia  a  vein  known  as  the  epigastric  (anterior  abdominal) 
carries  blood  from  the  hind  limbs  into  the  hepatic  portal  system. 
This  vein  also  receives  blood  from  the  allantoic  bladder,  a  fact 
which  is  of  great  theoretical  importance.  In  all  higher  Verte- 
brates in  embryonic  life,  the  blood  from  the  allantois  passes 
through  the  liver,  and  to  a  greater  or  less  extent  into  its  capil- 
laries, on  its  way  to  the  heart.  In  Reptiles,  the  allantoic  veins 
persist  throughout  life  as  the  epigastric  vein  or  veins.  In  Birds 
and  Mammals,  on  the  other  hand,  they  atrophy  completely  at  the 
close  of  fcetal  life.  In  Birds,  however,  a  vein  is  developed  which 
connects  the  veins  coming  from  the  posterior  region  with  the 
allantoic  veins,  this  persists  when  the  remainder  of  the  allantoic 
veins  atrophy,  and  thus  in  Birds  as  in  Amphibia  there  is  a  connec- 
tion between  the  components  of  the  inferior  vena  cava  and  the 
portal  system.  In  Mammals  no  such  connection  occurs. 

According  to  many  authorities,  the  vascular  system  is  developed 
in  the  mesoblast  from  the  hollowing  out  of  strands  of  cells,  the 
outer  cells  forming  the  walls  of  the  vessels,  the  inner  forming 
the  constituents  of  the  blood.  According  to  some,  however,  the 
endoderm  plays  an  important  part  in  the  process.  Thus,  in 
Elasmobranch  fishes,  the  aorta  and  the  sinus  venosus  arise  directly 
from  the  archenteron,  and  the  cardinal  veins  arise  from  the  fusion 
of  segmental  outgrowths  of  the  aorta. 

Associated  with  the  vascular  system  is  the  spleen,  which 
appears  to  be  an  area  for  the  multiplication  of  blood  cor- 
puscles. It  is  usually  believed  to  be  of  mesodermic  origin, 
but  there  are  some  facts  which  point  to  its  being  endodermic. 

Developed  in  mesoblastic  spaces,  and  continuous  with 
the  body  cavity  on  the  one  hand,  and  the  blood  vessels  on 
the  other,  is  the  system  of  lymphatic  spaces  and  vessels  (see 
Chap.  II.). 


458  STRUCTURE   OF    VERTEBRATA. 

Respiratory  System. 

In  BalanoglossuS)  Tunicates,  and  Amphioxus,  the  walls  of 
the  pharynx  bear  slits,  between  which  the  blood  is  exposed 
in  superficial  blood  vessels  to  the  purifying  and  oxygenating 
influence  of  the  water. 

In  Cyclostomata,  Fishes,  all  young  and  some  adult  Am- 
phibians, there  are  not  only  clefts  on  the  walls  of  the 
pharynx,  but  gills  associated  with  these.  On  the  large 
surface  of  the  feathery  or  plaited  gills,  the  blood  is  exposed 
and  purified. 

In  Reptiles,  Birds,  and  Mammals,  traces  of  gill  clefts 
occur  in  the  embryos,  but  without  lamellae  or  respiratory 
function.  In  the  embryo  the  blood  is  purified,  as  will 
be  explained  afterwards,  by  aid  of  the  foetal  sac  known  as 
the  allantois  ;  and  after  birth  the  animals  breathe  by  lungs. 
All  adult  Amphibians  also  have  lungs,  to  which  the  lung 
or  swim-bladder  of  Dipnoi  is  physiologically  equivalent. 

The  gill  clefts  arise  as  outgrowths  of  the  endodermic  gut 
which  meet  the  ectoderm  and  open.  The  ventral  paired 
lungs  arise  from  an  outgrowth  of  the  gut,  and  such  also  is 
the  swim-bladder  of  many  Fishes,  though  that  usually  lies 
on  the  dorsal  surface,  has  rarely  more  than  a  hydrostatic 
function,  and  has  a  blood  supply  different  from  that  of  the 
lungs.  That  lung  and  swim-bladder  are  homologous  is  by 
no  means  certain,  but  the  comparison  is  plausible. 

Excretory  System. 

The  development  of  this  is  always  complicated.  In  the 
embryos  of  Vertebrates  at  an  early  stage  there  are  always 
traces  of  a  pronephros  or  so-called  head  kidney.  In  its 
most  developed  condition  this  consists  of  3-7  pairs  of 
segmentally  arranged  tubules,  which  communicate  on  the 
one  hand  with  the  body  cavity,  and  on  the  other  by  a 
segmental  duct  with  the  exterior.  The  tubules  are  com- 
parable to,  and  perhaps  homologous  with,  the  nephridia  of 
Annelids.  The  pronephros  persists,  although  apparently  in 
a  somewhat  degenerate  condition,  in  Myxine  and  Bdello- 
stoma  ;  in  Bony  Fishes  and  Amphibia  it  forms  a  large  and 
functional  organ  in  early  life;  in  Elasmobranchs  and  onwards 
it  is  from  the  first  rudimentary  and  functionless. 


EXCRETORY  SYSTEM. 


459 


The  segmental  duct  has  possibly  an  epiblastic  origin, 
it  grows  gradually  backwards  from  its  place  of  origin,  and 
it  seems  probable  that  the  original  excretory  opening  was 
far  forwards.  At  a  late  period  in  those  types  in  which  the 
pronephros  is  a  functional  larval  organ,  but  much  earlier  in 
the  higher  Vertebrates,  another  set  of  tubules  are  differenti- 
ated from  the  mesoblast,  nearer  the  posterior  end  of  the 
body,  and  acquiring  a  connection  with  the  segmental  or 
pronephric  duct,  constitute  the  mesonephros  or  mid  kidney. 
Below  the  Amniota  this  forms  the  permanent  excretory 
organ. 

In    higher    forms    another   series  of  nephridial   tubules 

s.c. 


a.r.b. 


FIG.  147. — Transverse  section  through  a  Vertebrate 
Embryo.     (After  SEMON.) 

sc.,  Spinal  cord  ;  My.,  Myotomes  ;  N. ,  notochord  ;  a.,  aorta,  with 
vessels  to  two  glomeruli ;  g. ,  rudiment  of  genital  organ;  p.n.d., 
pronephric  or  segmental  duct ;  N. ,  nephrostome.  The  letters 
a.r.b.  lie  in  the  top  part  of  the  gut. 

arises  still  further  back  in  the  body,  and  forms  the  meta- 
nephros,  or  permanent  kidney. 

In  each  case,  when  typically  developed,  the  tubules 
consist  (a)  of  an  internal  ciliated  funnel  (nephrostoma) 
opening  into  the  body  cavity,  but  only  rarely  persistent ; 
(£)  of  a  dilatation  (Malpighian  body),  into  which  blood 
vessels  project ;  and  (c)  of  a  coiled  tube  in  part  excretory,  in 
part  a  conducting  canal  for  the  waste  filtered  from  the  blood. 


460 


STRUCTURE   OF   VERTEBRATA. 


EXCRETORY  SYSTEM. 


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462  STRUCTURE   OF    VERTEBRATA. 

The  segmental  or  pronephric  duct  on  each  side  is,  at 
any  rate  in  some  of  the  lower  Vertebrates,  divided  into  two 
ducts,  the  Miillerian  duct  and  the  mesonephric  or  Wolffian 
duct.  In  the  Amniota  the  origin  of  the  Miillerian  duct 
is  still  somewhat  obscure.  It  becomes  the  genital  duct  or 
oviduct  of  the  female,  while  in  the  male  the  Wolffian  duct 
becomes  the  genital  duct  or  vas  deferens. 

The  ureters  or  ducts  from  the  persistent  functional 
kidneys  are  either  the  original  archinephric  or  segmental 
ducts  (e.g.,  in  Cyclostomata),  or  the  Wolffian  ducts  (in 
Amphibians),  or  special  posterior  derivations  of  the  latter. 

Suprarenal  Bodies. — These  are  found  in  most  Vertebrates  near  the 
reproductive  organs  and  kidneys.  Structurally,  each  shows  a  distinction 
into  a  cortical  and  a  medullary  zone.  It  is  usually  asserted  that  these 
two  areas  have  a  different  origin,  the  medullary  region  being  derived 
from  the  sympathetic  nervous  system,  the  cortex  from  the  pronephros. 
On  the  other  hand,  some  investigators  consider  that  the  medulla  is 
derived  from  metamorphosed  cortical  cells.  Nor  is  the  origin  of  the 
cortical  part  beyond  dispute,  for  by  some  it  is  said  to  originate  from  a 
degeneration  of  the  most  anterior  portion  of  the  germinal  epithelium,  or 
from  this  in  association  with  a  part  of  the  primitive  kidney. 

With  regard  to  function,  there  is  even  more  uncertainty.  The  supra- 
renal bodies  are  relatively  very  large  in  embryonic  life,  but  fail  to 
maintain  their  primitively  rapid  rate  of  growth.  It  has  been  suggested 
that  they  assist  in  breaking  down  or  disposing  of  waste  pigment. 

Reproductive  System. 

The  ovaries  and  testes  are  developed  from  a  ridge  formed 
by  a  part  of  the  epithelium  lining  the  abdominal  cavity, 
this  ridge  constituting  the  so-called  germinal  epithelium. 

In  the  male,  the  proliferating  germinal  epithelium  is 
divided  by  embryonic  connective  tissue  into  numerous 
follicles.  The  cells  of  the  follicles  form  seminal  mother 
cells,  which,  by  their  ultimate  divisions,  give  rise  to  sper- 
matozoa. From  the  mesonephros,  tubules  grow  out  to  the 
embryonic  testes  ;  these  form  the  collecting  tubes  of  the 
organ  and  open  into  the  mesonephric  duct,  the  vas  deferens 
of  the  adult.  This  is  the  most  typical  Vertebrate  condition, 
but  is  not  universal.  (See  Table,  page  461.) 

In  the  female,  the  ovary  is  similarly  divided  up  into 
follicles.  In  this  case,  however,  differentiation  sets  in 
among  the  originally  equivalent  cells  of  the  follicle.  One 
cell  in  each  follicle  is  more  successful  than  its  neighbours, 


REPRODUCTIVE  SYSTEM. 


463 


which  are  sacrificed  to  form  an  envelope  of  follicular  cells 
around  the  single  large  ovum  cell.  The  ova  are  usually 
shed  into  the  body  cavity,  and  pass  thence  to  the  exterior 
by  the  Miillerian  ducts  or  oviducts. 

In  many  cases  between  the  follicular  cells  and  the  ovum  there  is  a 
membrane,  the  zona  radiata,  which  is  traversed  by  fine  pores,  and,  in 
consequence,  has  a  striated  appearance  ;  other  egg  membranes,  more  or 
less  transitory  in  nature,  also  occur.  In  the  lower  Vertebrates  the  layer 
of  follicle  cells  is  single,  but  in  Mammals  it  is  multiple,  and  a  quantity 
of  clear  fluid  accumulates  between  the  cells  and  the  ovum.  The  whole 
forms  a  "  Graafian  follicle,"  which  bursts  when  the  ovum  is  liberated. 

Before  fertilisation  takes  place  the  ovum  undergoes  a  process  of 
maturation,  during  which  extrusion  of  polar  bodies  typically  occurs ; 
the  technical  difficulties  in  the  way  of  the  definite  observation 
of  this  fact  are,  however,  often  very  great.  The  ova  are  fertilised 

outside  the  body  in  Cyclostomata, 
Ganoids,  Teleosteans,  Dipnoi,  and 
tailless  Amphibians  ;  internally  in 
the  other  Vertebrates. 

Hermaphroditism  occurs  as  a 
normal  state  in  Tunicata,  most  of 
which  are  first  functionally  female 
and  then  male  (protogynous)  ;  in 
Myxine  (q.  v.),  which  is  first  male 
and  then  female  (protandrous) ;  in 
some  species  of  the  Teleostean 
genera  Chrysophrys  and  Serranus, 
of  which  the  latter  is  regularly 
self- fertilising  ;  and  in  a  solitary 
Batrachian.  It  occurs  casually  in 
some  Selachians,  in  the  sturgeon, 
in  about  a  score  of  Teleosteans, 
e.g.,  cod,  in  various  Amphibians, 
and  more  rarely  in  Amniota. 
There  are  also  embryological  facts 


FIG.  149. — Mammalian  Ovum. 
(After  HERTWIG.) 

ov.,  Ovum  ;  f.,  follicular  capsule  ; 
fz.,  follicle  cells;  f.c.t  follicle  cells 
forming  discus  proligerus  ;  f.L,  cavity 
occupied  by  liquor  folliculi. 


which  suggest  that  the  embryos  of  higher  Vertebrates  pass  through  a 
state  of  hermaphroditism  before  the  unisexual  condition  is  reached. 
On  these  grounds  it  has  often  been  suggested  that  the  original  Verte- 
brate animals  were  hermaphrodite. 

The  quantity  of  yolk  present  in  the  egg  varies  very  greatly  in 
Vertebrates,  and  its  presence  or  absence  exercises  a  profound  influence 
upon  the  processes  of  development.  Following  Hertwig  we  may  notice 
that  the  presence  of  yolk  has  both  a  physiological  and  a  morphological 
effect.  Physiologically,  the  presence  of  a  store  of  nutriment  enables 
the  developmental  process  to  be  carried  on  uninterruptedly,  and  the 
period  of  independent  life  to  be  postponed  until  more  or  less  complexity 
of  organisation  has  been  attained.  Again,  morphologically,  yolk  acts 
as  a  check  to  the  activity  of  the  protoplasm,  and  by  substituting  an 
embryonic  mode  of  nutrition  for  that  for  which  the  adult  organism  is 


464  STRUCTURE    OF    VERTEBRATA. 

fitted,  tends  to  prevent  a  speedy  establishment  of  the  adult  form. 
When  much  yolk  is  present  it  usually  forms  a  hernia-like  yolk  sac,  hang- 
ing down  from  the  embryonic  gut.  As  a  further  consequence,  we  may 
notice  the  tendency  to  the.  production  of  embryonic  organs  useful  only 
during  embryonic  life.  We  must  consider  the  formation  of  an  organic 
connection  between  mother  and  unborn  young  as  a  further  step  in  the 
same  direction  as  the  acquisition  of  yolk.  This  is  hinted  at  in  some 
Fishes  and  Reptiles,  but  culminates  in  the  placental  Mammals.  It  may 
be  looked  at  in  two  different  ways.  On  the  one  hand,  the  diversion 
of  the  nourishment  from  the  ovary,  during  the  period  of  gestation, 
tends  to  starve  the  remaining  ovarian  ova,  and  this  check  to  fertility 
is  further  prolonged  during  lactation  (Ryder)  ;  on  the  other  hand,  the 
chance  of  survival  is  much  increased,  and  the  maternal  sacrifice  finds 
its  justification  in  the  increased  specialisation  of  the  offspring. 

In  accordance  with  the  effect  of  the  presence  of  yolk  as  noted  above, 
we  find  that  segmentation  is  total  (holoblastic)  in  the  ova  of  the  lam- 
prey, the  sturgeon,  Ceratodus,  Amphibians,  and  all  Mammals  except  the 
Monotremes.  In  the  ova  of  Elasmobranchs,  Teleosteans,  Reptiles, 
Birds,  and  Monotremes,  the  activity  of  the  protoplasm  is  not  sufficient 
to  overcome  the  inertia  of  the  yolk,  and  segmentation  is  partial 
(meroblastic). 

Similarly  we  find  that  a  gastrula  is  formed,  in  part  at  least,  by  distinct 
invagination  in  the  development  of  the  lamprey,  the  sturgeon,  and 
Amphibians  (recently  the  occurrence  of  invagination  has  been  denied  for 
the  frog)  ;  it  is  more  modified  in  Teleosteans  and  Elasmobranchs,  whose 
ova  have  more  yolk  ;  it  is  much  disguised  in  Sauropsida  and  Mammals. 

Most  Vertebrates  lay  eggs  in  which  the  young  are  hatched, 
and  these  animals  are  usually  called  oviparous,  though 
all  animals  do  of  course  produce  eggs.  In  some  sharks, 
a  few  Teleosteans,  some  tailed  Amphibians,  a  few  lizards 
and  snakes,  the  young  are  hatched  before  they  leave  the 
body  of  the  mother  animal.  To  such  the  awkward  term 
ovo-viviparous  is  sometimes  applied,  but  there  is  no  real 
distinction  between  this  mode  of  birth  and  that  called 
oviparous,  and  both  may  occur  in  one  animal  (e.g.,  in  the 
grass  snake)  in  different  conditions.  In  the  placental  Mam- 
mals, there  is  a  close  organic  connection  between  the  unborn 
young  and  the  mother,  and  the  parturition  in  this  case  is 
usually  called  viviparous.  But  this  term  is  also  objection- 
able, since  all  animals  are  in  a  sense  viviparous. 


CHAPTER    XXI. 

CLASS    CYCLOSTOMATA. 
(SvN.  MARSIPOBRANCHII.) 

THE  hag  (Myxine),  the  lamprey  (Pctromyzon\  and  a  few 

others  like  them,  are  so  different  from  Fishes  that  they  must 
be  ranked  in  a  distinct  class.  They  seem  to  represent  an 
old-fashioned  type,  whose  interest  has  been  enhanced  by  the 
discovery  of  Palceospondylus  in  the  Old  Red  Sandstone. 

GENERAL  CHARACTERS. —  Unlike  all  higher  Vertebrates 
(Gnathostomata\  the  Cyclostomata  have  round  mouths  without 
distinctly  developed  jaws.  They  are  also  without  paired  fins 
and  without  scales.  Their  respiratory  system  consists  of 
6-7  pairs  of  gill  pouches,  to  which  the  term  Marsipobranch 
refers.  In  the  extant  forms  the  skeleton  is  wholly  cartilaginous,  . 
and  the  notochord  persists  unconstricted.  The  "  nostril"  is' 
single^  there  is  no  sympathetic  nervous  system,  no  conus 
arteriosus,  no  pancreas,  no  spleen,  and  the  segmental  duct 
is  unsplit. 

FIRST  TYPE.     Myxine — The  Hag. 

The  glutinous  hag  (Myxine  glutinosd)  is  not  uncommon 
off  the  coast  of  Scotland,  N.  England,  Scandinavia,  &c., 
living  in  the  mud  at  depths  of  40  to  300  fathoms.  It  often 
lies  buried  with  only  the  nostril  protruding  from  the  mud,  or 
it  may  swim  about  in  search  of  prey.  It  eats  the  bait  off  the 
fisherman's  long  lines,  and  it  also  enters  and  devours  the 
cod,  &c.,  which  have  been  caught  on  the  hooks.  According 
to  some  reports,  the  hag  also  attacks  free  swimming  fishes, 
boring  its  way  into  them,  but  the  evidence  is  not 
satisfactory.  Mr.  J.  T.  Cunningham  discovered  that  the 
young  animals  are  hermaphrodite,  containing  immature  ova 

30  -~4&  *<• 

TT'.  >^i 


466  CYCLOSTOMATA. 

and  ripe  spermatozoa,  while  older  forms  produce  ova  only, 
and  Nansen  has  corroborated  this.  Of  the  development 
and  early  history  nothing  is  known.  They  are  said  to 
spawn  in  late  autumn. 

Form. 

The  body  is  eel-like,  measuring  15-24  inches  in  the 
adult.  There  is  a  slight  median  fin  around  the  tail ; 
beside  the  mouth  and  nostrils  are  four  pairs  of  barbules. 
There  are  no  paired  fins. 

The  Skin. 

The  skin  is  scaleless,  and  rich  in  goblet  cells,  which 
secrete  so  much  mucus  that  the  ancients  said  the  hag  "could 
turn  water  into  glue."  Besides  the  diffuse  goblet  cells,  there 
is  a  double  row  of  glandular  pits  -arranged  segmentally  on 
each  side  of  the  ventral  surface  along  the  entire  length  of 
the  animal.  Each  opens  by  a  distinct  pore. 

Muscular  System. 

The  muscle  segments  or  myomeres  are  to  some  extent 
traceable.  Working  the  rasping  teeth  is  a  powerful 
muscular  structure,  sometimes  called  a  "tongue." 

The  Skeleton. 

The  skeleton  is  wholly  cartilaginous.  The  notochord 
persists  unsegmented  within  a  firm  sheath,  the  skull  is  a 
simple  unroofed  trough,  jaws  are  not  distinctly  developed, 
there  is  only  a  hint  of  the  complicated  basket  work 
which  supports  the  gill  pouches  of  the  lamprey,  but  the 
tongue,  the  barbules,  &c.,  are  supported  by  cartilaginous 
rods.  The  end  of  the  notochord  in  the  tail  is  quite  straight 
(protocercal  or  diphycercal). 

Nervous  System. 

The  brain  has  the  usual  parts,  but  is  small  and  simple.  It 
is  much  compressed,  with  practical  obliteration  of  ventricles, 
the  fore  brain  seems  to  agree  with  that  of  Ganoids  and 
Teleosteans  in  having  a  non-nervous  roof.  The  spinal  cord 
is  somewhat  flattened.  Throughout  at  least  a  portion  of  the 
cord  there  are  two  dorsal  roots  for  each  ventral  root.  The 


ALIMENTARY  AND  RESPIRATORY  SYSTEMS.      467 

union  of  dorsal  and  ventral  roots  is  only  partial,  and  there 
is  no  sympathetic  system. 

The  eye  is  degenerate  (e.g.,  without  a  lens  or  iris),  and  is 
hidden  beneath  the  skin ;  the  ear  has  only  one  semicircular 
canal ;  the  single  nasal  sac  opens  dorsally  at  the  apex  of 
the  head,  and  communicates  posteriorly  with  the  pharynx 
by  a  naso-palatine  duct.  The  absence  of  pigment  and 
sensory  structures  in  the  skin,  and  the  degeneracy  of  the  eye 
may  be  associated  with  the  hag's  mode  of  life. 

Alimentary  System. 

The  mouth  is  suctorial.  There  is  a  median  tooth  above, 
and  two  rows  of  teeth  are  borne  on  each  side  of  the 
muscular  "tongue."  These  teeth  are  "horny,"  but  sharp. 
Into  the  mouth,  just  in  front  of  a  fringed  velum  which 


FIG.  150. — Median  longitudinal  section  of  anterior  end 
ofMyxine.     (After  RETZI us.) 

Showing  barbules,  nasal  passage,  mouth  cavity,  brain. 

separates  it  from  the  pharynx,  the  nasal  sac  opens.  Thus 
water  passes  from  the  nostril  into  the  pharynx.  It  may 
be,  as  Beard  suggests,  that  this  passage  is  a  persistent  u  old 
mouth."  From  the  gullet  open  six  respiratory  pouches, 
each  of  which  has  an  efferent  tube,  but  the  six  efferent 
tubes  of  each  side  unite  and  have  a  common  exhalent 
orifice.  The  gut  is  straight  and  uniform,  with  longitudinal 
ridges  internally,  with  a  two-lobed  liver  and  a  gall  bladder, 
but  without  the  usual  pancreas. 

Respiratory  System. 

Water  enters  by  the  nasal  sac,  passes  into  the  pharynx, 
down  the  gullet,  into  the  six  pairs  of  respiratory  pouches, 


468 


CYCLOSTOMATA. 


out  by  their  efferent  tubes  at  a  single  aperture  on  each  side. 
The  respiratory  pouches  have  much  plaited  internal  walls, 
on  which  the  blood  vessels  are  spread  out.  On  the  left  side, 
behind  the  sixth  pouch,  a  tube  opens  from  the  oesophagus 
to  the  exhalent  aperture. 

Vascular  System. 

The  blood  contains  nucleated  red  corpuscles.  It  is 
collected  from  the  body  in  anterior  and  posterior  cardinals, 
passes  through  a  sinus  venosus 
into  the  auricle  of  the  heart, 
thence  to  the  ventricle,  thence 
along  a  ventral  aorta  which 
gives  off  arches  to  the  respira- 
tory pouches.  From  these  the 
purified  blood  passes  dorsal- 
wards  in  efferent  branchial 
vessels,  which  unite  posteriorly 
to  form  the  dorsal  aorta, 
while  from  the  most  anterior 
a  branch  goes  to  the  head. 


FIG.  151. — Respiratory  Sys- 
tem of  Hag,  from  ventral  sur- 
face. 

g.  Gullet;  g.p.)  first  gill  pouch;  e.t., 
exhalent  tube  of  first  gill  pouch,  unit- 
ing with  those  from  the  other  five 
pouches  ;  #./.,  exhalent  aperture  ;  v., 
ventricle  of  heart ;  v.a.,  ventral  aorta, 
carrying  blood  to  gill  pouches. 


Excretory  System. 

The  segmental  or  archi- 
nephric  ducts  remain  unsplit, 
and  the  kidney  or  nephridial 
system  is  represented  by  a 
series  of  small  segmental 
tubules  attached  to  the  ducts. 
The  pronephros  or  fore  kid- 
ney persists,  apart  from  the 
functional  mesonephros,  in  a  degenerate  state  on  each  side 
of  the  pericardium.  The  segmental  ducts  are  said  to  end 
much  in  the  same  way  as  they  do  in  the  lamprey. 

Reproductive  System. 

Myxine  is  a  protandrous  hermaphrodite,  spermatozoa 
being  formed  at  an  early  period,  and  ova  afterwards.  The 
reproductive  organ  is  simple,  unpaired,  and  moored  by  a 
median  dorsal  mesentery.  Owing  to  the  large  size  of  the 
ova,  the  ovary  is  very  conspicuous  in  full  grown  forms. 


THE  LAMPREY.  469 

When  the  ova  are  freed  from  the  ovary  they  pass  into  the 
body  cavity.  Each  has  an  oval  horny  case,  with  a  circlet 
of  knobbed  processes  at  each  end.  By  these  they  become 
entangled  together.  There  are  no  genital  ducts,  and  the 
expulsion  of  the  products  requires  to  be  investigated.  The 
development  is  still  unknown. 

Besides  Myxine  glutinosa,  two  other  species  are  known,  one  from 
Japan,  another  from  the  Magellan  Straits.  The  genus  Bdellostoma,  from 
Southern  seas  (off  the  Cape  of  Good  Hope,  &c. ),  is  nearly  allied  ;  it  has 
six  or  more  gill  pouches  which  open  apart  from  one  another. 


SECOND  TYPE  OF  CYCLOSTOMATA. 
Petromyzon — The  Lamprey. 

There  are  three  British  species,  the  sea  lamprey  (Petro- 
myzon marinus)  over  three  feet  in  length,  the  river  lampern 
(P.  fluviatilis)  nearly  two  feet  long,  and  the  small  lampern 
or  "  stone-grig "  (P.  branchialis  or  planeri).  They  eat 
worms,  small  crustaceans,  insect  larvae,  dead  animals,  &c., 
but  they  also  fix  themselves  to  living  fishes  and  scrape 
holes  in  their  skin.  As  their  names  suggest,  they  also  fix 
their  mouths  to  stones,  and  some  draw  these  together  into 
nests. 

The  spawning  takes  place  in  spring,  usually  far  up  rivers. 
Before  laying  the  eggs,  the  lamprey  seems  to  fast  (cf. 
salmon,  Protopterus^  frog),  and  its  muscles  undergo  a 
granular  degeneration  (cf.  ProtopUrus,  tadpole,  &c.).  Soon 
after  spawning  the  adults  of  both  sexes  die.  For  reproduc- 
tion is  often  the  beginning  of  death  as  well  as  of  life — 
though  in  higher  animals  the  nemesis  is  often  slow.  The 
young  are  in  many  ways  unlike  the  parents,  and  after  two 
years  or  three  years  pass  through  a  metamorphosis.  To 
the  larvae  before  metamorphosis  the  old  name  Ammoc&tes 
is  often  applied. 

Form. 

The  body  is  eel-like,  with  two  unpaired  dorsal  fins, 
and  another  round  the  tail.  Two  ridges,  one  on  each 
side  of  the  anus,  Dohrn  compares  to  rudimentary  pelvic 
fins.  Otherwise  there  is  no  trace  of  limbs. 


470  CYCLOSTOMATA. 

The  Skin. 

The  skin  is  scaleless,  slimy,  and  pigmented.  Its  structure, 
like  that  of  Myxine,  is  complex.  Sensory  structures  occur 
on  the  head  and  along  the  sides. 

Muscular  System. 

The  muscle  segments  are  well  marked.  The  suctorial 
mouth  and  the  rasping  "  tongue  "  are  very  muscular. 

The  Skeleton. 

The  skeleton  is  wholly  cartilaginous.  The  notochord 
persists  unsegmented,  but  its  firm  sheath  forms  rudimentary 
neural  arches.  The  skull  is  imperfectly  roofed.  There  are 
no  distinct  jaws,  but  a  cartilaginous  ring  supports  the  lips  of 
the  mouth.  There  is  a  complex  basket  work  around  the 
gill  pouches,  and  it  is  likely  that  its  elements  correspond  to 
visceral  arches.  Fin  rays  support  the  dorsal  and  caudal  fins, 
and  other  skeletal  parts  occur  about  the  "  tongue."  The 
caudal  end  of  the  notochord  is  quite  straight. 

Nervous  System. 

The  brain  has  the  usual  parts,  but  is  small  and  simple ; 
the  roof  of  the  fore  brain  is  composed  of  non-nervous 
epithelium  ;  there  is  a  distinct  pineal  body ;  the  oral  part  of 
the  hypophysis  is  developed  from  in  front  of  the  mouth,  in 
close  connection  with  the  involution  of  epiblast  which 
forms  the  nostril.  The  spinal  cord  is  flattened  ;  the  anterior 
and  posterior  roots  of  the  spinal  nerves  do  not  unite  ;  there 
is  no  sympathetic  system. 

Though  the  larva  sometimes  receives  the  name  of  "  nine- 
eyes  " — which  expresses  a  popular  estimate  of  the  branchial 
apertures — it  is  blind,  for  the  eyes  are  rudimentary  and 
hidden.  In  the  adult  they  rise  to  the  surface,  and  are  fairly 
well  developed.  The  ear  has  only  two  semicircular  canals 
instead  of  the  usual  three.  The  single  nasal  sac  does  not 
open  posteriorly  into  the  mouth  as  it  does  in  Myxine ; 
though  prolonged  backwards  it  ends  blindly.  Its  external 
opening  is  at  first  ventral,  but  is  shunted  dorsally. 

Alimentary  System. 

The  oral  funnel,  at  the  base  of  which  the  mouth  lies, 
has  numerous  horny  teeth.  It  is  applied  to  the  lamprey's 


RESPIRATORY  AND    VASCULAR  SYSTEMS.        471 

victim,  and  adheres  like  a  vacuum  sucker ;  the  toothed 
"  tongue  "  works  like  a  piston  ;  both  flesh  and  blood  are 
thus  obtained.  From  the  floor  of  the  pharynx  a  groove  is 
constricted  off  (cf.  p.  450). 

From  the  gullet  of  the  young  larva  seven  gill  pouches 
open  directly  to  the  exterior ;  but  in  the  adult  this  larval 
gullet  becomes  wholly  a  respiratory  tube.  -  It  is  closed 
posteriorly,  and  opens  anteriorly  into  the  gullet  of  the  adult, 
which  is  a  new  structure.  At  the  junction  of  the  re- 
spiratory tube  with  the  gullet  of  the  adult,  lie  two  flaps 
or  vela. 

The  rest  of  the  gut  is  straight  and  simple,  with  a  single- 
lobed  liver,  but  without  a  pancreas.  There  is  a  slight  fold 


FIG.  152. — Longitudinal  vertical  section  of  anterior  end 
of  larval  lamprey.     (After  BALFOUR). 

w.,  Mouth  ;  th.,  thyroid  ;  g*p>,  one  of  the  gill  pouches  ;  v.a.0.,  ven- 
tral aorta  ;  h.,  heart ;  N.,  notochord  ;  S.C.,  spinal  cord  ;  £.,  audi- 
tory vesicle;  cb.,  cerebellum;  /.£.,  pineal  body;  ch.,  cerebral 
hemispheres  ;  olf.,  olfactory  involution. 

in  the  intestine,  which  may  be  compared  with   the  spiral 
valve  of  Elasmobranchs. 

Respiratory  System. 

Seven  gill  pouches  with  plaited  walls  open  directly  to  the 
exterior  on  each  side,  and  communicate  indirectly  with  the 
gullet  as  already  described. 

When  the  lamprey  is  sucking  a  victim,  and  perhaps  at 
other  times,  water  passes  in  as  well  as  out  by  the  external 
openings  of  the  gill  pouches.  In  the  larva  there  is  an  eighth 
most  anterior  pouch  which  does  not  open  to  the  surface.  It 
corresponds  to  the  spiracle  of  Elasmobranchs. 


472  CYCLOSTOMATA. 

Vascular  System. 

The  vascular  system  is  essentially  the  same  as  in  the 
hag.  The  red  blood  cells  are  biconcave,  circular,  nucleated 
discs. 

Excretory  System. 

There  are  two  elongated  kidneys  (mesonephros),  each 
with  a  wide  ureter.  The  ureters  open  terminally  into  a 
urinogenital  sinus,  the  external  aperture  of  which  lies  behind 
the  anus  and  in  the  same  depression. 

Reproductive  System. 

The  sexes  are  separate.  The  reproductive  organ  is 
elongated,  unpaired,  and  moored  by  a  median  dorsal 
mesentery.  There  are  no  genital  ducts.  The  ova  and 
spermatozoa  are  liberated  into  the  body  cavity,  and  seem 
to  pass  by  two  pores  into  the  urinogenital  sinus,  and  thence 
to  the  exterior.  In  the  male  there  is  an  ejaculatory  structure, 
or  so-called  "  penis."  There  are  many  more  males  than 
females. 

Development  of  P.  planeri. — In  the  ripe  ovum,  which  has  a  consider- 
able quantity  of  yolk,  the  nuclear  substance  of  the  germinal  vesicle  is 
expanded  like  a  cup  at  the  "  animal  pole,"  forming  the  so-called  "  pole 
plasma. "  Outside  of  this  is  a  clear  cupola,  which  several  spermatozoa 
may  enter,  though  only  one  really  penetrates  into  the  egg.  After  a 
spermatozoon  has  begun  to  make  its  way  inwards,  two  polar  bodies  are 
formed.  The  elements  of  the  sperm  nucleus  combine  as  usual  in  an  inti- 
mate manner  with  those  of  the  reduced  nucleus  of  the  ovum,  and  a  seg- 
mentation nucleus  is  formed  about  three  hours  after  fertilisation. 

Segmentation  is  total,  but  slightly  unequal  owing  to  the  yolk  ;  a 
blastosphere  results  which  is  invaginated  into  a  gastrula.  The  blasto- 
pore  or  mouth  of  the  gastrula  persists  as  the  anus  of  the  animal,  and 
there  is  no  neurenteric  canal. 

The  formation  of  the  central  nervous  system  is  peculiar,  for  the  sides 
of  the  epiblastic  infolding  remain  in  contact  instead  of  forming  an  open 
medullary  canal. 

In  the  head  region,  where  the  gut  is  not  surrounded  by  yolk  cells,  the 
mesoblast  is  formed  from  hollow  folds  in  "  enterocoelic  "  fashion  ;  but  in 
the  trunk  region  the  cushions  of  hypoblastic  yolk  cells  change  gradually 
into  mesoblast,  and  acquire  a  coelome  cavity  in  "  schizocoelic  "  fashion. 
Thus  the  two  main  ways  in  which  a  body  cavity  arises,  (a)  from  ccelome 
pouches  of  the  archenteron,  (b]  from  a  splitting  of  solid  mesoblast  rudi- 
ments, are  here  combined. 

The  time  between  fertilisation  and  the  hatching  of  the  larva,  or 
AmmocceteS)  varies  with  the  temperature,  being  seventeen  days  in  North 
Germany,  but  only  eight  at  Naples. 


REPRODUCTIVE  SYSTEM. 


473 


The  larvae  live  wallowing  in  the  sand  or  mud  of  streams,  and  feed  on 
minute  animals.  Those  of  P.  planeri  are  so  unlike  the  adults  that  they 
were  once  referred  to  a  distinct  genus  Ammoccetes,  and  though  a  S trass- 
burg  fisherman,  Baldner,  is  said  to  have  discovered  their  true  nature 
about  200  years  ago,  the  fact  was  overlooked  until  August  Miiller 
traced  the  metamorphosis  in  1856.  In  the  small  lampern  the  change 
to  the  adult  state  is  sometimes  postponed  until  the  autumn  of  the  fourth 
or  fifth  year,  when  it  completes  itself  rapidly.  Less  is  known  about  the 
metamorphosis  of  the  other  species. 

In  the  AmmocceteS)  or  larva  before  metamorphosis,  the  head  is  small  ; 
the  dorsal  fin  is  continuous,  the  upper  lip  is  semicircular,  the  lower  lip 
is  small  and  separate  ;  the  mouth  is  toothless  and  not  suctorial ;  the 
brain  is  long  and  narrow ;  the  eyes  are  half  made  and  hidden  beneath 
the  skin ;  the  future  gullet,  as  distinguished  from  the  respiratory  tube, 
is  not  yet  developed. 

Lampreys  are  distributed  in  the  rivers  and  seas  of  north  and  south 
temperate  regions.  They  are  often  used  as  food.  Besides  Petromyzon 
there  are  several  related  genera,  e.g.,  Mcrdacia  and  Geotria,  from  the 
coasts  of  Chili  and  Australia,  and  Ichthyomyzon,  from  the  west  coast  of 
N.  America.  Certain  structures  called  "  conodonts  "  from  very  ancient 
(Silurian)  strata  have  been  interpreted  as  teeth  of  lampreys  or  hags. 


HAG  (Myxine). 


CYCLOSTOMATA. 

LAMPREY  (Petromyzon}, 


Exclusively  marine. 

The  fin  is  confined  to  the  tail. 

Numerous  large  glands  in  the 
complex,  slimy  skin. 

Mouth  with  barbules,  no  lips,  few 
teeth. 

Skull  without  any  roof. 
Skeletal   system    less    developed 
han  in  the  lamprey. 


Eyes  hidden  and  rudimentary. 

Ear  with  one  semicircular  canal. 
Nasal  sac  opens  posteriorly  into 
the  pharynx. 

Six  pairs  of  gill  pouches,  opening 
directly  into  the  gullet,  less  directly 
to  the  exterior. 

Longitudinal  ridges  in  the  intes- 
tine. 

Ova  large  and  oval,  with  attach- 
ing threads. 

(Development  unknown). 


In  rivers  and  seas. 

Two  unpaired  dorsal  fins. 

Sensory  structures  in  the  complex, 
slimy,  pigmented  skin. 

No  barbules,  but  lips,  and  many 
teeth. 

Skull  very  imperfectly  roofed. 
Hints  of  vertebral  arches. 
Cartilaginous  basket  work  around 
gill-pouches. 

Eyes  hidden  and  retarded  in  the 
larva,  exposed  and  complete  in  adult. 
Ear  with  two  semicircular  canals. 
Nasal  sac  ends  blindly. 


Seven  pairs  of  gill  pouches,  open- 
ing directly  to  the  exterior,  less 
directly  into  the  adult  gullet. 

A  slight  spiral  fold  in  the  intestine. 


Ova  small  and  spherical. 
Development  with  metamorphosis. 


474 


CYCLOSTOMATA. 


Palceospondylus  gunni. 

Under  this  title,  Dr.  Traquair  has  recently  described  a 
remarkable   fossil   form   from 
the   Old    Red    Sandstone   of 
Caithness.     He   speaks  of  it 
as  a  "strange  relic   of  early  c^?| 

vertebrate  life." 

It  is  a  dainty  little  creature, 
somewhat  tadpole-like  at  first 
sight,  usually  under  an  inch  in 
length.  The  following  char- 
acters point  strongly  to  its 
affinities  with  Cyclostomata : — 

(i.)  uThe  skull  is  apparently 
formed  of  calcified  cartilage,  and 
devoid  of  discrete  ossifications." 
An  anterior  part  is  comparable  to 
the  trabecular  and  palatal  region  of 
a  lamprey's  skull ;  a  posterior  part 
is  comparable  to  the  parachordal 
region  and  auditory  capsules, 

(2.)  "There  is  a  median  opening 
or  ring,  surrounded  with  cirri,  and 
presumably  nasal,  in  the  front  of  the 
head"  (n.,  Fig.  153). 

(3.)  "  There  are  neither  jaws  nor 
limbs." 

(4.)  "The  rays  which  support  the 
caudal  fin  expansion,  apparently 
springing  from  the  neural  and  hoemal 
arches,  are  dichotomised  (at  least 
the  neural  ones),  as  are  the  corre- 
sponding rods  in  the  lamprey. " 

Just  behind  the  head  lie  two  small 
oblong  plates  (.%•.,  Fig.  153),  closely 
apposed  to  the  commencement  of 
the  vertebral  column,  one  on  each 
side.  The  notochordal  sheath  is 
calcified  in  the  form  of  ring-shaped 
or  hollow  vertebral  centra  with 
neural  arches.  Towards  the  tail, 
the  arches  are  produced  into  slender  neural  spines,  opposite  which 
are  shorter  haemal  ones. 


FIG.  153. — Restored  skeleton 
of  Pakeospondylus.  (After 
TRAQUAIR.) 

d.c.,  cirri  of  dorsal  margin  ;  I.e., 
long  lateral  cirri  ;  v.c.,  cirri  of  ventral 
margin  ;  «.,  nasal  ring  ;  t.p.,  anterior 
trabeculo-palatine  part  of  cranium  ;  b., 
anterior  depression  or  fenestra ;  c., 
posterior  depression  or  fenestra  ;  «., 
lobe  divided  off  from  anterior  part ; 
p. a.,  posterior  or  parachordal  part  of 
cranium  ;  x.>  post  occipital  plates. 


CHAPTER    XXI 
K*HiJkx>»    IV  "  ^xx-»-4-^**-^»    LfH-j'tMj}  pl£; 
t-J-J         .CLASS    PISCES  —  FISHES. 


Order  i.  —  Elasmobranchii  or  Selachii,  cartilaginous  fishes,  e.g.,  skates 

and  sharks.     With  these  may  perhaps  be  ranked  the 

-^  Holocephali  (Chimera  and  Callorhynchtts.} 

(jz.  —  Ganoidei,  such   as   sturgeon   (Acipenser]  and   bony 

(Lepidosteus]  ;    numerous    extinct   genera,    only   seve 
extant. 

3.  —  Teleostei,    bony   fishes,    such    as    cod,    herring,    salmon, 

flounder,  eel. 

4.  —  Dipnoi,  mud  fishes  :  Ceratodus^  Protopterus^  Lepidosiren. 

The  Dipnoi,  or  double  breathers,  are  so  distinct  that  some  would  re- 
move them  from  among  Fishes,  and  place  them  as  an  independent  class 
between  Fishes  and  Amphibians. 

FISHES  form  the  first  markedly  successful  class  of  Verte- 
brates. For  while  the  Tunicates  are  numerous,  most  of 
them  are  degenerate  ;  the  level  attained  by  the  lancelet  is 
represented  by,  at  most,  two  or  three  closely  related  genera  ; 
and  the  Cyclostomata  are  few  in  number  and  partially  re- 
trogressive. 

In  the  possession  of  a  Vertebrate  axis  and  central  nervous 
system,  in  the  general  integration  of  their  structure,  and  in 
their  great  fecundity,  Fishes  have  an  easy  pre-eminence  over 
their  Invertebrate  inferiors.  As  successfully  adapted  forms 
—  with  typically  wedge-like  bodies,  supple  muscular  tails, 
fin-like  limbs,  and  the  like  —  they  may  well  compare  with 
Birds  in  their  mastery  of  the  medium  in  which  they  live. 

Their  success  may  be  read  in  the  immense  number  of  in- 
dividuals, species,  and  genera,  not  only  now  but  in  the  past  ; 
in  the  geological  record  which  shows  how  the  cartilaginous 
Elasmobranchs  have  persisted  strongly  from  Silurian  ages, 
or  how  the  mysterious  decadence  of  the  Ganoid  order  has 


, 


Wuu   !^< 
476  FISHES. 

been  followed  by  a  yet  richer  predominance  of  the  modern 
Bony  Fishes ;  and,  furthermore,  in  the  plasticity  with  which 
many  types  appear  to  have  assumed  particular  specialisations, 
such  as  that  evolution  of  lungs  which,  in  the  double  breath- 
ing Dipnoi,  prophesies  the  epoch-making  transition  from 
water  to  terra  fir  ma. 

GENERAL  CHARACTERS. — Pishes  are  aquatic  Vertebrates, 
breathing  by  gills  attached  to  bony  or  gristly  arches  on  the 
sides  of  the  pharyngeal  gill  clefts.  In  Dipnoi,  a  single  or 
double  outgrowth  from  the  gut — the  air-  or  swim-bladder — 
.  functions  as  a  lung,  air  being  inspired  at  the  surface  of  the 
water.  In  Ganoids  and  in  most  Teleosteans  the  same 
structure  is  present,  but  though  occasionally  of  some  slight 
usefulness  in  respiration,  usually  serves  as  a  hydrostatic 
organ. 

Two  pairs  of  limbs,  in  the  form  of  fins,  are  usually  present, 
and  there  are  also  unpaired  median  fins,  supported  by  fin- 
rays.  There  are  two  great  types  of  paired  fin.  In  Dipnoi, 
and  in  some  extinct  forms,  the  fin  has  a  central  segmented 
axis,  which  (e.g.,  Ceratodus)  bears  on  each  side  a  series  of 
radial  pieces.  In  other  fishes  the  radials  diverge  ouhuards 
from  several  basal  pieces,  and  there  is  no  median  axis. 

The  skin  usually  bears  numerous  scales,  in  great  part  due 
to  the  dermis,  but  covered  by  a  layer  of  epidermis,  which  may 
produce  enamel.  They  vary  greatly  in  form  and  texture,  are 
suppressed  in  eels  and  electric  fishes,  and  rudimentary  in  some 
other  forms.  Numerous  glandular  cells  occur  in  the  skin, 
but  these  are  not  compacted  into  multicellular  glands,  except 
in  Dipnoi  and  a  few  poisonous  fishes.  The  skin  also  bears 
sensory  structures,  usually  aggregated  on  the  head,  and 
arranged  in  one  or  more  "  lateral  lines  "  along  the  trunk. 
There  are  no  muscular  elements  in  the  dermis  or  cutis. 

In  many  the  gut  ends  in  a  cloaca,  or  a  distinct  anus  may 
lie  in  front  of  the  genital  and  urinary  aperture,  or  apertures. 

The  heart  is  tivo-chambered,  and  contains  only  venous 
blood,  except  in  the  Dipnoi,  where  it  shows  hints  of  becoming 
three-chambered  and  receives  pure  blood  from  the  lungs  as 
well  as  impure  blood  from  the  body.  Apart  from  the  Dipnoi, 
the  heart  has  a  single  auricle,  receiving  impure  blood  from 
the  body,  and  a  ventricle  which  drives  this  through  a  ventral 
aorta  to  the  gills,  whence  the  purified  blood  flows  to  the 


THE  SKATE.  477 

head  and  by  a  dorsal  aorta  to  the  body.  In  addition  to  the 
two  essential  chambers  of  the  heart,  there  is  a  sinus  venosus, 
which  serves  as  a  porch  to  the  auricle,  and  there  is  often  a 
muscular  conus  arteriosus  in  front  of  the  ventricle,  or  a  bulbus 
arteriosus  at  the  base  of  the  ventral  aorta.  Except  in  Dipnoi, 
there  is  no  vein  which  exactly  corresponds  to  what  is  known 
in  all  higher  Vertebrates  as  the  inferior  vena  cava,  i.e.,  a 
single  vessel  which  receives  hepatic  veins  from  the  liver,  renal 
veins  from  the  kidneys,  and  other  veins  from  the  posterior 
organs. 

There  is  no  distinct  indication  of  an  outgrowth  from  the 
hind  end  of  the  gut  comparable  to  that  which  forms  the 
bladder  of  Amphibians  or  the  allantois  of  higher  Vertebrates. 

Most  fishes  lay  eggs  which  are  fertilised  in  the  water. 

First  type  of  FISHES.     The  Skate  (Raja) — of  the  order 
Elasmobranchii. 

Various  species  of  Raja — the  grey  skate  (R.  batis],  the 
thornback  (R.  clavatd),  and  the  ray  (R.  maculata) — are 
common  off  the  coast  of  Britain.  They  are  comparatively 
sluggish  but  very  voracious  fishes,  and  live  along  the  bottom 
at  considerable  depths. 

Form. 

The  body  is  flattened  from  above  downwards  or  dorso- 
ventrally,  unlike  that  of  the  bony  flat  fishes,  such  as  plaice 
and  flounder,  which  are  flattened  from  side  to  side.  The 
skate  rests  on  its  ventral  surface,  the  flounder  on  its  side. 
The  triangular  snout,  the  broad  pectoral  fins,  the  long  tail 
with  small  unpaired  fins,  are  obvious  features.  On  the 
dorsal  surface  the  skin  is  pigmented  and  studded  with 
placoid  scales  or  skin  teeth ;  on  the  top  of  the  skull  there 
are  two  unroofed  areas  or  fontanelles ;  there  are  numerous 
jointed  radials  in  the  pectoral  fins.  Behind  the  lidless  eyes 
are  the  spiracles — the  first  of  the  obvious  gill  slits,  opening 
dorsally,  containing  a  rudimentary  gill,  and  communicating 
posteriorly  with  the  mouth  cavity.  On  the  ventral  surface, 
are  seen  the  sensory  mucous  canals  extending  over  the  skin, 
the  transverse  mouth  and  the  nostrils  incompletely  separated 
therefrom  as  if  in  double  hare  lip,  the  five  pairs  of  gill 
apertures,  the  cloacal  aperture  and  two  abdominal  pores 


478  FISHES. 

beside  it.  We  may  feel  the  pectoral  and  pelvic  girdles 
supporting  the  fore  and  hind  fins.  In  the  male,  the  latter 
are  in  part  modified  into  copulatory  "  claspers." 

The  Skin. 

On  the  dorsal  pigmented  surface,  embedded  in  the  dermis, 
there  are  many  "  skin  teeth,"  or  "  dermal  denticles,"  or 
"  placoid  scales."  Each  is  based  in  bone,  cored  with 
dentine  or  ivory,  tipped  with  enamel,  the  latter  being  due 
to  the  ectoderm  (epidermis),  the  rest  to  the  mesoderm 
(dermis  or  cutis)  of  the  skin,  the  whole  arising  as  a  skin 
papilla.  On  the  ventral  unpigmented  surface  are  numerous 
mucus  canals  or  jelly  tubes,  ending  in  ampullae.  These 
are  also  present  on  the  dorsal  aspect,  especially  about  the 
head.  They  have  a  sensory  function.  Most  of  the  slime 
which  exudes  on  the  surface  comes  from  glandular  goblet 
cells  in  the  epidermis. 

Muscular  System. 

In  the  posterior  part  of  the  body  and  in  the  tail,  the 
segmental  arrangement  of  the  muscles  may  be  recognised. 
The  large  muscles  which  work  the  jaws  are  noteworthy. 
Professor  Cossar  Ewart  has  described  a  rudimentary  electric 
organ  in  the  tail  region  of  Raja  batis  and  R.  davata^ 
apparently  too  incipient  to  be  of  any  use. 

Electric  organs  are  best  developed  in  two  Teleostean  fishes — a  S. 
American  eel  (Gymnotus]  and  an  African  Siluroid  (Malapterurus],  and 
in  the  Elasmobranch  Torpedo.  In  Gymnotus  they  lie  ventrally  along 
the  tail,  in  Malapterurus  they  extend  as  a  sheath  around  the  body,  in 
Torpedo  they  lie  on  each  side  of  the  head,  between  the  gills  and  the 
anterior  part  of  the  pectoral  fin.  In  other  cases  where  they  are  slightly 
developed,  both  in  Elasmobranchs  and  Teleosteans,  they  lie  in  the  tail. 
Separated  from  one  another  by  connective  tissue  partitions,  are  numerous 
"  electric  plates,"  which  consist  of  strangely  modified  muscle  substance 
and  numerous  nerve  endings.  The  electric  discharge  is  very  distinct  in 
the  three  forms  noted  above,  and  is  controlled  in  some  measure  at  least 
by  the  animal. 

The  Skeleton. 

The  skeleton  is  for  the  most  part  cartilaginous,  but  here 
and  there  ossification  has  taken  place,  as  a  crust  over  many 
parts,  but  more  deeply  in  the  vertebral  bodies,  in  the  teeth, 
and  in  the  tooth-like  scales. 


THE  SKELETON. 


479 


The  vertebral  column  consists  of  an  anterior  plate  not 
divided  into  vertebrae,  and  of  a  posterior  series  of  distinct 
vertebral  bodies.  Each  of  these  has  a  biconcave  or 
amphiccelous  centrum.  From  each  side  of  the  centrum  a 
transverse  process  projects  backwards,  and  bears  a  minute 
hint  of  a  rib.  From  the  dorsal  surface  of  each  centrum  rise 
two  neural  processes,  which  arch  upwards  on  each  side  of  the 
spinal  cord ;  the  arch  is  continued  upwards  in  inter-neural 
plates  which  meet  in  a  neural  spine  on  the  top.  On  the 


FlG.  154. — Under  surface  of  skull  and  arches  of  skate. 
(After  W.  K.  PARKER). 


h.br.    1-5,    hypo-branchials  ;    c.br.   5,    fifth  cerato-branchial  ; 
cerato-hyal  ;  /.  2-4,  labial  cartilages. 

caudal  vertebrae,  what  seem  to  be  the  transverse  processes 
are  directed  downwards,  to  form  a  haemal  arch  enclosing  an 
artery  and  a  vein.  In  the  lozenge-shaped  spaces  between 
the  vertebrae  lie  gelatinous  remains  of  the  notochord.  The 


480  FISHES. 

vertebral  column  develops  from  the  mesodermic  sheath  of 
the  endodermic  notochord. 

The  skull  is  a  cartilaginous  case,  with  a  spacious  cavity 
for  the  brain,  a  large  posterior  aperture  or  foramen  magnum 
through  which  the  spinal  cord  passes,  a  large  ear  capsule  on 
each  side  posteriorly,  a  similar  nose  capsule  on  each  side 
anteriorly,  a  long  snout  or  rostrum  projecting  in  front,  two 
incomplete  regions  or  fontanelles  on  the  roof.  Compared 
with  the  skull  of  a  cod  or  of  a  higher  Vertebrate,  that  of  a 
skate  is  simple ;  it  is  not  ossified,  nor  divided  into  distinct 
regions,  nor  has  it  anything  corresponding  to  the  investing 
membrane  bones,  which  in  higher  animals  are  added  to 
the  original  foundations  of  the  skull,  nor  do  the  visceral 
arches  in  the  skate  take  part  in  forming  the  skull,  which 


FIG.  155. — Side  view  of  skate's  skull.     (After  W.  K. 
PARKER.) 

71,  First  labial  cartilage;  n.c.,  nasal  capsule;  a.o.,  ant-orbital ; 
p.pt.q.,  palato-pterygo-quadrate  ;  M.c.,  Meckel's  cartilage  ;  h.m., 
hyo-mandibular  ;  e,h.,  epi-hyal:  c.k.,  cerato-hyal ;  h.h.,  hypo-hyal ; 
h.br.  1-5,  hypo-branchial ;  c.br. ,  cerato-branchial ;  e.br.,  epi-branchial ; 
p.br* '.,  first  pre-branchial  ;  z'.^z.,  inter-hyal  ;  m.pt.^  meta-pterygoid  ; 
2>  5>  7?  foramina  of  these  nerves. 

arises,  as  usual  (see  p.  427),  from  parachordals,  trabeculae, 
sense  capsules,  &c. 

The  visceral  arches  are  primitively  supports  for  the  wall  of 
the  anterior  part  of  the  food  canal,  but  at  least  two  of  them 
are  much  modified  alike  in  position  and  function. 

The  upper  jaw  of  the  skate  is  a  strong  transverse  bar, 
formed  from  the  union  of  two  palato-pterygo-quadrate 
cartilages.  The  lower  jaw  is  a  similar  bar  formed  from 
the  union  of  two  MeckeFs  cartilages. 


h.m 


s.t. 


st 


-pu 


31 


FIG.  156. — Skeleton  of  Skate.     (From  a  preparation.) 

h.vi.,  Hyo-mandibular  ;  s.t.,  sensory  tube  ;  h.br.,  hypo-branchial, 
No.  5  ;  v.pl. ,  vertebral  plate  ;  c.,  coracoid  region  ;  s.v.,  spiral  valve  ; 
s.c.,^  scapular  region;  n.c.,  nasal  capsule;  p.q.,  palato-quadrate- 
cartilage  (upper  jaw)  ;  M.,  Meckel's  cartilage  (lower  jaw)  ;  /.//., 
pro-pterygium  ;  m.pt.,  _meso-pterygium  ;  vit.pt.,  meta-pterygium  ; 
st.,  stomach  ;  pu.,  pubic  bar. 


482  FISHES. 

From  the  ear  capsule  to  the  articulation  of  upper  and 
lower  jaw  there  extends  on  each  side  a  club  shaped  cartilage 
known  as  the  hyo-mandibular.  Attached  to  this  is  a  slender 
three  jointed  rod — the  hyoid. 

Then  follow  five  branchial  arches,  each  primarily  four- 
jointed,  forming  the  framework  of  the  gill-bearing  region. 

Of  less  importance  are  four  labial  cartilages  about  each 
nose  capsule,  an  antorbital  cartilage  uniting  the  nose  capsule 
with  the  end  of  the  pectoral  fin,  and  a  spiracular  cartilage 
supporting  the  rudimentary  gill  in  the  spiracle. 

The  dorsal  or  scapular  region  of  the  pectoral  girdle  is 
fixed  on  each  side  to  the  crest  of  the  vertebral  plate  by 
means  of  a  supra-scapula.  The  ventral  region  of  the  girdle 
is  distinguished  as  the  coracoid  portion.  The  outer  edge 
bears  three  facets,  to  which  the  three  basal  pieces  of  the 
pectoral  fin  are  fixed. 

Of  these  three  basal  pieces  the  anterior  or  propterygium 
and  the  posterior  or  metapterygium  are  large,  the  median  or 
mesopterygium  is  small.  All  bear  jointed  radials.  The 
true  fin  rays,  comparable  to  the  dermal  rays  in  the  fins  of 
Bony  Fishes,  are  represented  by  "  horny  "  fibres. 

The  pelvic  girdle  is  simpler  than  the  pectoral,  and  is  not 
fixed  to  the  vertebral  column.  Its  dorsal  region  is  pro- 
longed into  an  iliac  process,  while  anteriorly  a  pre-pubic 
process  projects  from  the  ventral  (pubic)  bar.  The  girdle 
bears  two  articulating  facets,  to  the  posterior  of  which  the 
the  strong  basal  piece  or  metapterygium  of  the  hind  limb  is 
attached.  From  this,  and  from  the  anterior  facet  of  the 
girdle,  the  jointed  radials  proceed.  The  claspers  of  the 
males  are  closely  connected  with  the  hind  limb,  and  have  a 
very  complex  cartilaginous  skeleton  and  an  associated  gland. 

The  Brain. 

The  brain  (see  p.  435)  has  the  following  parts  : — 

(1)  The  fused  cerebral  hemispheres  or  prosencephalon,  with  a 

nervous  roof,  and  without  ventricles. 

(2)  The  thalamencephalon  or  region  of  the  optic  thalami,  with 

a  thread-like  pineal  body  above,  infundibulum  and  pituit- 
ary body  below,  thinly  roofed  third  ventricle  within. 

(3)  The  mesencephalon  or  mid-brain  with  the  optic  lobes  above, 

the  crura  cerebri  below,  the  iter  passing  between. 
(4)  The  cerebellum  with  an  anterior  and  a  posterior  lobe,  both 
marked  by  ridges  and  grooves. 


CRANIAL  NERVES.  483 

(5)  The  medulla  oblongata,  with  thin  vascular  roof,  with  dorso- 

lateral  extensions  called  "  restiform  bodies." 

The  region  beneath  the  thalamencephalon  bears  (a)  two  ovoid  inferior 
lobes,  (b}  the  infundibulum  which  carries  the  pituitary  body,  and  (c]  a 
thin-walled  three-lobed  saccus  vasculosus  situated  between  the  pituitary 
body  and  the  inferior  lobes. 

Cranial  Nerves.1 

Owing  to  the  flat  form  of  the  skate  and  its  frequently  large 
size,  the  dissection  of  the  cranial  nerves  is  perhaps  easier 
than  in  any  other  Vertebrate.  Expecting  practical  verifica- 
tion, we  shall  describe  their  distribution  in  some  detail, 
following  in  regard  to  certain  points  the  investigations  of 
Professor  Cossar  Ewart. 

I.  The  olfactory,  rising  from  the  olfactory  lobes  of  the 
cerebral  hemispheres,  extend  to  the  nostrils,  and 
there  expand  in  olfactory  bulbs,  which  give  off 
nerves  to  the  nostrils. 

II.  The  of  tic,  leaving  the  region  of  the  optic  thalami, 
cross  in  an  optic  chiasma,  and  extend  to  the 
retina  of  the  eye. 

III.  The  oculomotor  or  ciliary,  arising  from  the  crura 
cerebri,  near  the  mid-ventral  line,  supply  four  of 
the  six  muscles  of  the  eye.  There  is  a  ciliary 
ganglion  in  connection  with  III.,  and  also  with 
the  ganglion  of  the  ophthalmicus  profundus. 
IV.  The  pathetic  or  trochlear  are  small  nerves  arising 
dorsally  from  between  the  mid  and  hind  brain, 
and  supplying  the  superior  oblique  muscles  of  the 
eye.  It  is  possible  that  they  really  belong  to  V. 
V.  The  trigeminal,  or  nerve  of  the  "  mouth-cleft,"  aris- 
ing from  the  medulla  oblongata  (as  do  all  that 
follow),  has  a  (Gasserian)  ganglion  on  its  root, 
and  three  main  branches — the  sensory  maxillary, 
which  unites  with  the  inner  buccal  of  VII. ;  the 
motor  mandibular,  which  innervates  the  muscles 
of  the  jaws ;  and  the  sensory  superficial  ophthal- 
mic (or  orbitonasal),  which  runs  over  the  eye  to 
the  snout,  and  comes  into  close  relations  with  a 

1  I  have  to  acknowledge  indebtedness  to  Dr.  Beard  for  his  kindness  in 
helping  me  to  state  the  distribution  of  these  nerves  correctly,  or  as  cor- 
rectly as  is  at  present  possible. 


484  FISHES 

similar  branch  of  VII.  Internal  to  the  mandi- 
bular  branch  lies  the  ganglionated  ophthalmicus 
profundus,  which  sends  branches  to  the  eyeball, 
snout,  &c.,  and  is  referred  by  some  to  III.,  by 
others  to  V.,  and  is  regarded  by  others  as  an 
independent  nerve. 


FIG.  157. — Dissection  of  nerves  of  Skate. 

nix.,  Maxillary  of  V. ;  inn.,  mandibular  of  V. ;  pn.,  ophthalmicus 
profundus;  s.o.,  superficial  ophthalmic  of  V.;  s.01.,  superficial  ophthal- 
mic of  VII.;  V.,  VII.,  VIII.,  IX.,  X.,  these  nerves;  1-5.,  gill  clefts  ; 
#./.,  optic  lobes  ;  Cb.,  cerebellum  ;  N.,  nostril ;  e.,  eye  ;  M.,  muscles 
of  jaws;  sp.,  spiracle;  E.,  ear;  m.o.,  medulla  oblongata;  Br.p. 
brachial  plexus;  I.,  II.,  III.,  IV.,  VI.,  VII.,  these  nerves. 


CRANIAL  NERVES.  485 

VI.  The  abducens,  a  slender  nerve,  arising  near  the  mid- 
ventral  line,  adjacent  to  V.  and  VIII.,  and  hidden 
by  the  former,  supplies  the  external  rectus  muscle 
of  the  eye. 

VII.  The  facial^  morphologically  the  nerve  of  the  spir- 
acular  cleft,  supplies  all  the  five  groups  of 
ampullae  on  the  head,  and  has  seven  main 
branches. 

1.  The  ophthalmicus  superficialis  runs  over  and  past 

the  eye,  unites  with  a  similar  branch  of  V.,  and 
supplies  ampullae  on  the  snout. 

2.  The  inner  buccal  runs  under  the  eye,  through  the 

nasal  capsule,  to  inner  buccal  ampullae.  The 
outer  buccal  runs  under  the  eye,  over  the  antor- 
bital  cartilage,  to  outer  buccal  ampullae. 

3.  The  hyomandibular  runs  directly  outwards  behind 

the  spiracle  to  hyoid  ampullae. 

4.  The  external  mandibular  runs  behind  and  under 

the  spiracle  along  the  jaw  to  mandibular  am- 
pullae, and  is  a  branch  of  the  hyo-mandibular. 

5.  The  palatine  runs  over  the  spiracle  to  the  roof  of 

the  mouth. 

6.  The  "facial  proper"  supplies  the  muscles  of  the 

hyoid  arch,  and  gives  off — 

7.  The  "  chorda  tympani,"  which  runs  under  the  spir- 

acle to  the  inner  side  of  the  jaw. 
With  the  loss  of  the  sensory  ampullae,  the  seventh 

nerve  of  higher  Vertebrates  becomes  restricted  to 

the  last  three  branches. 
A  recurrent  branch  of  the  facial  also  runs  under 

the  auditory  capsule  to  IX.,  and  is  equivalent  to 

Jacobson's  anastomosis  in  higher  forms. 

VIII.  The  auditory,  arising  just  behind  VII.,  is  the  nerve 

of  the  ear. 

IX.  The  glossopharyngeal,  the  most  typical  of  all,  is  the 
nerve  of  the  first  functional  gill  cleft.  Its  root 
passes  through  the  floor  of  the  auditory  capsule, 
and  bears  a  ganglion  above  the  cleft.  Its 
branches,  as  named  by  Beard,  are : — 

(a)  Post-branchial,  to  the  muscles  of  the  first  branchial 

arch  ; 

(b]  prae-branchial,  arches  over  the  cleft  and  runs  along 

its  front  wall ; 

(c]  intestinal  or  visceral,  to  the  pharynx  ; 

(d)  supra-branchial  or  dorsal  to  a  few  sense  organs  on 

the  mid  dorsal  line  of  the  head. 


486  FISHES. 

X.  The  vagus )  apparently  made  up  of  at  least  four 
cranial  nerves,  has  five  roots,  and  divides  into 
six  main  ganglionated  portions,  which  supply 
the  four  posterior  clefts  and  arches,  the  posterior 
jelly-tubes,  and  the  heart  and  stomach.  It  thus 
consists  of: — 

(a)  A  ganglionated  root  to  the  clefts  and  arches  (2  to  5 

inclusive),  with  post  -  branchial,  prae- branchial, 
and  pharyngeal  branches,  as  in  IX. 

(b]  A  ganglionated  root,  arising  in  front  of  all  the 

others,  from  which  arises  the  lateral  branch  in- 
nervating all  the  posterior  sensory  tubes, 
(r)  From  the  fourth  branchial  branch  arises  the  gan- 
glionated intestinal  which  innervates  the  heart 
and  the  stomach. 


FIG.  158. — Side  view  of  chief  cranial  nerves  of  Elasmo- 
branchs.     (Slightly  modified  from  COSSAR  EWART.) 

olf.)  Over  olfactory  nerve;  ck.,  over  cerebral  hemispheres;  cb., 
over  cerebellum;  m.o.,  over  medulla  oblongata ;  ;;z.,  mouth;  vix., 
maxillary  branch  of  5  ;  mn.$.,  mandibular  branch  of  5  ;  vm.j.,  mandi- 
bular  branch  of  seventh  nerve;  #.1-5.,  groups  of  ampullae;  o.s.$., 
superficial  ophthalmic  of  5;  o.p.,  ophthalmicus  profundus ;  o.s.'j., 
superficial  ophthalmic  of  7  ;  N.t  nostril;  3.,  oculomotor ;  e.g.,  ciliary 
ganglion  ;  5.,  trigeminal ;  /.£.,  inner  buccal ;  o.b.,  outer  buccal ;  7.^., 
buccal  of  7  ;  /,  palatal  of  7;  sp.,  spiracle;  ch.,  chorda  tympani ; 
j.hm.,  hyomandibular  of  7  ;  8.,  auditory  ;  £.,  ear;  9.,  glossopharyn- 
geal ;  10.,  roots  of  vagus;  /.io.,  lateral  nerve  of  vagus  ;  z'.io.,  intes- 
tinal nerve  of  vagus  ;  I'-s',  gill  clefts. 

The  spinal  cord  lies  in  the  cartilaginous  neural  archway 
above  the  vertebral  column,  is  divided  by  deep  dorsal  and 
ventral  fissures,  and  gives  off  numerous  spinal  nerves, 
formed  as  usual  from  the  union  of  dorsal  (sensory)  and 


SENSE   ORGANS—ALIMENTARY  SYSTEM, 


487 


ventral  (motor)  roots.  The  first  sixteen  or  eighteen  nerves 
form  the  brachial  plexus,  converging  and  uniting  in  a  trunk 
which  supplies  the  pectoral  fin. 

The  sympathetic  system  consists  of  a  longitudinal  ganglion- 
ated  cord  along  each  side  of  the  vertebral  column.  The 
ganglia  of  these  cords  are  connected  with  the  spinal  nerves. 

Sense  Organs. 

(a)  The  Eyes  (see  p.  445).  The  iris  has  a  beautifully  fringed  upper 
margin. 

(1}}  The  Ears  (see  p.  444).  The  vestibule  is  connected  with  the  sur- 
face by  a  delicate  canal — the  aqueductus  vestibuli — a  remnant 
of  the  original  invagination.  A  small  part  of  the  wall  of  the 
auditory  capsule  is  covered  only  by  the  skin  forming  a  kind  of 
tympanum.  Within  the  vestibule  are  calcareous  otolithic  par- 
ticles surrounded  by  a  jelly. 

(c)  The  Nasal  sacs  are  cup-like  cavities  with  plaited  walls. 

(d)  The  Sensory  tubes  are  best  seen  on  the  ventral  surface,  where  they 

lie  just  under  the  skin.      They  end  in  ampulloe,    containing 
sensory  cells. 

Alimentary  System. 

The  mouth  is  a  transverse  aperture ;  the  teeth  borne  by 
the  jaws  are  numerous,  and  those  worn  away  in  front  are 
replaced  by  fresh  teeth  from  be- 
hind; naso-buccal  grooves  connect 
the  nostrils  with  the  corners  of  the 
mouth ;  the  spiracles,  which  open 
dorsally  behind  the  eyes,  communi- 
cate with  the  buccal  cavity;  from  the 
gullet  five  gill  clefts  open  ventrally 
on  each  side;  the  stomach  lying 
rather  to  the  left  is  bent  upon  itself ; 
the  large  brownish  liver  is  trilobed, 
and  has  an  associated  gall  bladder, 
from  which  the  bile  duct  extends 
to  the  duodenum  —  the  part  of 
the  gut  immediately  succeeding  the 
stomach ;  the  whitish  pancreas  lies 
in  the  duodenal  loop  between 
stomach  and  intestine,  and  its  duct 
opens  opposite  the  bile  duct ;  the  intestine  contains  an  in- 
ternal spiral  fold — a  membrane  which  increases  the  absorp- 


FIG.  159. — Spiral  valve 
of  Skate.  (After  T.  J. 
PARKER.  ) 


488 


FISHES. 


tive  surface ;  a  small  rectal  gland  of  unknown  significance 
is  attached  to  the  terminal  or  rectal  portion  of  the  gut ;  the 
end  of  the  gullet  and  the  anterior  portion  of  the  stomach 
and  the  rectum  are  supported  by  folds  of  peritoneum, — the 
membrane  which  lines  the  body  cavity, — the  rest  of  the  gut 
lies  freely ;  into  the  terminal  chamber  or  cloaca  the  rectum, 
the  ureters,  and  the  genital  ducts  all  open ;  an  abdominal 
pore  opens  on  each  side  of  the  cloacal  aperture.  Excepting 
mouth-cavity  and  cloaca,  the  gut  is  lined  by  endoderm. 

Respiratory  System. 

The  first  apparent  gill  clefts — the  spiracles — open  dorsally 
behind  the  eyes.     Each  contains  a  rudimentary  gill  on  the 


FIG.  1 60. — Upper  part  of  the  dorsal  aorta  in  the  Skate. 
(After  MONRO.) 

d.a.,  Dorsal  aorta;  c.,  coeliac  artery;  m.,  superior  mesenteric ; 
s.cl.)  subclavian  ;  e.b.,  efferent  branchial  vessels,  three  formed  from 
the  union  of  nine ;  v.,  vertebral ;  c.,  carotid. 

anterior  wall,  supported  by  a  spiracular  cartilage.     Through 
the  spiracles  water  may  enter  or  leave  the  mouth. 

There  are  five  pairs  of  gill  cavities,  separated  by  parti- 
tions, and  with  ventral  apertures.  The  first  cavity  is 
bounded  anteriorly  by  the  hyoid  arch,  posteriorly  by  the 
first  branchial  arch.  The  hyoid  arch  bears  branchial  fila- 
ments on  its  posterior  surface;  the  first  four  branchial 
arches  bear  gill  filaments  on  both  surfaces ;  the  fifth 


CIRCULATORY  SYSTEM. 


489 


branchial  arch  bears  none.  Each  of  the  first  four  branchial 
arches  bears  a  half  gill  on  each  side;  thus,  including  the 
gill  filaments  borne  on  the  posterior  side  of  the  hyoid,  there 
are  four  and  a  half  gills.  The  absence  of  an  operculum  or 
gill  cover  is  obvious.  The  gills  are  outgrowths  from  the 
wall  of  the  gut,  and  therefore  endodermic. 

Circulatory  System. 

The  impure  blood  from  the  body  enters  the  heart  by  a 
bow-shaped  sinus  venosus,  which  leads  into  a  large  thin- 
walled  auricle.  Thence  through  a  bivalved  aperture  the 


FIG.  161. — Heart  and  adjacent  vessels  of  Skate.     (In  part 
after  MONRO.) 

v.,  Ventricle  ;  c.a.,  conus  arteriosus ;/./.,  posterior  innominate; 
v.a.,  ventral  aorta;  a.i.,  anterior  innominate;  Th.,  thyroid;  in., 
mouth  ;  «.,  auricle  ;  s.z>.,  sinus  venosus  ;  s.c.,  precaval  sinus  or  sinus 
of  Cuvier;  k.s.,  hepatic  sinus;  /.,  jugular;  6r.,  brachials ;  cd.,  car- 
dinal ;  epg. ,  epigastric. 

blood  passes  into  the  smaller  muscular  ventricle,  and  from 
this  it  is  driven  through  a  contractile  conus  arteriosus, 
with  three  longitudinal  rows  of  five  valves,  into  the  ventral 
aorta. 

The  ventral  aorta  gives  off  a  pair  of  posterior  innominate 
arteries,  which  take  blood  to  the  three  posterior  gills,  and  a 


490  FISHES. 

pair  of  anterior  innominate  arteries,  which  supply  the  anterior 
gill  and  the  hyoid  half  gill  on  each  side. 

The  purified  blood  passes  from  each  half  gill  by  an 
efferent  branchial  artery.  To  begin  with,  there  are  nine 
of  these  on  each  side,  but  by  union  they  are  reduced  to 
three  efferent  trunks,  which  combine  to  form  the  dorsal 
aorta. 

From  the  efferent  branchial  of  the  hyoid  arch,  a  carotid 
arises,  which  divides  into  internal  and  external  branches 
supplying  the  brain  and  head.  From  the  first  of  the 
efferent  trunks,  a  vertebral  arises  which  supplies  the  brain 
and  spinal  cord. 

The  dorsal  aorta  gives  off — (i)  a  subclavian  to  each 
pectoral  fin ;  (2)  a  coeliac  to  the  stomach,  duodenum,  and 
liver ;  (3)  a  superior  mesenteric  to  the  intestine,  pancreas, 
and  spleen ;  (4)  spermatic  arteries  to  the  reproductive 
organs ;  (5)  an  inferior  mesenteric  to  the  rectum  ;  (6)  renal 
arteries  to  the  kidneys;  (7)  arteries  to  the  pelvic  fins.  It 
ends  in  the  caudal  artery. 

At  each  end  of  the  bow-shaped  sinus  venosus,  there  is 
a  pre-caval  sinus.  This  receives  venous  blood  as  follows  : — 

(a)  from  the  head  by  a  jugular  vein ;    (b)  from  the  liver 
by  a  hepatic  sinus,  which  runs  from   one  pre-caval  sinus 
to  the  other  like  the  string  of  the  bow;  (c)  from  a  large 
posterior  cardinal  sinus  (between  the  reproductive  organs) 
by  a  cardinal  vein  on  each  side ;    (d)  from  the   hind-fin 
by  an  epigastric,  with  which  brachials  from  the  fore-limb 
unite  anteriorly.     The  great  cardinal  sinus  receives  blood 
from  the  hind  limbs,  the  kidneys,  and  other  posterior  parts. 

Blood  passes  into  the  liver  (a)  from  the  coeliac  artery,  and 

(b)  by  portal  veins  from   the  intestine  (the  hepatic  portal 
system) ;  blood  leaves  the  liver  by  hepatic  veins  which  enter 
the  hepatic  sinus. 

Blood  passes  into  the  kidneys  (a)  from  the  renal  arteries, 
and  (b)  by  renal  portal  veins  from  the  caudal,  pelvic,  and 
lumbar  regions  (the  renal  portal  system) ;  blood  leaves  the 
kidneys  by  posterior  cardinal  veins,  which  enter  the  cardinal 
sinus. 

Into  the  pre-caval  sinus  there  also  opens  the  lymphatic 
trunk,  with  nutritive  fluid  from  the  intestine. 

The  heart  lies  in  a  pericardial  cavity,  which  is  connected 


EXCRETORY  SYSTEM. 


491 


with  the  abdominal  cavity  by  two  fine  canals,  and  is  an 
anterior  part  of  the  coelome.  The  blood  contains,  as  usual, 
red  blood  corpuscles  and  leucocytes. 

The  dark  red  spleen  lies  in  the  curve  of  the  stomach. 
The  red  thyroid  gland  lies  just  in  front  of  the  anterior  end 
of  the  ventral  aorta.  The  whitish  thymus  gland  is  a  paired 
structure  lying  dorsally  above  the  gills. 

Excretory  System. 

Excretory  System  (see  pp.  458-462). — The  elongated,  dark 
red  kidneys  lie  posteriorly  on  each  side  of  the  vertebral 


FIG.  162. — Urinogenital  organs  01  male  Skate.     (From  a 
specimen  in  Edinburgh  Museum  of  Science  and  Art.) 

7\,  Testis  ;  Ep.,  epididymis  ;  v.d.,  vas  deferens  :  K.>  kidney ;  v.s.,    . 
seminal  vesicle;   s.s.,  sperm   sac;    u.g.s.,  urinogenital  sinus;   C7., 
cloaca. 

column.  They  are  developed  from  the  hind  part  of  the 
mesonephros.  Several  tubes  from  each  kidney  combine  to 
form  a  ureter.  The  two  ureters  of  the  male  open  into  the 
urinogenital  sinus,  whence  the  waste  products  pass  out  by 


492  FISHES. 

the  cloaca ;  in  the  female  they  open  into  little  bladders — the 
dilated  ends  of  the  Wolffian  ducts,  and  thence  by  a  common 
aperture  into  the  cloaca. 

The  archinephric  or  segmental  duct  of  each  side  divides 
into  a  Wolffian  and  a  Mlillerian  duct.  The  Wolffian  duct 
becomes  in  the  male  the  vas  deferens,  in  the  female  an 
unimportant  mesonephric  duct ;  the  Miillerian  duct  becomes 
in  the  female  the  oviduct,  and  in  the  male  a  mere  rudiment. 

The  muscles  and  other  organs  of  Elasmobranchs  retain 
considerable  quantities  of  nitrogenous  waste  products. 


FIG.  163. — Urinogenital  organs  of  female  Skate.     (In 
part  after  MONRO.) 

«/.,  Aperture  of  united  oviducts;  W.D.>  Wolffian  duct;  <??./., 
ovary;  O.D.G.,  oviducal  gland;  £.,  egg  in  mermaid's  purse;  BL, 
bladder  at  base  of  Wolffian  ducts  (arrow  into  cloaca);  K.,  kidney 
(arrow  from  base  of  oviduct  into  cloaca). 

Reproductive  System. 

The  male  organs  or  testes  lie  on  each  side  of  the  cardinal 
sinus,  moored  by  a  fold  of  peritoneum.  Spermatozoa  pass 
from  the  testis  by  vasa  efferentia  into  a  tube  surrounded 


DE  VEL  OPMENT.  493 

anteriorly  by  an  epididymis.  The  tube  of  the  epididymis  is 
continued  into  the  vas  deferens,  which  is  dilated  posteriorly 
into  a  seminal  vesicle  and  an  adjacent  sperm  sac.  Finally, 
the  two  vasa  deferentia  open  into  the  urinogenital  sinus, 
through  which  the  spermatozoa  pass  into  the  cloaca.  In 
copulation,  the  complex  "claspers"  of  the  male  are  inserted 
into  the  cloaca  of  the  female. 

The  female  organs  or  ovaries  lie  on  each  side  of  the  car- 
dinal sinus,  moored  by  a  fold  of  peritoneum.  In  young 
skates  they  are  like  the  young  testes,  but  in  the  adults  they 
are  covered  with  large  Graafian  follicles,  each  containing  an 
ovum.  The  ripe  ova  burst  into  the  body  cavity,  and  enter 
the  single  aperture  of  the  oviducts,  which  are  united  an- 
teriorly behind  the  heart.  About  the  middle  of  each  oviduct 
there  is  a  large  oviducal  gland,  which  secretes  the  "  purse ; " 
the  elastic  lower  portions  open  into  the  cloaca. 

Development. 

The  ripe  ovum  which  bursts  from  the  ovary  is  a  large 
sphere  of  yolk,  with  the  formative  protoplasm  concentrated 
at  one  pole. 

In  Elasmobranchs  formation  of  polar  bodies  (maturation) 
takes  place  at  an  early  stage. 

In  the  upper  part  of  the  oviduct  the  ovum  is  fertilised. 
It  is  said  by  some  that  numerous  spermatozoa  often  enter 
the  Elasmobranch  ovum,  although  only  one  is  actually 
concerned  in  fertilisation. 

As  the  ovum  descends  further  it  is  surrounded  by 
albuminous  material,  and  by  the  four-cornered  "  mermaid's 
purse "  secreted  by  the  walls  of  the  oviducal  gland.  This 
purse  is  composed  of  keratin — a  common  skeletal  substance 
which  occurs  for  instance  in  hair  and  nails.  Its  corners  are 
produced  into  long  elastic  tendrils,  which  may  twine  round 
sea  weed,  and  thus  moor  the  egg.  Rocked  by  the  waves, 
the  embryo  develops,  and  the  young  skate  leaves  the  purse 
at  one  end. 

The  segmentation  is  meroblastic,  being  confined  to  the 
disc  of  formative  protoplasm.  From  the  edge  of  the  blasto- 
derm, or  segmented  area,  some  nuclei  (so-called  "mero- 
cytes  ")  are  formed  in  the  outer  part  of  the  subjacent  yolk 
(Fig.  164,  «.).-  According  to  some,  these  yolk  nuclei  after- 


494  FISHES. 

wards  share  in  the  making  of  the  embryo.  On  the  other 
hand,  it  has  been  said  that  they  do  nothing,  even  that  they 
are  the  heads  of  numerous  spermatozoa  which  have  suc- 
ceeded in  entering  the  ovum. 

At  the  close  of  segmentation  the  blastoderm  is  a  lens- 
shaped  disc  with  two  strata  of  cells.  It  is  thicker  at  one 
end — where  the  embryo  begins  to  be  formed.  Towards  the 
other  end,  between  the  blastoderm  and  the  yolk,  lies  a  seg- 
mentation cavity  (Fig.  164,  sg.c.). 

At  the  embryonic  end,  the  outer  layer  or  epiblast  under- 
goes a  slight  invagination  (Fig.  164,  x.),  beginning  to  form 
the  roof  of  the  future  gut  (^.),  in  other  words  establishing 
the  hypoblast.  This  inflected  arc  of  the  blastoderm  corres- 
ponds to  the  blastopore  or  mouth  of  the  gastrula,  which  is 


sg.c. 


FIG.  164. — Elasmobranch  development.     (After  BALFOUR.) 

Uppermost  figure  shows  blastoderm  at  an  early  stage.  Ep., 
epiblast ;  sg-.c.,  segmentation  cavity  ;  n.,  yolk  nuclei. 

Middle  figure  shows  the  invagination  which  forms  the  gut.  x., 
blastopore;  g. ,  archenteron.  Mesoderm  dark. 

Lowest  figure,  a  longitudinal  section  at  a  later  stage.  Ep. , 
epiblast;  n.c.,  neural  canal ;  ne.c.,  neurenteric  canal ;  g.,  gut ;  »., 
notochord.  Mesoderm  dark. 

much  disguised  by  the  presence  of  a  large  quantity  of  yolk. 
As  the  invagination  proceeds,  the  segmentation  cavity  is 
obliterated.  The  floor  of  the  gut  is  formed  by  infolding  of 
the  lateral  walls. 

Along  the  mid  dorsal  line  of  the  epiblast  a  medullary 
groove  appears  —  the  beginning  of  the  central  nervous 
system.  Its  sides  afterwards  arch  towards  one  another,  and 
meet  to  form  a  medullary  canal  (Fig.  164,  n.c.}.  A  posterior 


FORM  AND  EXTERNAL  FEATURES.  495 

communication  between  this  dorsal  nervous  tube  above  and 
the  ventral  alimentary  tube  persists  for  some  time  as  the 
neurenteric  canal  (Fig.  164,  n.e.c.}. 

The  mesoblast  arises  as  two  lateral  plates,  one  on  each 
side  of  the  medullary  groove.  The  plates  seem  to  arise  as 
a  pair  of  solid  outgrowths  from  the  wall  of  the  gut.  They 
are  afterwards  divided  into  segments.  Between  the  meso- 
blast plates,  along  the  mid  dorsal  line  of  the  gut,  the  noto- 
chord  is  established  (Fig.  164,  n.). 

Besides  the  internal  establishment  and  differentiation  of 
layers,  there  are  two  important  processes,  (a)  the  growth  of 
the  blastoderm  around  the  yolk,  (b]  the  folding  off  of  the 
embryo  from  the  yolk.  The  yolk  is  thus  enclosed  in  a  yolk 
sac,  with  which  the  embryo  is  finally  connected  only  by  a 
thin  stalk — the  umbilical  cord.  Through  the  canal  of  this 
cord  nutriment  is  absorbed  into  the  gut,  and  blood  vessels 
also  effect  absorption. 

What  the  different  layers  form,  and  how  the  organs  arise, 
may  be  inferred  from  the  general  conclusions  stated  else- 
where. 


Second  type  of  FISHES.  The  Haddock  (Gadus  czglefinus] 
— A  type  of  Teleosteans  with  closed  swim  bladder 
(Physoclisti). 

Form  and  External  Features. 

The  elongated  wedge-like  form  is  well  adapted  for  rapid 
swimming.  The  terminal  mouth  bears  a  short  barbule; 
this  is  long  in  the  cod  (G.  morrhua),  and  absent  in  the 
whiting  (G.  merlangus).  The  nostrils,  situated  near  the 
end  of  the  snout,  have  double  apertures.  The  eyes  are  lid- 
less,  but  covered  with  transparent  skin.  Over  the  gill 
chamber  and  the  four  gills  lies  the  operculum,  supported  by 
several  bones.  Distinct  from  one  another,  but  closely 
adjacent,  are  the  anal,  genital,  and  urinary  apertures, — 
named  in  order  from  before  backwards.  Along  the  sides  of 
the  body  runs  the  dark  lateral  line  containing  sensory  cells. 
There  are  three  dorsal  and  two  anal  fins,  and  an  apparently 
symmetrical  tail  fin. 


496 


FISHES. 


Skin. 

The  small  scales  which  cover  the  body  are  developed  in 
the  dermis,  and  are  without  any  bone  cells.  Their  free 
margin  is  even,  a  characteristic  to  which  the  term  cycloid  is 
applied,  in  contrast  to  ctenoid,  which  describes  those  scales 
which  have  a  notched  or  comb-like  free  margin.  Over  the 
scales  extends  a  delicate  partially  pigmented  epidermis. 

Appendages. 

The  pectoral  fins  are  attached  to  the  shoulder  girdle  just 
behind  the  branchial  aperture.  The  pelvic  or  ventral  fins, 
attached  to  what  is  at  most  a  rudiment  of  the  pelvic  girdle, 
lie  below  and  slightly  in  front  of  the  pectorals — far  from  the 
normal  position  of  hind  limbs. 


FIG.  165. — External  characters  of  a  Teleostean — a  carp. 
(After  LEUNIS.) 

R.,  Dorsal  unpaired  fin  ;  S^.,  homocercal  caudal  fin  :  A.,  anal  fin  ; 
B.B.)  Pectoral  and  pelvic  paired  fins.  Note  also  the  lateral  line  and 
barbule. 

Muscular  System. 

The  main  muscles  of  the  body  are  disposed  in  segments, 
— myotomes  or  myomeres,  separated  by  partitions  of  con- 
nective tissue. 

Skeleton. 

The  Vertebral  column  consists  of  biconcave  or  amphi- 
ccelous  bony  vertebrae.  Each  centrum  in  the  trunk  region 
bears  superior  neural  processes,  uniting  in  a  neural  arch 
crowned  by  a  neural  spine,  and  transverse  processes  pro- 


SKELETON. 


497 


jecting  from  each  side.  Articulated  to  the  distal  ends  of 
the  transverse  processes  are  the  downward  curving  ribs,  and 
also  more  delicate  intermuscular  bones  which  curve  upwards. 
In  the  caudal  vertebrae,  the  centra  (c.)  bear  not  only 
superior  neural  processes  (n.a.),  but  also  inferior  haemal 
processes  (h.a.\ 

At  the  end  of  the  vertebral  column  lies  a  fan-shaped 
hypural  bone  which  helps  to  support  the  tail.     The  fin  rays 
are  jointed  flexible  rods,  which  in  the  dorsal  and  anal  fins  are 
attached  to  the  ends  of  interspinous  bones  alternating  with 
the  neural  and  haemal  spines,  and 
attached  to  them  by  fibrous  tissue. 
The  Skull  includes  the  following 
bones,   which  may  be  grouped  in 
the    following  regions   (the   mem- 
brane bones  in  italics)  : — 

(a)  Around    the    foramen    magnum  ; 
basi-occipital,  two  ex-occipitals,  and 
a  supra- occipital. 

(b)  Along    the    roof;    .r^ra-occipital, 
parietals,    frontals,      mesethmoid, 
nasals.      Beneath  the  parietals  lie 
the  alisphenoids. 

(c)  Along    the    floor ;     basi-occipital, 
parasphenoid,  vomers. 

(d)  Around    the    ear    on    each    side  ; 
sphenotic,     pterotic,      and     epiotic 
above,      prootic      and      opisthotic 
(beneath). 

(e)  In  front  of  and  around  the  orbit ; 
Parethmoid,  lachrymal,  orbitals. 

The  first  or  mandibular  arch  is  believed  by  many  to  form 
Meckel's  cartilage  beneath,  and  the  palato-pterygo-quadrate 
cartilage  above.  Meckel's  cartilage  becomes  the  foundation 
of  the  lower  jaw,  and  bears  a  large  tooth-bearing  membrane 
bone — the  dentary,  a  small  corner  bone — the  angular,  while 
the  articular  element  is  a  cartilage  bone.  Of  the  bones 
associated  with  the  upper  part,  the  palatine  lies  in  front,  the 
quadrate  articulates  with  the  lower  jaw ;  while  between 
palatine  and  quadrate  lie  the  pterygoid,  the  mesopterygoid, 
and  the  metapterygoid. 

The  second  or  hyoid  arch  is  believed  by  many  to  form 
the  hyomandibular  and  the  symplectic  above,  and  various 

32 


FIG.  1 66. — Caudal  verte- 
bra of  haddock. 
n.a.,  Neural  arch  ;  c.,  centrum  ; 
h. a.,  haemal  arch. 


498 


FISHES. 


hyoid  bones  beneath.  The  hyomandibular  and  its  inferior 
segment  the  symplectic  connect  the  quadrate  with  the  side 
of  the  skull.  Of  the  six  hyal  bones,  the  largest  and  most 
important  is  the  ceratohyal,  which  bears  seven  long  branchio- 
stegal  rays. 

The  toothed  premaxilla  forms  the  upper  part  of  the  gape, 


Er. 


p.op. 


FIG.  167. — Disarticulated  skull  of  Cod.     (From  Edinburgh 
Museum  of  Science  and  Art. ) 

S.O.,  Supra-occipital ;  Pa.,  parietal ;  Fr.,  frontal ;  M.E.,  meseth 
moid;  N.,  nasal;  P.E.,  parethmoid  ;  Of.,  otics ;  E.O.,  ex-occi 
pital ;  B.O.,  basi-occipital ;  Pa.S.,  parasphenoid  ;  V.,  vomer ;  L. 
lachrymal ;  orb.,  orbitals  ;  H.M.,  hyomandibular  ;  S.,  symplectic 
Q.,  quadrate  ;  Pt.,  pterygoid  ;  uit.pt.,  metapterygoid  ;  ms.pt. 
mesopterygoid  ;  PL,  palatine;  MX.,  maxilla;  Pmy.,  premaxilla 
Ar.,  articular;  An.,  angular;  D.,  dentary ;  u.h.,  urohyal ;  h.h. 
hypohyal ;  c.h.,  ceratohyal;  ep.h.,  epihyal  ;  ih.,  inter-hyal;  Op.. 
opercular  ;  S.op.,  sub-opercular ;  i.op.,  inter-opercular ',  p.op.,  prae- 
opercular. 

while  the  maxilla  which  articulates  dorsally  with  the  vomer, 
and  nearly  reaches  the  quadrate  posteriorly,  does  not  enter 
into  the  gape.  Both  are  membrane  bones. 

In  the  opercular  fold  are  four  membrane  bones. 


SKELETON. 


499 


The  branchial  arches  are  divided  into  various  parts,  of 
which  the  most  interesting  are  the  two  superior  pharyngeal 
bones  which  lie  in  the  roof  of  the  pharynx  and  bear  teeth, 
and  their  counterpart,  the  inferior  pharyngeal  bone,  which 
lies  on  the  floor  of  the  pharynx,  and  is  likewise  toothed. 

The  Limbs  and  Girdles. — The  dermal  rays  of  the  pectoral 
fin  are  attached  to  four  small  brachial  ossicles ;  these  articu- 
late with  a  dorsal  scapula  and  a  more  ventral  coracoid; 
both  of  these  are  attached  to  the  inner  face  of  a  large 
clavicle,  which  almost  meets  its  fellow  of  the  other  side 


FIG.  1 68. — Pectoral  girdle  and  fin  of  Cod.     (From 
Edinburgh  Museum  of  Science  and  Art.) 

y.r.,  Fin  rays;  b.o»  brachial  ossicles  ;  cor.,  coracoid  ;  sc.,  scapula  ; 
cl.,  clavicle;  p.cl.,  post-clavicle;  s.cl.,  supra-clavicle ;  p.t.,  post- 
temporal. 

in  the  mid-ventral  line  of  the  throat.  From  the  clavicle  a 
slender  post- clavicle  extends  backwards  and  downwards ; 
while  a  stout  supra-clavicle  extends  from  the  dorsal  end  of 
the  clavicle  upwards  to  articulate  with  a  forked  post- 
temporal,  which  articulates  with  the  back  of  the  skull.  It 
must  not  be  assumed  that  the  elements  of  this  girdle  are 
directly  comparable  to  those  of  a  higher  Vertebrate,  although 
the  nomenclature  is  the  same. 


500  FISHES. 

The  fin  rays  of  each  pelvic  fin  are  attached  to  a  thin 
innominate  bone,  which  may  be  a  basal  element  of  the  fin, 
or  the  rudiment  of  a  pelvic  girdle. 

Nervous  System. 

The  relatively  small  cerebral  hemispheres,  the  thalamen- 
cephalon  with  its  inferior  lobes  and  infimdibulum,  the  large 
optic  lobes,  the  tongue-shaped  cerebellum  which  conceals 
most  of  the  medulla  oblongata,  have  their  usual  general 
relations.  Each  of  the  olfactory  nerves  is  at  first  double ; 
their  bulb-like  terminations  lie  far  from  the  brain  behind 
the  nasal  sacs.  The  large  optic  nerves  cross  one  another 
without  fusion  at  a  slight  distance  from  their  origin,  other- 
wise the  nerves  generally  resemble  those  of  the  skate. 

In  the  large  eyes,  the  different  parts  will  be  readily  identi- 
fied; the  small  nasal  sacs  with  plaited  walls  have  double 
anterior  apertures ;  the  vestibule  of  the  ear  contains  a  large 
otolith,  and  another  very  small  one  in  a  posterior  chamber. 
The  dark  lateral  line,  covered  over  by  modified  scales,  lodges 
sensory  tubes,  and  is  innervated  by  a  branch  of  the  vagus. 

Alimentary  System. 

Teeth  are  borne  by  the  premaxillse,  the  vomer,  and  the 
superior  pharyngeal  bones  above,  by  the  dentaries  and  the 
inferior  pharyngeal  bone  beneath.  There  are  no  salivary 
glands,  nor  spiracles,  nor  posterior  nares.  A  small  tongue 
is  supported  by  a  ventral  part  of  the  hyoid  arch.  Five  gill 
clefts  open  from  the  pharynx;  their  inner  margins  are  fringed 
by  horny  gill  rakers  attached  to  the  branchial  arches  and 
serving  as  strainers.  The  gullet  leads  into  a  curved  stomach; 
at  the  junction  of  stomach  and  duodenum  numerous  tubular 
pyloric  caeca  are  given  off;  into  the  duodenum  opens  the 
bile  duct  from  the  gall  bladder  and  liver;  the  intestine 
passes  gradually  into  the  rectum,  which  has  an  aperture 
apart  from  those  of  the  genital  and  urinary  ducts.  A 
pancreas  is  absent ;  perhaps  the  pyloric  caeca  take  its 
place.  The  peritoneal  membrane  which  lines  the  abdominal 
cavity  is  darkly  pigmented. 

Respiratory  System. 

Water  that  passes  in  by  the  mouth  may  pass  out  by  the 
gill  clefts ;  the  branchial  chamber  is  also  washed  by  water 


CIRCULATORY  SYSTEM. 


which    passes   both    in    and 


-PCX 


out  under  the  operculum. 
The  gill  filaments  borne  on 
the  four  anterior  branchial 
arches  are  long  triangular 
processes,  whose  free  ends 
form  a  double  row.  As  there 
are  no  partitions  between 
the  five  gill  clefts,  the  fila- 
ments project  freely  into  the 
cavity  covered  by  the  oper- 
culum. Along  each  arch  and 
filament  there  are  blood 
vessels,  bringing  the  impure 
blood,  and  removing  it  puri- 
fied. On  the  internal  surface 
of  the  operculum  lies  a  red 
patch,  the  pseudobranch  or 
rudimentary  hyoidean  gill. 

The  swim  bladder  lies 
along  the  dorsal  wall  of  the 
abdomen;  the  duct  which 
originally  connected  it  with 
the  gut  has  been  closed. 
The  dorsal  wall  of  the 
bladder  is  so  thin,  that  the 
kidneys  and  vertebrae  are 
seen  through  it ;  the  ventral 
wall  is  thick,  and  bears 
anteriorly  a  large  vascular 
rete  mirabile,  which  receives 
blood  from  the  mesenteric 
artery  and  returns  blood  to 
the  portal  vein. 

Circulatory  System. 

The  heart  lies  within  a  peri- 
cardial  chamber,  separated  by 
a  partition  from  the  abdo- 
minal cavity.  The  blood  from 
the  body  and  liver  enters 
the  heart  by  the  sinus  venosus,  passes  into  the  thin-walled 


FIG.  169. — Diagram  of 
Teleostean  circulation.  (After 
NUHN.) 

The  venous  system  is  dark.  A., 
auricle  ;  V.,  ventricle  ;  b.a.,  bulbus 
arteriosus ;  v.a.,  ventral  aorta;  a. fir., 
afferent  branchials  ;  e.br.,  efferent 
branchials ;  c-c.,  cephalic  circle  ;  c., 
carotids  ;  A.c.v.,  anterior  cardinal  veins  ; 
P.C.V.,  posterior  cardinal  veins;  d.c., 
ductus  Cuvierii  ;  d.a.,  dorsal  aorta  ;  c.v., 
caudal  vein  ;  c.a.,  caudal  artery ;  k., 
kidney. 


502  FISHES. 

auricle,  and  thence  to  the  muscular  ventricle.  From  the 
ventricle  it  is  driven  up  the  ventral  aorta,  the  base  of  which 
forms  a  white  non-contractile  bulbus  arteriosus. 

The  ventral  aorta  gives  off  on  each  side  four  afferent 
branchial  vessels  to  the  gills.  Thence  the  blood  is  collected 
by  four  efferent  trunks,  which  unite  on  each  side  in  an 
epibranchial  artery.  The  two  epibranchials  are  united 
posteriorly  to  form  the  dorsal  aorta,  while  anteriorly  they 
give  off  the  carotids  which  are  united  by  a  transverse  vessel 
closing  the  "cephalic  circle." 

Blood  enters  the  sinus  venosus  by  two  vertical  precaval 
veins,  and  by  hepatics  from  the  liver.  Each  precaval  vein  is 
composed  dorsally  of  a  jugular  from  the  head  and  a  cardinal 
from  the  body.  The  cardinals  extend  along  the  kidneys 
and  are  continuous  posteriorly  with  the  caudal  vein,  but  the 
middle  part  of  the  left  cardinal  is  obliterated. 

Excretory  System. 

The  kidneys  are  very  long  bodies,  extending  above  the 
swim  bladder  under  the  vertebral  column.  The  argest 
parts  lie  just  in  front  of  and  just  behind  the  swim  bladder. 
From  the  posterior  part  an  unpaired  ureter  extends  to 
the  urinary  aperture,  before  reaching  which  it  gives  off  a 
small  bilobed  bladder.  The  pronephros  degenerates ;  the 
functional  kidney  is  a  mesonephros. 

Reproductive  System. 

The  testes  are  long  lobed  organs,  conspicuous  in  mature 
males  at  the  breeding  season.  The  ovaries  of  the  female 
are  more  compact  sacs,  more  restrictedly  posterior  in  position. 

Two  vasa  deferentia  combine  in  a  single  canal.  The 
likewise  single  oviduct  is  continuous  with  the  cavity  of  the 
ovaries.  The  genital  aperture  in  either  sex  is  in  front  of, 
but  very  close  to,  that  of  the  ureter.  According  to  some 
authorities  the  genital  canals  in  Teleosteans  are  secondary 
structures,  unconnected  with  the  archinephric  or  segmental 
ducts,  but  the  researches  of  Jungersen  have  made  this  very 
doubtful. 

Development. 

The  ova  of  the  haddock,  like  those  of  other  Teleosteans, 
contain  a  considerable  quantity  of  yolk,  are  fertilised  after 


THE  HERRING.'  503 

they  have  been  laid,  and  undergo  meroblastic  segmentation. 
The  eggs  float,  i.e.,  are  pelagic  ;  while  those  of  the  herring 
sink,  i.e.,  are  dimersal. 

At  one  pole  of  a  transparent  sphere  of  yolk,  lies  a  disc  of  formative 
protoplasm  of  a  light  terra  cotta  colour.  The  ovum  is  surrounded  by  a 
firm  vitelline  membrane.  After  fertilisation,  the  formatiye  disc  divides 
first  into  two,  then  into  four,  then  into  many  cells  which  form  the  blasto- 
derm. From  the  edge  of  the  blastoderm  certain  yolk  nuclei  or  peri- 
blast  nuclei  are  formed  which  afterwards  have  some  importance.  At  the 
end  of  segmentation,  the  blastoderm  lies  in  the  form  of  a  doubly  convex 
lens  in  a  shallow  concavity  of  the  yolk. 

The  blastoderm  extends  for  some  distance  laterally  over  the  yolk  ; 
the  central  part  raises  itself,  and  thus  forms  a  closed  segmentation 
cavity ;  one  radius  of  the  blastoderm  becomes  thicker  than  the  rest,  and 
forms  the  first  hint  of  the  embryo  ;  an  inward  growth  from  the  edge  of 
the  blastoderm  forms  an  invaginated  layer — the  dorsal  hypoblast  or  roof 
of  the  gut ;  the  periblast  forms  the  floor  of  the  gut,  and  afterwards  aids 
the  mesoblast  which  appears  between  epiblast  and  hypoblast ;  the 
medullary  canal  is  formed  as  usual  in  the  dorsal  epiblast.  It  is  likely 
that  the  edge  of  the  blastoderm  represents  the  blastopore  or  mouth  of 
the  gastrula,  much  disguised  by  the  presence  of  yolk. 

The  newly  hatched  larva  is  still  mouthless,  and  lives  for  a  while  on 
the  residue  of  yolk,  which,  by  its  buoyancy,  causes  the  young  fish  to  be 
suspended  in  the  water  back  downwards. 

Third  Type  of  FISHES.  The  Herring — Clupea  harengus.  A 
type  of  those  Teleosteans  which  have  the  swim  bladder 
communicating  with  the  gut  (Physostomi). 

In  habit  the  herring  is  pelagic  and  gregarious.  It  is  found 
in  the  North  Sea,  the  temperate  and  colder  parts  of  the 
Atlantic,  the  Baltic,  and  the  White  Sea.  A  similar  species 
lives  in  the  N.  Pacific. 

External  Characters. 

The  herring  has  the  typical  "fish"  shape.  Externally  it 
differs  from  the  haddock  in  the  following  features  : — there  is 
no  barbule ;  the  maxilla  is  divided  into  three  parts ;  the 
nostrils  have  a  single  aperture  on  each  side;  there  is  no 
lateral  line ;  the  pelvic  fins  are  abdominal,  not  jugular  in 
position ;  there  is  one  dorsal  and  one  anal  fin ;  the  body  is 
more  compressed  ;  the  ventral  edge  is  covered  by  sharply 
keeled  bony  scales. 


504  FISHES. 

Nervous  System. 

The  brain  has  very  small  cerebral  hemispheres  and  large 
optic  lobes.  The  ear  has  a  peculiar  connection  with  the 
swim  bladder.  External  to  each  of  the  well  developed  eyes 
are  two  immovable  transparent  folds  of  skin,  with  a  vertical 
slit  between. 

Alimentary  System. 

The  mouth  has  a  narrowed  gape.  The  upper  jaw  moves 
downwards  and  forwards  when  the  mouth  is  opened.  Small 
visible  teeth  are  borne  on  the  tongue  and  on  the  vomer, 
but  those  on  both  jaws  are  inconspicuous.  The  food — 
chiefly  small  crustaceans — is  probably  in  part  crushed  by 
the  gill  rakers,  which  also  prevent  it  passing  out  by  the  gill 
clefts.  From  the  posterior  end  of  the  gullet  a  caecum  or 
crop  is  given  off.  A  narrow  communication  leads  from 
beneath  this  crop  to  a  thick-walled,  gizzard-like,  muscular 
organ  directed  forwards.  This  in  turn  has  an  opening  into 
the  intestine,  which  runs  straight  to  the  anus.  About  twenty 
digestive  caeca  open  into  the  beginning  of  the  intestine. 
The  swim  bladder  has  a  silvery  exterior,  and  lies  close  under 
the  back  bone.  The  herring  differs  from  most  Physostomi 
as  regards  the  connection  between  the  swim  bladder  and 
gut,  for  the  bladder  does  not  communicate  with  the  gullet 
but  with  the  caecum  by  means  of  a  narrow,  twisted  canal. 
Anteriorly  on  each  side  the  swim  or  air  bladder  gives  off  a 
thin  duct  which,  passing  through  the  walls  of  the  skull, 
divides  into  two  branches,  each  ending  in  a  dilatation  close 
to  the  ear.  Posteriorly  the  swim  bladder  has  a  duct  opening 
externally  on  the  left  side  of  the  anus. 

Respiratory  System,  &c. 

The  gill  filaments  are  fixed  in  a  double  row  on  the  outer 
edge  of  each  branchial  arch.  The  specially  wide  opening 
behind  the  gill  cover  permits  of  a  free  current  of  water  for 
respiration.  The  heart,  kidneys,  &c.,  are  much  the  same 
as  in  the  haddock. 

Reproductive  System. 

The  testes  or  milt  of  the  male  and  the  ovaries  or  roe  of 
the  female,  lie  on  each  side  of  the  abdominal  cavity.  In 


THE   ORDERS   OF  FISHES.  5°5 

each  sex  there  is  a  single  external  opening  behind  the  anus. 
The  number  of  mature  eggs  spawned  at  one  time  by  the 
female  has  been  variously  estimated  at  from  10,000  to 
30,000. 

In  British  waters  there  is  a  spring  as  well  as  an  autumn  spawning — 
probably,  however,  by  distinct  shoals  of  herrings.  When  about  to 
spawn,  the  herring  come  near  the  coasts  into  water  of  from  ten  to  twenty 
fathoms  depth.  While  the  eggs  are  being  shed  by  the  females,  the 
spermatic  fluid  is  passed  into  the  water  by  the  males,  and  the  eggs  are 
thus  fertilised  before  reaching  the  bottom,  where  they  adhere  to  stones, 
zoophytes,  and  even  crustaceans.  The  hatching  of  the  eggs  takes 
from  8  to  40  days,  according  to  the  temperature. 

Development. — The  young  herring  on  emerging  from  the  egg  has  the 
yolk  sac  attached  ;  its  skeleton  is  rudimentary ;  it  has  no  scales  ;  the 
ventral  fins  are  undeveloped  ;  one  continuous  fin  passes  along  the  back, 
round  the  tail  to  the  anus.  A  month  after  hatching,  the  larva  is  about 
two-thirds  of  an  inch  long,  and  has  absorbed  all  its  yolk.  About  the 
third  month  the  scales  appear,  and  though  only  two  inches  in  length, 
the  form  is  then  that  of  the  adult.  Growth  continues  at  the  rate  of  less 
than  half  an  inch  per  month,  and  at  the  end  of  eighteen  months  the 
herring  is  sexually  mature. 

Closely  allied  to  the  herring  are  the  sprat,  the  shad,  and  the  pilchard. 
Thames  "whitebait"  are  herring  not  six  months  old. 


THE    ORDERS    OF    FISHES. 

(See  Table,  pp.  518-19). 

Order  I.   ELASMOBRANCHII — Cartilaginous  Fishes. 

Synonyms.      Selachii.      Plagiostomata  (with  transverse 
ventral  mouth). 

Sharks  and  skates  represent  the  two  distinct  types  included 
in  this  order.  They  are  voracious  carnivorous  fishes.  The 
scales  are  "  skin  teeth."  There  is  no  cover  over  the  (5-7) 
gill  apertures ;  anterior  to  these  there  is  often  a  spiracle, — 
the  first  gill  cleft — with  a  rudimentary  gill.  The  fins  are 
large.  The  skeleton  is  mostly  cartilaginous.  The  tail  is 
asymmetrical  or  heterocercal.  The  mouth  extends  trans- 
versely on  the  under  side  of  the  head.  The  nostrils  are  also 
ventral.  A  spiral  fold  extends  along  the  internal  wall  of  the 
large  intestine.  Into  the  terminal  chamber  (or  cloaca)  of 
the  gut,  the  genital  and  urinary  ducts  also  open.  The 
ventricle  of  the  heart  has  an  anterior  auxiliary  region — a 
contractile  conus  arteriosus.  The  males  are  provided  with 
copulatory  modifications  of  the  hind  limb,  known  as  claspers. 


5o6 


FISHES. 


Fertilisation  is  internal.  The  ova  are  few  and  large.  Large 
egg  purses  are  common,  but  some  Elasmobranchs  are 
viviparous.  The  embryos  have  external  gills. 

Subdivisions. — The  shark  and  the  skate  are  types  of  two  distinct 
suborders  :  —  ( I )  the  older 
Selachoidei,  with  approxi- 
mately cylindrical  bodies  and 
lateral  gill  openings,  as  in 
shark  and  dog  fish  ;  (2)  the 
more  modified  Batoidei,  with 
flattened  bodies  and  ventral 
gill  openings,  as  in  skates  or 
rays. 

Special  Forms.  — Mustelus, 
Carcharias,  Squalus,  Torpedo, 
Acanthias,  and  others,  are 
viviparous  ;  Raja,  Scy  Ilium, 
Cestracion,  and  others,  are 
oviparous.  In  two  species 
of  the  genera  first  named, 
there  is  a  placenta-like  connec- 
tion between  the  yolk  sac  of 
the  embryo  and  the  uterus  of 
the  mother.  Zygcena  has  a 
peculiar  hammer-like  head 
expansion ;  Pristis  has  the 
snout  prolonged  into  a  tooth- 
bearing  saw  ;  Torpedo  has 
a  powerful  electric  organ. 

History.  —  The  Elasmo- 
branchs appear  in  the  Upper 
Silurian,  are  very  abundant 
from  the  Carboniferous  on- 
wards, but  are  now  greatly 
out-numbered  by  the  Bony 
Fishes.  An  increasing  calci- 
fication of  the  axial  skeleton 
is  traceable  through  the  ages, 
and  in  some  of  the  ancient 
forms  the  exoskeleton  was 
greatly  developed,  often  in- 
cluding long  spines  or  ichthyo- 
dorulites  firmly  fixed  on  the  isj^^^j^?^0^^'/  wtSSal 
dorsal  fins  or  on  the  neck,  gflfe  ;*.,  cloaca ;  *^' daspcrs.  . 
Among  the  most  remarkable 

extinct  genera  is  Pleuracanthiis,  from  Carboniferous  to  lower  Permian. 
It  had  a  terminal  mouth,  a  naked  body,  a  continuous  dorsal  fin,  a 
symmetrical  tail,  and  pectoral  fins  with  an  arrangement  of  rays 
resembling  that  in  the  biserial  "  archi-pterygium." 


FIG.  170. — Young  Skate. 
(From  BEARD.) 


GANOIDEL  507 

The  Holocephali  are  represented  by  the  sea  cat  or  Chim&ra  from 
northern  seas,  and  Callorhynchus  from  the  south.  There  is  a  fold  or 
operculum  covering  the  gill  clefts  and  leaving  only  one  external  opening 
on  each  side  ;  the  jaws  are  rigidly  fixed  to  the  cartilaginous  skull ;  the 
skin  is  naked  ;  the  anus,  the  Miillerian  and  urinary  ducts  open  sepa- 
rately. Otherwise  the  Holocephali  resemble  Elasmobranchs,  and  may 
be  regarded  as  a  suborder.  In  some  respects,  however,  e.g.,  in  the 
structure  of  the  skull,  they  suggest  Dipnoi,  and  in  this  connection  it  is 
interesting  to  notice  that  there  is  an  auricular  septum  in  Chinuzra. 

Teeth  (of  Ptyctodtis,  Rhynchodus,  &c.),  which  have  been  referred  to 
Chimseroids  occur  in  Devonian  rocks,  and  some,  at  least  of  the  detached 
spines  of  Carboniferous  age,  may  have  belonged  to  fishes  of  this  order  or 
sub-order.  Undoubted  Mesozoic  Chimseroids  are  Squaloraja^  Myria- 
canthus,  Chinmropsis,  Ischyodus,  &c.,  while  others,  including  the  recent 
genus  ChimcBra,)  are  found  in  strata  of  Tertiary  age.  The  other  recent 
genus,  Callorhynchus ,  is  also  represented  by  a  Cretaceous  species, 
C.  Hectori. 

Another  interesting  but  quite  extinct  group,  whose  position  was  for 
long  a  matter  of  dispute,  but  which  is  now  usually  referred  to  the 


FIG.  171. — Outline  of  Acanthodes  subcatus.      (After 
TRAQUAIR.) 

/.,  Pectoral  fins  ;  v.,  ventrals  ;  a.,  anal ;  d.,  dorsal. 

Elasmobranchii,  is  that  of  the  Acanthodei.  These  flourished  principally 
in  Devonian  times,  but  lived  on  also  through  the  Carboniferous  to  the 
Lower  Permian.  These  are  usually  rather  small  fishes,  with  minute 
rhomboidal  shagreen-like  scales,  and  a  strong  spine  in  front  of  each 
fin,  except  the  caudal.  In  some  genera  (Parexus,  Climatius]  there  are 
two  rows  of  small  intermediate  spines  between  the  proper  pectorals  and 
the  ventrals. 

Order  II.     GANOIDEI. 

This  ancient  order  of  armoured  fishes  flourished  in 
Devonian  and  Carboniferous  ages,  but  is  now  represented 
by  only  seven  genera,  of  which  the  Sturgeon  (Acipenser)  and 
the  Bony  Pike  (Lepidosteus)  are  the  most  familiar. 

The  skin  bears  large  scales,  or  bony  scutes.  The  tail  is 
either  heterocercal  or  homocercal.  Membrane  bones  invest 


508  FISHES. 

the  skull  and  shoulder  girdle.  The  endoskeleton  is  in  great 
part  cartilaginous  in  Acipenser^  Scaphirhynchus,  and  Spatu- 
laria,  but  is  ossified  in  Lepidosteus,  Polypterus,  Calamoichthys^ 
and  Amia.  In  the  first  three  the  notochord  is  uncon- 
stricted ;  in  the  others  there  are  distinct  vertebral  bodies, 
opisthoccelous  in  Lepidosteus,  amphiccelous  in  the  other 
three  genera.  The  fore -brain  has  a  non- nervous  roof. 
There  is  a  spiral  valve  in  the  intestine,  but  it  is  very  small 
in  Lepidosteus.  The  food  canal  ends  apart  from  and  in 
front  of  the  urinogenital  aperture.  There  are  also  abdo- 
minal pores.  An  air  bladder  is  present  with  a  persistent 
open  duct.  The  openings  of  the  gill  clefts  are  covered 
by  an  operculum  supported  by  bones ;  in  some  of  the 
genera  there  is  a  spiracle.  A  conus  arteriosus  is  associated 
with  the  ventricle.  The  archinephric  or  segmental  ducts 
do  not  divide ;  thus  no  Miillerian  ducts  are  formed ;  the 
pronephros  completely  degenerates.  The  ova  are  small, 
and  are  fertilised  in  the  water;  they  have  comparatively 
little  yolk,  and  so  far  as  we  know,  their  segmentation  is 
holoblastic. 

Genera. — The  sturgeon  (Acipenser]  is  one  of  the  more  cartilaginous 
Ganoids.  The  skin  bears  five  rows  of  large  bony  scutes ;  the  tail  is 
asymmetrical  or  heterocercal ;  the  notochord  is  unsegmented.  A  snout, 
bearing  pendent  barbules,  extends  in  front  of  the  ventral  mouth,  which 
is  rounded  and  toothless.  Sturgeons  feed  on  other  fishes,  which  they 
swallow  whole.  They  are  the  largest  fishes  found  in  fresh  water,  for 
A.  sturio  may  attain  a  length  of  18  feet,  and  a  weight  of  600  pounds, 
while  the  A.  huso  of  Southern  Russia  may  measure  25  feet,  and  weigh 
nearly  3000  pounds  !  Most  of  the  species  are  found  both  in  the  sea  and 
in  rivers  or  lakes.  The  flesh  is  edible,  except  in  the  case  of  the  green 
sturgeon,  A.  medirostris,  of  the  Pacific  coasts,  which  is  said  to  be 
poisonous ;  the  roes  or  ovaries  form  caviare  ;  the  gelatinous  internal 
layer  of  the  swim  bladder  is  used  as  isinglass.  The  genus  Scaphi- 
rhynchus  is  represented  in  Asia  and  the  United  States ;  Polyodon  or 
Spatularia  spatula  is  the  paddle  fish  or  spoon  bill  of  the  Mississippi. 
In  PolypteruS)  from  the  Nile  and  other  African  rivers,  the  dorsal  fin  is 
divided  into  many  parts,  the  nasal  sac  has  a  complex  labyrinthine 
structure,  the  swim  bladder  arises  from  the  ventral  side  of  the  gullet, 
the  young  are  said  to  have  external  gills.  In  Old  Calabar  there  is  a 
related  genus  Calamoichthys.  The  gar  pike  or  bony  pike — Lepidostetts 
— is  covered  with  rows  of  enamelled  scales  ;  the  whole  skeleton  is  well 
ossified,  and  the  vertebral  bodies  are  opisthoccelous  or  concave  behind  ; 
the  swim  bladder  is  like  a  lung  in  structure,  and  to  a  slight  extent  in 
function.  The  bow  fin,  Amia  calva,  frequenting  still  waters  in  the 
United  States,  has  a  similar  lung-like  swim  bladder. 


TELEOSTEL  509 

The  fossil  Ganoids  appear  in  the  Silurian  about  the  same  time  as  the 
Elasmobranchs,  they  are  abundant  from  the  Devonian  to  the  Upper 
Cretaceous  when  the  Teleosteans  begin  to  become  numerous.  It  is  very 
doubtful  whether  the  primitive  armoured  fishes  (Tremataspis^  Pteraspis, 
Cephalaspis,  Pterichthys,  &c,)  have  any  claim  to  be  considered  as  Ganoids 
at  all.  They  constitute  the  group  of  Ostracodermi,  which,  commencing 
in  the  Upper  Silurian,  seems  to  have  become  extinct  at  the  conclusion 
of  the  Devonian  era. 

Fishes  allied  to  the  Ganoids  of  the  present  day  appear  in  the  Middle 
Devonian,  and  are  found  in  abundance  until  the  close  of  the  Jurassic 
era,  when  they  gave  way  to  the  more  specialised  Teleostei.  In  Devonian 
and  Carboniferous  rocks  these  Ganoids  may  be  classed  in  two  series  : — 
Crossopterygii  (Holoptychiidge,  Rhizodontidse,  Osteolepidse,  Ccelacan- 
thidse),  allied  to  the  living  Polypterus,  and  the  Acipenseroidei  (Paloe- 
oniscidae),  allied  to  the  Sturgeons.  But  already  in  the  Permian  era  we 
begin  to  find  representatives  of  that  great  semi-heterocercal  series, 
which  is  represented  at  the  present  day  by  Lepidosteus  and  Amia,  and 
which,  in  reality,  passes  gradually  into  the  Physostomous  Teleostei. 
These,  represented  by  such  forms  as  Lepidotus,  Dapedius,  Eugnathus^ 


FIG.  172. — Pterichthys  Milleri.     (Lateral  View.     Restored 
by  TRAQUAIR). 

&c.,  become  very  abundant  in  Jurassic  rocks,  while  the  Crossopterygii 
and  Acipenseroidei  dwindle  away.  So  does  the  Lepidosteid  series  in 
the  Cretaceous  era,  and  in  Tertiary  times  the  Ganoids  were,  as  now, 
nearly  a  thing  of  the  past. 


Order  III.     TELEOSTEI — the  "Bony'Fishes." 

This  order  includes  most  of  the  fishes  now  alive. 
Though  comparatively  modern  fishes,  they  are  older  than 
was  formerly  supposed,  as  several  Jurassic  genera  (Thrissops^ 
Leptolepis,  &c.),  which  used  to  be  classed  as  Ganoids,  must 
be  considered  as  actual  Clupeoids,  or  Herring-like  Teleostei. 
It  is,  however,  not  until  the  Upper  Cretaceous  and  Tertiary 


510  FISHES. 

epochs  that  they  assume  among  fishes  that  overwhelming 
preponderance  in  numbers  which  they  possess  at  the  present 
day.  The  physostomous  type  of  Teleostean  is  the  most 
ancient,  and  probably  stands  in  a  continuous  genetic  line 
with  the  Lepidosteoid  Ganoids. 

The  skeleton  is  well  ossified,  with  numerous  investing 
bones  on  the  skull,  others  in  the  operculum,  and  on  the 
shoulder  girdle.  The  tail  is  sometimes  quite  symmetrical 
or  diphycercal,  but  in  most  cases  it  is  heterocercal  at  first, 
and  acquires  a  secondary  symmetry  termed  homocercal,  for 
while  the  end  of  the  notochord  in  the  young  forms  is  bent 
upwards  as  usual,  the  subsequent  development  of  rays  pro- 
duces an  apparent  symmetry.  The  scales  are  in  most  cases 
relatively  soft.  As  in  Ganoids,  the  roof  of  the  fore-brain  is 
without  nervous  matter.  The  optic  nerves  are  remarkable, 
because  they  cross  one  another  without  fusing  (decussate). 
As  in  Ganoids,  the  partitions  between  the  gill  clefts  dis- 
appear, so  instead  of  the  pouches  seen  in  Elasmobranchs, 
there  is,  on  each  side,  one  branchial  chamber,  covered  over 
by  an  opercular  fold.  Into  this  chamber  the  gill  lamellae 
borne  by  the  branchial  arches  project  freely.  In  most,  a 
swim  bladder  is  developed  from  the  dorsal  side  of  the  gullet. 
There  is  no  spiral  valve  in  the  intestine,  and  the  food  canal 
ends  in  front  of  and  separate  from  the  genital  and  urinary 
apertures  or  aperture.  The  base  of  the  ventral  aorta  is 
swollen  into  a  non-contractile  bulbus  arteriosus,  but  there 
is  no  conus,  unless  very  exceptionally,  as  in  Butirinus. 
According  to  some  authorities,  the  archinephric  duct  is 
unsplit,  and  there  is  no  Miillerian  duct ;  according  to 
Jungersen,  the  oviduct  is  a  true  Miillerian  duct.  The 
pronephros  degenerates ;  the  ova  are  numerous,  and  are 
fertilised  in  the  water. 

Classification  of  Tekostei.     (After  Giinther.) 

f  Acanthopteri.     Example — Perch/ 
Dorsal,  anal,  and   pelvic  J 

fins  in  part  spiny.  J  Pharyngognathi.         Example  — 


Wrass. 
fAnacanthini ;   the  pelvic  fins  are 


The     dorsal,     anal,     and        situated  far  forward.    Examples 
pelvic      fins       without^      -Cod,  Flounder. 


Physoclisti,  —  duct 
of  swim  bladder  is 
closed. 


pelvic      nns       witnoutx 

I  Physostomi ;   duct  of  swim  bladder  remains  open.     Ex- 
V.    amples — Herring,  Salmon,  Carp,  Eel. 


DIPNOI.  511 

Besides  these  chief  sub-orders,  there  are  two  sets  of  aberrant  forms: — 

(a)  The  sea  horses,  such  as  Hippocampus  and  Phyllopteryx,  and  the 

pipe  fishes,  such  as  Syngnathus^  are  distinguished  as  Lopho- 
branchii.  The  gills,  instead  of  being  rows  of  filaments,  are 
tufts  of  rounded  lobes  ;  the  gill  cover  is  a  simple  plate,  leaving 
a  small  aperture  ;  the  skin  is  more  or  less  protected  by  large 
dermal  plates  ;  the  toothless  mouth  is  at  the  end  of  a  prolonged 
snout. 

(b]  The  globe  fishes,  such  as  Tetrodon  and  Diodon,  the  trunk  fishes — 

Ostracion,  the  sun  fish — Orthagoriscus,  and  others,  are  distin- 
guished as  Plectognathi.  The  body  is  globular  or  compressed 
sideways  ;  the  skin  bears  bony  scutes  or  spines,  or  is  naked  ; 
the  skeleton  is  incompletely  ossified,  and  the  vertebrae  are  few  ; 
the  bones  of  the  upper  jaw  are  more  or  less  fused  ;  the  pelvic 
fins  are  absent  or  reduced  to  spines ;  the  gills  are  comb-like ; 
the  swim  bladder  has  no  duct. 

It  is  likely  that  some  of  the  loosely-built  deep-sea  fishes,  such  as  the 
pelican  fish  Eurypharynx,  are  not  referable  to  the  orders  usually  recog- 
nised. 

Order  IV.     DIPNOI— "  Mud  Fishes." 

The  Dipnoi,  whose  name  means  double  breathers,  are 
now  represented  by  three  genera — Ceratodus,  from  two 
rivers  of  Queensland ;  Protopterus,  from  certain  African 
rivers,  e.g.,  the  Gambia ;  and  Lepidosiren,  from  the  Amazons. 
The  wide  distribution  is  noteworthy. 

They  are  very  ancient  forms,  for  Ceratodus  or  a  closely 
allied  form  has  lived  on  from  Mesozoic  times,  and  there 
were  also  undoubted  Dipnoi  far  back  in  Palaeozoic  times, 
such  as  Dipterus  and  Phaneropleuron  of  the  Devonian, — 
Ctenodus  and  Uronemus  of  the  Carboniferous.  According 
to  some,  the  remarkable  Devonian  Coccosteidae  are  also  to 
be  considered  as  an  aberrant  group  of  Dipnoi. 

Prof.  W.  N.  Parker  regards  them  as  "the  isolated  sur- 
vivors of  an  exceedingly  ancient  group,  which  was  probably 
nearly  allied  to  the  ancestors  of  existing  Amphibians  and 
Fishes,  more  particularly  Elasmobranchs,  though  the  Ganoid 
stock  most  likely  arose  not  far  off." 

Were  it  not  for  the  disadvantage  of  multiplying  classes, 
one  would  be  inclined  to  place  them  between  Pisces,  which 
they  resemble  in  having  cycloid  scales,  paired  fins,  a  spiral 
valve,  &c  ,  and  Amphibia,  which  they  approach  in  having 
lungs,  an  incipiently  three-chambered  heart,  a  vena  cava, 
a  pulmonary  vein,  posterior  nares,  and  multicellular  skin 
glands. 


512  FISHES. 

It  must  be  noted,  however,  that  it  does  not  follow  that 
the  Dipnoi  are  the  connecting  links  between  Fishes  and 
Amphibians  because  they  possess  certain  characters  of 
both  these  classes.  We  require  further  palseontological  and 
embryological  evidence.  The  Dipnoi  are  physiologically 
transitional  between  Fishes  and  Amphibians,  having,  for 
instance,  acquired  lungs  while  retaining  gills,  but  it  does 
not  follow  that  they  are  morphologically  transitional. 

(a)  Ceratodus. 

The  genus  Ceratodus  is  abundantly  represented  by  fossils 
in  the  Mesozoic  beds  of  Europe,  America,  Asia,  and 
Australia,  but  the  living  animal  is  now  limited  to  the  basins 
of  two  of  the  rivers  of  Queensland.  C.  forsteri,  the  best 
known  and  perhaps  the  only  species,  was  first  described  by 
Krefft  in  1870,  and  recently  (1891)  its  habits  have  been 
studied  by  Professor  Richard  Semon  of  Jena.  Like  that 
other  old-fashioned  animal  the  duckmole,  Ceratodus  fre- 
quents the  still  deep  places  of  the  river's  bed,  the  so-called 
"water-holes."  At  the  bottom  of  these  it  lies  sluggishly, 
occasionally  rising  to  the  surface  to  gulp  in  air.  Its  diet 
was  formerly  supposed  to  be  exclusively  vegetarian,  but 
Semon  holds  that  it  crops  the  luxuriant  vegetation  of  the 
river-banks  only  for  the  sake  of  the  animal  life — larvae  and 
eggs  of  insects,  worms,  molluscs,  amphibians,  and  fishes — 
contained  among  it.  Certain  it  is,  that  natives  and  colonists 
catch  it  by  means  of  animal  bait.  From  this  method  of 
angling  for  it,  and  from  its  rosy-tinted  flesh,  considerable 
confusion  has  arisen  between  Ceratodus  and  a  Teleostean 
fish,  the  true  Barramunda  or  Dawson  salmon,  found  in 
some  of  the  Queensland  rivers.  Ceratodus  is  quite  unable 
to  live  out  of  water,  but  its  air-breathing  powers  enable  it  to 
exist  in  water  which  is  laden  with  sand  or  rotten  vegetable 
matter.  According  to  Semon,  its  limited  distribution  is  to 
be  accounted  for,  first,  by  its  sluggish  nature,  for  it  comes 
of  a  dying  stock ;  and,  secondly,  by  the  fact  that  the  eggs 
are  very  readily  destroyed,  and  so  incapable  of  distribution 
by  any  of  the  ordinary  means.  Nothing  is  known  of  the 
process  of  fertilisation,  but  the  eggs,  which  are  surrounded 
by  a  jelly-like  envelope,  are  laid  singly  in  the  water.  The 
development  has  not  yet  been  fully  worked  out,  but  seg 


PROTOPTERUS. 


513 


mentation  is  complete  and  unequal,  and  is  followed  by 
gastrulation.  Segmentation  of  the  embryo  is  obvious  at  a 
very  early  period ;  there  is  no  trace  of  external  gills.  The 

early  stages  resemble  very 
closely  the  corresponding 
stages  in  the  development  of 
Amphibians. 

Ceratodus  sometimes  attains  a 
length  of  six  feet.  The  body  is 
elongated  and  compressed,  and  bears 
a  continuous  vertical  fin.  The  paired 
fins  are  trowel-like,  with  a  median 
jointed  axis,  from  which  rays  pro- 
ject on  each  side.  There  are  four 
gill  clefts,  four  internal  gills,  and  a 
hyoid  half  gill.  There  are  no  ex- 
ternal gills. 

The  swim  bladder  or  lung — for 
as  such  it  acts — is  single.  It  is  sup- 
plied with  blood  from  the  fourth 
aortic  arches,  as  is  the  swim  bladder 
of  the  Ganoids  —  Polypterus  and 
Amia.  It  arises  ventrally,  but  lies 
dorsally,  and  is  divided  into  com- 
partments. 

The  heart  has  only  one  auricle, 
with  a  dorsal  fibrous  ridge  hinting 
at  a  division.  The  conus  arteriosus 
is  peculiarly  twisted,  and  contains  a 
short  longitudinal  spiral  valve  and 
numerous  large  "  pocket "  (or 
"  Ganoid ")  valves.  The  septum 
in  the  conus  is  not  complete,  as  it 
is  in  the  other  Dipnoi,  thus  mixed 
blood  passes  into  the  first  two  pairs 
of  arches.  There  are  four  pairs  of 
these  arches  or  arteries  supplying 
the  gills  ;  the  efferent  vessels  (two 
from  each  gill,  as  in  Elasmobranchs), 
unite  to  form  epibranchials,  and 
these  to  form  the  dorsal  aorta.  The 

fourth  epibranchial  gives  off  the  pulmonary  artery.      The  pulmonary 

vein  enters  the  left  side  of  the  auricle. 


FIG.  173. — Skeleton  of  Cera- 
todus Fin.  (From  GEGEN- 
BAUR.) 


«.,   Central   axis  ; 
basal  piece. 


radial s 


(b}  Protopterus. 

Protopterus  lives  in  African  rivers  (Gambia,  Quilimane, 
&c.),  is  mainly  but  not  exclusively  carnivorous,  and  attains 
33 


514  FISHES. 

a  length  of  2  to  3  feet.  It  has  extraordinary  vitality,  surviv- 
ing severe  wounds,  long  fasting,  and  desiccation.  It  appears 
to  be  most  active  at  night,  and  to  prefer  shallow  water, 
swimming  rapidly  with  powerful  tail-strokes,  or  "walking" 
slowly  along  the  bottom  with  its  filamentous  fins  moving 
alternately  on  each  side,  somewhat  like  the  legs  of  a  newt. 
At  short  intervals  it  comes  to  the  surface  to  take  mouth- 
fuls  of  air,  which  passes  out  again  through  the  opercular 
aperture. 

As  the  dry  season  approaches,  Protopterus  burrows  into 
the  earth  to  a  depth  of  about  18  inches,  coils  itself  up, 


e.br 


pc.  I. 


FIG.  174. — Head  region  of  Protopterus.     (From  W.  N.  PARKER.) 

sn.t.,  Sensory  tubes  ;  /./.,  lateral  line  ;  e.br.,  external  gills  ;  pc.L, 
pectoral  fin  ;  op.,  operculum. 

and  secretes  abundant  mucus  from  its  skin  glands.  This 
secretion  forms  a  cocoon  or  capsule,  with  adherent  earth 
externally,  with  moist  slime  internally,  and  with  a  lid,  on 
which  there  is  always  a  small  aperture.  Thus  encapsuled, 
the  animal  may  remain  dormant  for  many  months,  e.g.,  from 
August  to  December.  "  The  animal  lies  coiled  up  in  such 
a  manner  that  the  head  lies  alongside  the  base  of  the  tail, 
which  from  this  point  is  again  bent  backwards  over  the 


PROTOPTERUS.  515 

head,  so  that  it  covers  the  head  and  body  like  a  veil." 
These  capsules,  with  the  surrounding  earth,  have  often  been 
transported  from  Africa  to  northern  Europe,  without  injury 
to  the  dormant  life  within.  On  emergence  the  animal 
makes  peculiar  sounds,  probably  due  to  the  forcible  expul- 
sion of  air  from  the  lungs  through  the  lips. 

Two  questions  of  much  interest  arise: — how  does  the  encapsuled 
animal  breathe,  and  how  is  it  nourished  ? 

Although  the  red  vascular  appearance  of  the  tail  led  Wiedersheim  to 
the  opinion  that  caudal  blood  vessels  might  be  the  seat  of  a  respiratory 
interchange  of  gases,  it  is  almost  certain  that  air  passes  directly  from 
the  mouth  of  the  burrow,  through  the  aperture  of  the  capsule-lid  (which 
is  produced  inwards  in  a  short  pipe)  to  the  external  nostrils,  and  thence 
to  the  lungs. 

The  nourishment  appears  to  be  derived  from  a  store  of  fat  deposited 
in  the  lymphoid  tissue  around  the  reproductive  organs  and  kidneys,  and 
among  the  lateral  muscles  of  the  tail  (cf.  fatty  bodies  in  caterpillars, 
amphibians,  &c.).  Moreover,  some  of  the  muscles  are  replaced  by  fat, 
and  others  undergo  a  pathological  granular  degeneration  (cf.  lamprey). 
To  a  certain  extent,  therefore,  the  dormant  animal  lives  on  its  own  tail. 
It  is  probable  that  leucocytes  aid  in  the  absorption  and  transportation 
of  the  degenerated  muscles  (cf.  tadpoles). 

A  few'  of  the  anatomical  characteristics  of  Protopterus  may  now  be 
noted,  following  Prof.  W.  N.  Parker. 

The  paired  fins  are  filamentous,  and  seem  degenerate  when  compared 
with  those  of  Ceratodus^  having  only  one  series  of  short  lateral  horny 
rays  on  the  cartilaginous  segmented  axis.  The  tail  is  symmetrical,  and 
ends  in  a  filament  which,  like  the  end  of  the  fins,  is  often  bitten  off; 
often,  however,  there  is  a  slight  upward  bending,  which  suggests  a 
heterocercal  condition.  Both  tail  and  fins  may  be  regenerated  after 
serious  injuries. 

In  the  skin  are  very  numerous  mucus-secreting  goblet  cells,  and  there 
are  also  (especially  on  the  snout)  multicellular  glands,  which  are  absent 
from  most  fishes,  though  common  in  Amphibians,  Reptiles,  and 
Mammals.  There  is  a  continuous  lateral  line,  and  apart  from  this  there 
are  other  integumentary  sense  organs  on  the  head  and  various  parts  of 
the  body.  There  are  taste  buds  on  tongue  and  palate,  olfactory  organs 
with  posterior  as  well  as  anterior  nares — the  latter  concealed  by  the 
overhanging  lips,  relatively  small,  lidless  eyes,  and  auditory  organs. 
"The  apparently  anomalous  position  of  the  nostrils  is  probably  to  be 
explained  as  an  adaptation  to  the  habits  of  the  animal  in  connection 
with  its  summer  sleep." 

There  is  a  spiral  valve  in  the  large  intestine ;  the  cloaca  has  an 
associated  "  caecum  ;  "  the  pancreas  surrounds  the  bile-duct,  and  though 
large,  is  almost  hidden  within  the  walls  of  the  gut ;  the  spleen  is  also 
large,  but  inconspicuous.  Cilia  are  present  throughout  the  stomach  and 
intestine,  and  there  are  no  differentiated  gastric  or  intestinal  glands. 
There  is  an  unusually  abundant  investment  of  lymphoid  tissue  associated 
with  the  gut,  "which,  during  the  period  when  Protopterus  is,  as  it 


516  FISHES. 

were,  parasitic  upon  itself,  is  probably  of  especial  importance,  not  only 
in  the  formation  of  leucocytes  and  in  the  destruction  of  dying  cells,  but 
also  in  the  processes  of  metabolism." 

Behind  the  hyoid  are  five  rudimentary  .branchial  arches.  There  are 
five  gill  clefts,  covered  by  an  operculum,  outside  which  are  three 
external  epidermic  gills.  Of  the  true  internal  gills  the  arrangement 
is  as  follows: — the  hyoid  has  a  small  half  row,  the  next  two  arches  bear 
none,  the  third  and  fourth  have  the  usual  double  rows  of  lamellae,  and 
the  fifth  has  a  single  row. 

The  lungs  are  paired  along  almost  their  entire  length,  and  extend  under 
the  notochord  to  the  end  of  the  body  cavity.  The  glottis  lies  as  usual 
on  the  median  ventral  floor  of  the  pharynx,  and  by  means  of  a  vestibule 
ascending  on  the  right  side  communicates  with  the  unpaired  anterior 
end  of  the  lungs.  Thus,  although  the  lungs  lie  dorsally,  they  probably 
arise  as  a  ventral  diverticulum,  as  in  higher  animals. 

The  blood  is  remarkable  for  the  large  size  of  its  elements  and  for  the 
predominance  of  white  over  red  corpuscles.  In  general  structure  the 
heart  is  like  that  of  Ceratodus.  There  is  but  one  auricle,  but  a  dorsal 
fibrous  ridge  hints  at  its  division.  The  conus  arteriosus  has  a  long 
spiral  longitudinal  valve  and  minute  pocket-like  valves.  From  the  cone 
four  branchial  arteries  arise  on  each  side,  and  pass  to  the  first  four 
branchial  arches,  and  the  effect  of  the  longitudinal  valve  is  that  the 
anterior  pair  contain  blood  already  purified  in  the  lungs ;  the  posterior 
pair  carry  almost  unmixed  venous  blood.  The  efferent  branchials  unite 
in  a  transverse  trunk,  and  then  form  the  dorsal  aorta,  and  from  the  root 
of  the  aorta  a  paired  pulmonary  artery  arises,  the  left  supplying  the 
ventral,  and  the  right  the  dorsal  aspect  of  the  lungs.  In  regard  to  the 
veins,  there  is  a  single  true  postcaval,  or  inferior  vena  cava,  along  with 
a  persistent  left  posterior  cardinal.  There  is  a  single  caudal  vein  giving 
rise  to  a  right  and  left  renal  portal.  Two  pulmonary  veins  unite  near 
the  front  of  the  lung  in  a  single  vessel,  which  enters  the  left  side  of  the 
auricle. 

The  urinogenital  organs  are  surrounded  by  lymphoid  and  fatty  tissue  ; 
the  kidneys  probably  represent  the  mesonephros,  and  their  duct  the 
Wolffian  duct ;  nephrostomes  are  absent.  The  vas  deferens  appears  to 
be  a  special  duct,  probably  formed  in  connection  with  the  testes,  quite 
independently  of  the  excretory  apparatus,  and,  therefore,  to  a  certain 
extent  comparable  to  that  of  Teleosteans  ;  it  opens  into  the  base  of  the 
Miillerian  duct,  the  rest  of  which  gradually  aborts  in  the  male.  The 
ovaries  are  strikingly  like  those  of  Amphibians  ;  the  oviduct  seems  to 
be  the  Miillerian  duct.  Ureters  and  genital  ducts  open  beside  one 
another  into  the  cloaca. 

(c]  Lepidosiren. — Relatively  little  is  known  in  regard  to  the  third 
type — Lepidosiren — from  the  Amazons.  It  has  an  eel-shaped  body  with 
a  continuous  vertical  fin.  The  limbs  are  reduced  to  cylindrical  stems 
without  any  radials.  There  are  no  external  gills.  The  air  bladder  or 
lung  is  double,  and  its  relations  to  blood  vessels  are  like  those  in  Proto- 
pterus. 

There  is  an  imperfect  muscular  septum  dividing  the  auricle  into  two, 
and  there  is  a  similarly  incomplete  septum  in  the  ventricle.  The  conus 
resembles  that  of  Protopterus. 


FORM  AND  MOVEMENT— COLOUR.  517 

GENERAL  NOTES  ON  FISHES. 
Form  and  Movement. 

A  fish  may  well  compare  with  a  bird  in  its  mastery  of  the 
medium  in  which  it  lives.  Thus  a  salmon  is  said  to  travel 
at  the  rate  of  about  eight  yards  in  a  second,  or  over  sixteen 
miles  an  hour.  The  motion  depends  mainly  on  the  power- 
ful muscles  which  produce  the  lateral  strokes  of  the  tail  and 
posterior  part  of  the  body.  It  may  be  roughly  compared 
to  the  motion  of  a  boat  propelled  by  an  oar  from  the 
stern.  So  energetic  are  the  strokes  that  a  fish  is  often  able 
to  leap  from  the  water  to  a  considerable  height.  In  some 
cases  undulating  movements  of  the  unpaired  fins,  and  even 
the  rapid  backward  outrush  of  water  from  under  the  gill 
cover,  seem  to  help  in  movement.  The  paired  fins  are 
chiefly  used  in  ascending  and  descending,  in  steering  and 
balancing,  and  some  observers  state  that  the  pectoral  fins  of 
the  flying  fish  are  distinctly  moved  during  the  long  skimming 
leaps.  In  a  few  cases,  as  in  the  climbing  perch,  and  in  the 
strange  Periophthalmus,  which  clambers  on  the  mangrove 
roots,  the  fore  fins  and  tail  are  used  in  scrambling. 

The  characteristic  form  of  the  body,  as  seen  in  herring  or 
trout,  is  an  elongated  laterally  compressed  spindle,  thinning 
off  behind  like  a  wedge.  In  most  cases  the  trunk  passes 
quite  gradually  into  head  and  tail.  It  is  evident  that  this 
form  is  well  adapted  for  rapid  progression  through  the  water. 
Flat  fishes,  whether  flattened  from  above  downwards,  like 
the  skate,  or  from  side  to  side  like  the  plaice  and  sole, 
usually  live  more  or  less  on  the  bottom  ;  eel-like  forms  often 
wallow  in  the  mud,  or  creep  in  and  out  of  crevices ;  globe 
fishes,  like  Diodon  and  Tetrodon,  often  float  passively. 
There  are  many  strange  fishes,  such  as  the  sea  horses  (e.g., 
Hippocampus),  which  play  among  the  sea-weeds  in  warm 
seas.  Some  of  the  deep-sea  fishes  have  very  quaint  shapes. 

Colour. 

The  colours  of  Fishes  are  often  very  bright.  They  de- 
pend partly  on  pigments  in  the  cells  of  the  skin,  partly  on 
the  physical  structure  of  the  scales.  The  common  silvery 
colour  is  often  due  to  small  crystals  on  the  scales.  In  many 
cases  the  colours  of  the  male  are  brighter  than  those  of  his 


5i8  FISHES. 

mate,  witness  the  gemmeous  dragonet  (Callionymus  lyrd) 
and  the  stickleback  (Gasferasteus),  and  this  is  especially  true 
at  the  breeding  season.  The  colours  of  many  fishes  change 
with  their  surroundings.  In  the  plaice  and  some  others  the 
change  is  rapid.  Surrounding  colour  affects  the  eye,  the 
influence  passes  from  eye  to  brain,  and  from  the  brain  down 
the  sympathetic  nervous  system,  thence  by  peripheral  nerves 
to  the  skin,  where  the  distribution  of  the  pigment  granules 
in  the  cells  is  altered.  In  shallow  and  clear  water  this  power 
of  colour  change  may  be  of  much  protective  value,  but  it 
seems  likely  that  this  has  been  exaggerated.  An  appre- 
ciation of  the  protective  value  of  colouring  demands  careful 
attention  to  the  habits  and  habitat  of  the  animals,  to  the 
nature  of  the  light  in  which  they  live,  and  to  the  enemies 
which  are  likely  to  attack  them. 

Food. 

The  food  of  Fishes  is  very  diverse — from  Protozoa  to 
Cetaceans.  Sharks  and  many  others  are  voraciously  carni- 
vorous, many  engulf  worms,  crustaceans,  insects,  molluscs, 
or  other  fishes ;  others  browse  on  sea-weeds,  or  swallow  mud 
for  the  sake  of  the  living  and  dead  organisms  which  it  con- 
tains. Their  appetite  is  often  enormous,  and  cases  are 
known  (e.g.,  Chiasmodon  niger),  where  a  fish  has  swallowed 
another  larger  than  its  own  normal  size.  Many  fishes  follow 
their  food  by  sight ;  many  by  a  diffuse  sensitiveness,  to 
which  it  is  difficult  to  give  a  name ;  and  others,  it  would 
seem,  by  a  localised  sense  of  smell. 

Some  Points  of  Strnctttre — Fins. — Along  the  median  line  of  the 
dorsal  and  ventral  surfaces  of  some  fishes,  e.g.,  flounder,  there  is  a  con- 
tinuous fin — a  fold  of  skin  with  fin  rays  and  underlying  skeletal  supports. 

In  the  embryos  of  many  fishes,  the  same  continuous  fringe  is  seen, 
while  the  adults  have  only  isolated  median  fins.  There  is  no  doubt  that 
these  isolated  median  fins — of  which  there  may  be  two  dorsals,  a  caudal, 
and  an  anal  or  ventral — arise  or  have  arisen  from  a  modification  of  a 
once  continuous  fin,  which  is  suppressed  at  one  part  and  increased  at 
another. 

Now,  the  paired  fins,  which  correspond  to  limbs,  often  resemble 
unpaired  fins  in  their  general  structure,  and  in  their  mode  of  origin. 
In  some  Elasmobranch  embryos,  Balfour  showed  that  the  pectoral  and 
pelvic  fins  were  connected  by  transitory  lateral  ridges.  It  is  therefore 
likely  that  the  paired  fins  have  arisen  by  a  localisation  of  two  once  con- 
tinuous lateral  folds.  Why  there  should  be  only  two  pairs  we  do  not 
know. 


FINS— TAIL— SCALES.  519 

Two  types  of  fish  fin  are  distinguishable — (a)  that  best  illustrated 
among  living  fishes  by  Ceratodus,  in  which  a  median  jointed  axis  bears 
on  each  side  a  series  of  radial  rays — a  form  often  called  an  archiptery- 
gium  ;  and  (b]  the  commoner  type,  in  which  the  radials  arise  from  a 
number  of  basal  pieces  (an  ichthyopterygium).  Experts  do  not  seem  to 
have  yet  come  to  a  decision  as  to  which  of  these  types  is  the  more 
ancient,  or  as  to  how  they  are  related  to  one  another. 

Professor  Huxley  suggested  that  the  fingered  limb  (cheiropterygium) 
of  higher  Vertebrates  might  arise  from  a  limb  of  the  Ceratodus  type  by 
an  atrophy  of  its  proximal  fore-and-aft  radials,  and  the  hypertrophy  of 
its  distal  radials.  Thus  the  axis  becomes  the  middle  digit,  while  the 
other  four  digits  are  the  terminations  of  the  two  distal  radials  on  each 
side.  But  it  seems  just  as  easy  or  as  difficult  to  trace  the  digitate  limb 
to  an  ichthyopterygium. 

Another  interesting  subject  of  inquiry  is  as  to  the  origin  of  the  girdles, 
whether  as  ingrowths  from  the  bases  of  the  limbs,  or  from  modifications 
of  branchial  arches,  or  from  both  or  neither. 

Tail. — In  Dipnoi  and  a  few  Teleosteans,  e.g.,  the  eels,  the  vertebral 
column  runs  straight  to  the  tip  of  the  tail,  dividing  it  into  two  equal 
parts.  This  perfectly  symmetrical  condition  is  called  diphycercal  or 
protocercal,  but  it  is  not  quite  certain  that  its  thorough  symmetry  is 
primitive. 

In  Elasmobranchs,  Holocephali,  cartilaginous  and  many  extinct 
Ganoids,  the  vertebral  column  is  bent  dorsally  at  the  end  of  the  tail, 
and  the  ventral  part  of  the  caudal  fin  is  smaller  than,  and  at  some  little 
distance  from,  the  upper  part.  This  asymmetrical  condition  is  called 
heterocercal. 

In  most  Teleostei,  and  in  extant  bony  Ganoids,  the  end  of  the  verte- 
bral column  is  also  bent  upwards,  but  the  apex  atrophies  and,  by  the 
disproportionate  development  of  rays  on  the  ventral  side,  an  apparent 
symmetry  is  produced.  The  vertebral  column  usually  ends  in  a  urostyle, 
— the  undivided  ossified  sheath  of  the  notochord.  Most  of  the  fin  really 
lies  to  the  ventral  side  of  this.  The  condition  is  termed  homocercal. 

Scales. — In  Elasmobranchs  the  scales  (placoid)  have  the  form  of  skin 
teeth  (dermal  denticles),  tipped  with  enamel,  cored  with  dentine,  and 
based  with  bone  sunk  in  the  dermis.  They  arise  from  skin  papillae,  the 
(ectodermic)  epidermis  forming  the  enamel,  the  (mesodermic)  dermis 
forming  the  rest.  It  has  been  recently  maintained,  however,  that  the 
ectoderm  forms  most,  if  not  all,  of  the  scale  (see  p.  426).  In  other 
fishes  the  scales  are  almost  wholly  dermic,  in  marked  contrast  to  those 
of  Reptiles. 

In  most  Teleosteans  the  scales  are  soft,  and  the  epidermic  covering  is 
very  thin.  They  are  called  cycloid  or  ctenoid,  as  their  free  margins 
projecting  from  sacs  in  the  dermis  are  entire  or  notched.  But  bony 
scales  also  occur  in  many  Teleosteans. 

The  sturgeon  has  five  rows  of  bony  dermic  plates  (scutes) ;  the  scales 
of  the  Bony  Pike  (Lepidosteus\  Polypterus,  and  many  extinct  Ganoids 
are  covered  with  enamel. 

The  great  interest  of  these  exoskeletal  structures  is  that  those  of  Elas- 
mobranchs are  homologous  with  teeth,  and  that  many  bony  scales  often 
fuse  into  plates,  suggesting  the  manner  in  which  the  membrane  bones  of 


520  FISHES. 

the  skull  and  pectoral  girdle  (e.g.,  the  clavicle  of  Bony  Fishes)  are 
believed  to  have  originated. 

The  simplest  teeth  of  Elasmobranchs  are  precisely  homologous  with 
dermal  denticles.  But  just  as  the  skin  teeth  sometimes  fuse  in  groups, 
so  is  it  also  with  their  homologues  which  form  true  teeth.  Compound 
cuspidate  teeth  in  sharks  arise  from  the  fusion  of  adjacent  simple  cusps. 
But  the  fusion  may  go  further  ;  a  complex  crushing  dental  plate  may  be 
formed  from  the  coalescence  of  several  successional  teeth.  A  further 
complication  is  brought  about  by  the  multiplication  of  cusps  on  in- 
dividual teeth.  These  facts  are,  as  Mr.  A.  Smith  Woodward  points  out, 
of  much  interest,  because  it  is  by  similar  processes  of  fusion  and  of 
multiplication  that  the  complex  teeth  of  various  Mammals  arise. 

Swim  bladder. — The  swim  bladder  of  fishes  is  one  of  the  numerous 
outgrowths  of  the  gut.  It  is  absent  in  Elasmobranchs  and  some  Teleos- 
teans,  such  as  most  flat  fish,  and  it  forms  the  lung  of  Dipnoi.  Unlike  a 
lung,  it  opens  dorsally  into  the  gut,  except  in  Dipnoi  and  the  Ganoid 
Polypterus,  where  the  aperture  is  ventral.  The  original  duct  communi- 
cating with  the  gut  may  remain  open,  as  in  Physostomatous  Teleosteans, 
or  it  may  be  closed  as  in  Physoclystous  Teleosteans.  The  bladder  is  usu- 
ally single,  but  it  is  double  in  Protopterus,  Lepidosiren,  and  Polypterus. 

In  regard  to  the  use  of  the  swim  bladder,  there  is  still  considerable 
uncertainty.  Where  it  is  abundantly  supplied  with  impure  or  partially 
purified  blood,  as  in  Dipnoi,  Polypterus,  and  Amia,  and  where  the  gas 
within  is  periodically  emptied  and  renewed,  it  is  doubtless  respiratory. 
But  what  of  other  cases,  where  its  supply  of  blood  is  arterial,  and  what 
especially  where  it  is  entirely  closed  ?  In  such  cases  it  is  usual  to  speak 
of  its  function  as  hydrostatic. 

In  greater  detail,  the  function  of  the  air  bladder  is  (i)  to  render  the 
fish,  bulk  for  bulk,  of  the  same  weight  as  the  medium  in  which  it  lives ; 
moreover  (2)  the  volume  of  the  contained  gas  varies  with  increased 
secretion  and  absorption,  and  seems  to  adjust  itself  to  different  external 
pressures  as  the  fish  descends  or  ascends.  (3)  In  many  fishes  the  bladder 
may  help  indirectly  in  respiration  by  storing  the  superabundance  of 
oxygen  introduced  into  the  blood  by  the  gills.  (4)  There  is  in  several 
Teleosteans,  a  remarkable  connection  between  the  swim  bladder  and 
the  ear,  sometimes  by  an  anterior  process  of  the  bladder,  as  in  the 
herring  and  perch-like  fishes,  sometimes  by  a  chain  of  bones,  as  in 
Siluridae.  This  has  suggested  the  view  that  the  connection  serves  to 
make  the  fish  aware  of  the  varying  tensions  of  gas  in  the  bladder,  due  to 
the  varying  hydrostatic  pressure,  and  in  the  same  connection  it  is 
interesting  to  notice  the  theory  that  the  ear  of  fishes  has  to  do  through 
its  semicircular  canals  with  the  equilibration  and  orientation  of  the 
animal's  movements.  It  is  also  worthy  of  note  that  those  fresh  water 
fishes  (Ostariophysise),  which  have  the  adjusting  mechanism  above 
referred  to,  have  a  marked  ascendancy  over  all  other  fresh  water  species 
in  which  this  mechanism  is  awanting  (Bridge  and  Haddon). 

Flat  fishes. 

In  illustration  of  biological  problems,  let  us  briefly  discuss 
some  of  the  peculiarities  of  the  flat  fishes  (Pleuronectidae), 


FLAT  FISHES.  521 

such  as  flounder,  plaice,  sole,  and  turbot.  These  forms, 
we  at  once  perceive,  are  flattened  from  side  to  side, — un- 
like the  skates  and  rays,  which  are  flattened  from  above 
downwards. 

In  adult  life  they  swim  and  rest  on  one  (the  right  or  the 
left)  side,  and  the  hidden  side  is  unpigmented.  .  Moreover, 
the  eye  belonging  to  the  downward  side  has  come  to  lie 
beside  its  fellow  on  the  upward  side;  the  dorsal  fin  is 
extended  anteriorly,  separating  the  blind  side  of  the  head 
from  that  which  bears  the  eyes  ;  the  inter-orbital  parts  of  the 
frontal  bones,  which  should  be  median,  are  bent  to  the 
upward  side  and  compressed;  and  there  may  be  further 
asymmetry  in  the  skull,  as  in  the  greater  development  of 
jaws  and  teeth  on  the  downward  side.  The  skin  of  the 
downward  side  has  an  opaque  reflecting  layer  (argenteum) 
and  minute  reflecting  elements  (iridocytes),  but  no  pigment 
cells  (chromatophores) ;  all  three  contribute  to  the  colour 
of  the  upturned  surface. 

In  early  life  the  larvae  swim  for  some  time  near  the  surface, 
and  in  the  normal  position,  with  the  dorso-ventral  plane 
vertical.  Then  they  have  an  eye  and  chromatophores  on 
each  side.  As  they  grow  older  they  cease  to  swim  vertically ; 
one  eye  begins  to  move  round  the  edge  of  the  head  (in 
Plagusia  through  an  anterior  extension  of  the  dorsal  fin); 
the  body  is  held  in  a  slanting  position  so  that  the  line  join- 
ing the  eyes  is  kept  horizontal;  more  or  less  rapidly  the 
slant  increases ;  the  lower  eye  gets  quite  round  to  the 
upward  side ;  the  chromatophores  on  the  shaded  side  dis- 
appear ;  and  the  fish  rests  and  swims  on  one  side  at  the 
bottom.  In  the  turbot  the  right  side  is  normally  downward; 
in  the  flounder,  the  left  side,  but  reversed  specimens 
(especially  of  flounder)  often  occur.  Occasionally  these  flat 
fishes  are  pigmented  on  both  sides,  and  then  it  is  some- 
times noted  that  the  migrating  eye  has  not  completed  its 
movement. 

Turbot  and  brill  (species  of  Rhombus]  have  a  well 
developed  swim  bladder  during  metamorphosis,  and  swim 
near  the  surface  until  the  change  is  almost  complete ; 
flounder  and  other  species  of  Pleuronectes  have  no  swim 
bladder  during  metamorphosis,  and  begin  to  lie  on  the 
bottom  almost  as  soon  as  the  change  commences. 


522  FISHES. 

So  far  some  of  the  more  important  facts, — what  of  their 
interpretation  ?  That  these  asymmetrical  forms  have  been 
derived  from  symmetrical  ancestors  is  plainly  suggested  by 
their  development.  Of  the  original  cause  of  the  asymmetry 
we  are  quite  ignorant.  Did  changes  in  the  conditions  of  life 
induce  the  ancestral  forms  to  leave  the  surface  for  the 
bottom  ?  Or  was  the  change  due  to  certain  peculiarities  of 
structure — requiring,  of  course,  previous  explanation — such 
as  the  great  depth  of  the  body  and  the  degeneration  of 
the  swim  bladder  ?  Or  did  both  these  causes  operate  at 
once  ? 

But,  supposing  we  had  attained  to  some  clearness  in 
regard  to  the  change  of  habitat  and  loss  of  vertical  balance, 
we  should  then  have  to  consider  the  twisting  round  of  the 
downward  turned  eye  and  the  absence  of  pigment  cells  on 
the  downward  side. 

As  to  the  change  of  the  eye,  it  may  be  said  (i)  that  this 
has  gradually  resulted  from  the  efforts  of  the  fish  to  continue 
to  use  the  lower  eye,  a  possible  interpretation  if  acquired 
characters  can  be  transmitted.  (2)  It  may  be  said  by  those 
who  do  not  believe  in  "use  inheritance"  that  the  twisting 
round  of  the  lower  eye  is  not  a  result  of  a  transmitted  growth 
tendency  at  all,  but  is  wrought  out  by  effort  in  each  genera- 
tion de  novo.  But  young  turbot  and  brill  have  nearly 
completed  the  twisting  round  of  the  lower  eye  long  before 
they  have  abandoned  their  pelagic  habit.  (3)  It  may  be 
said  that  the  twisting  round  of  the  lower  eye  arose  as  a 
germinal  variation,  apart  from  any  direct  influence  of 
function  or  environment,  and  that  it  has  been  retained  and 
strengthened  in  the  usual  course  of  natural  selection. 

Again,  as  to  the  absence  of  chromatophores,  it  may  be 
supposed  that  this  also  is  a  useful  adaptive  character  per- 
sistent as  the  result  of  selection.  But,  apart  perhaps  from 
economy,  it  is  not  evident  in  what  the  advantage  consists. 
It  seems  more  likely  that  the  under  surface  is  unpigmented 
because  it  is  shaded,  and  Mr.  J.  T.  Cunningham,  who  has 
devoted  special  attention  to  the  problem  of  flat  fishes,  has 
proved  experimentally  that  artificial  illumination  of  the  lower 
sides  by  means  of  a  mirror  induces  the  development  of 
pigment  cells.  It  must  be  noted,  however,  that  pigmenta- 
tion of  both  sides  occurs  also  as  a  natural  variation. 


SENSES— KEPROD  UCTION.  523 

Senses,  &c. 

Fishes  do  not  seem  to  have  much  sense  of  taste  or  of 
smell,  but  diffuse  sensitiveness  to  touch,  chemical  stimuli, 
&c.,  is  well  developed,  especially  on  the  head  and  along  the 
lateral  line.  Though  there  is  no  drum,  and  the  ear  is 
deeply  buried,  they  certainly  hear;  thus  there  are  well  known 
cases  of  tame  fishes  coming  to  the  sound  of  a  bell  or  voice. 
Experiments  have  led  some  to  believe  that  the  semicircular 
canals  of  the  fish's  ear  are  indispensable  in  the  direction 
or  equilibration  of  movement,  and  it  is  obvious  that  this 
function  is  more  important  to  a  fish  than  the  luxury  of 
listening.  But  the  results  of  experiment  are  still  somewhat 
discordant.  The  sense  of  sight  is,  on  the  whole,  well 
developed.  As  to  the  intellectual  powers  of  their  small 
brains  we  know  little,  but  many  show  quickness  in  perceiving 
friends  or  foes,  and  many  of  their  instincts  are  complex. 
At  the  breeding  season  there  is  sometimes  an  elaborate 
expression  of  excitement,  well  seen  in  the  stickleback. 

Reproduction. 

The  sexes  are  separate,  except  in  Chrysophrys  and 
SerranuS)  two  hermaphrodite  bony  fishes,  or  when  abnor- 
mal hermaphroditism  occurs,  as  in  herring,  cod,  mackerel. 
In  many  cases  the  males  are  smaller,  brighter,  and  less 
numerous  than  the  females.  Courtship  is  illustrated  by  the 
sticklebacks  {Gasterosteus,  &c.),  the  paradise  fish  (Macro- 
podus\  and  others,  and  the  bent  lower  jaw  of  the  male 
salmon  reminds  us  that  some  male  fishes  fight  with  their 
rivals. 

Most  Fishes  lay  eggs  which  are  fertilised  and  develop  out- 
side of  the  body.  They  may  be  extruded  on  gravelly  ground, 
or  sown  broadcast  in  the  water.  Sturgeon,  salmon,  and 
some  others  ascend  rivers  for  spawning  purposes,  while  the 
eels  descend  to  the  sea.  In  the  case  of  trout,  Barfurth  has 
observed  that  the  absence  of  suitable  spawning  ground  may 
cause  the  fish  to  retain  its  ova.  This  results  in  ovarian 
disease,  and  in  an  inferior  brood  next  season,  a  fact  which 
should  be  compared  with  what  Hertwig  has  observed  in 
regard  to  Echinoderms,  that  ova  which  are  retained  beyond 
the  normal  period  become  over  ripe  and  pathological. 


524  FISHES. 

Except  in  Elasmobranchs  the  ova  are  relatively  small,  and 
large  numbers  are  usually  laid  at  once.  In  Elasmobranchs, 
the  egg  is  large,  and  in  the  oviparous  genera  it  is  enclosed 
in  a  "mermaid's  purse." 

Most  sharks  and  a  few  Teleosteans  are  viviparous,  the 
eggs  being  hatched  within  the  body  of  the  mother, — in  the 
lower  part  of  the  oviduct  in  sharks,  in  the  ovary  in  Teleos- 
teans. In  two  of  the  viviparous  sharks  (Mustclus  Icevis  and 
Carcharias  glaucus]  there  is  an  interesting  union  between 
the  yolk  sac  and  the  wall  of  the  oviduct,  which  should  be 
compared  with  a  similar  occurrence  in  two  lizards,  and  with 
the  placenta  of  most  Mammals. 

As  to  fertilisation,  the  usual  process  is  that  the  male 
deposits  spermatozoa  or  "milt"  upon  the  laid  eggs  or 
"  spawn,"  but  fertilisation  is  of  course  internal  when  the  eggs 
are  enveloped  in  a  firm  sheath,  or  when  they  are  hatched 
within  the  mother. 

Most  Fishes  have  a  great  number  of  offspring,  and  parental 
care  is  proportionately  little.  Moreover,  the  conditions  of 
their  life  are  not  suited  for  the  development  of  that  virtue. 
When  it  is  exhibited,  it  is  usually  by  the  males, — witness  the 
sea  horse  (Hippocampus]  and  the  pipe  fish  (Syngnathus), 
which  hatch  the  eggs  in  external  pouches,  and  uthe  male  of 
some  species  of  Anus,  who  carries  the  ova  about  with  him 
in  his  capacious  pharynx."  The  female  of  Aspredo  carries 
the  eggs  on  the  under  surface  of  the  body  until  they  are 
hatched,  much  in  the  same  way  as  the  Surinam  toad  bears 
her  progeny  on  her  back,  while  in  Solenostoma  a  pouch  for 
the  eggs  is  formed  by  the  ventral  fins  and  skin.  At  least  a 
dozen  kinds  of  fishes  make  nests,  of  which  the  most  familiar 
illustration  is  that  of  the  male  stickleback,  who  twines  grass 
stems  and  water  weeds  together,  glueing  them  by  mucus 
threads  exuded  as  semi -pathological  products  from  the 
kidneys,  which  are  compressed  by  the  enlarged  male  organs. 

Fishes  have  a  less  definite  limit  of  growth  than  most  other 
Vertebrates,  and  it  is  rare  for  a  fish  to  exhibit  any  of  the 
senile  changes  associated  with  old  age  in  other  Vertebrates. 
But  surroundings  and  nutrition  affect  their  size  and  colour 
very  markedly.  Some  marine  forms,  such  as  flounders,  may 
survive  being  shifted  to  fresh  water,  while  others,  such  as 
salmon  and  sturgeon,  pass  from  sea  to  rivers  at  spawning 


INTER-RELATIONS.  525 

time.  But  many  are  sensitive  to  changes  of  medium. 
Many  can  endure  prolonged  fasting,  and  some  may  survive 
being  frozen  stiff.  Lowered  temperature  may  induce  torpor, 
as  seen  in  the  winter  sleep  of  the  pike,  while  in  the  dry 
season  of  hot  countries  the  mud  fishes,  the  Siluroids,  and 
others,  encyst  themselves  in  the  mud,  and  remain  for  a  long 
time  in  a  state  of  "latent  life." 

In  ter-rela  tions. 

Commensalism  is  illustrated  by  some  small  fishes  which 
shelter  inside  large  sea  anemones,  and  by  Fierasfer,  which 
goes  in  and  out  of  sea  cucumbers  and  medusae.  On  the 
outside  or  about  the  gills  of  Fishes  parasitic  Crustaceans, 
fish  lice,  are  often  found ;  various  Flukes  are  also  common 
external  parasites,  and  many  Cestodes  in  bladderworm  or 
tapeworm  stage  infest  the  viscera.  The  immature  stages  of 
Bothriocephalus  latus  occur  in  pike  and  burbot ;  a  remark- 
able hydroid  (Polypodiuni)  is  parasitic  on  the  eggs  of  a 
sturgeon ;  the  young  of  the  fresh  water  mussel  are  tem- 
porarily parasitic  on  the  stickleback ;  and  the  young  of  the 
Bitterltng  (Rhodeus  amarus)  live  for  a  time  within  the  gills 
of  fresh  water  mussels. 

Distribution  in  Space. — There  are  about  2300  species  of  fresh 
water  fishes,  three  or  four  Dipnoi,  about  thirty  Ganoids,  and  the  rest 
Teleosteans,  over  a  half  being  included  in  the  two  families  of  carps 
(Cyprinidse)  and  cat  fishes  (Siluridse). 

Among  marine  fishes,  about  3500  species  frequent  the  coasts,  rarely 
descending  below  30x3  fathoms.  A  much  smaller  number,  including 
many  sharks,  live  and  usually  breed  in  the  open  sea.  About  loo 
genera  have  been  recorded  from  great  depths. 

In  regard  to  the  last,  Dr.  Giinther  has  shown  that  in  forms  living  at 
depths  from  80-200  fathoms,  the  eyes  tend  to  be  larger  than  usual,  as  if 
to  make  the  most  of  the  scanty  light ;  beyond  the  200  fathom  line 
small-eyed  forms  occur  with  highly  developed  organs  of  touch,  and 
large-eyed  forms  which  have  no  such  organs,  but  seem  to  follow  the 
gleams  of  "  phosphorescent "  organs ;  finally,  in  the  greatest  depths 
blind  fishes  occur  with  rudimentary  organs.  Many  of  these  abyssal 
fishes  are  phosphorescent ;  the  colouring  is  usually  simple,  mostly 
blackish  or  silvery ;  the  skin  exudes  much  mucus ;  the  skeleton  tends 
to  be  light  and  brittle ;  the  forms  are  often  very  quaint ;  the  diet  is 
necessarily  carnivorous. 


[TABLE. 


526 


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528  FISHES. 

The  relationships  of  Fishes. 

Balfour  regarded  the  Elasmobranchs  as  nearest  the  ancestral  stock  ; 
while  from  hypothetical  Proto-Ganoids  he  derived  on  the  one  hand  the 
Dipnoi,  on  the  other  hand  the  Ganoids,  and  thence  the  Teleosteans. 

But  it  must  be  noted  that  the  Dipnoi  are  markedly  separated  in  many 
ways  from  living  Ganoids.  Moreover,  the  extinct  Ganoids  form  a  very 
large  and  diverse  series,  which  cannot  be  fairly  appreciated  by  a  study 
of  the  few  survivors.  Nor  does  the  palceontological  evidence  bear  out 
the  separateness  of  Teleosteans  and  Ganoids. 

Giinther  distinguishes  the  sub -class  Teleostei  from  the  sub -class 
Palseichthyes,  including  under  the  latter  the  Chondropterygii  (Elasmo- 
branchs and  Holocephali)  and  the  Ganoidei  (along  with  which  Dipnoi 
are  ranked).  As  two  other  sub-classes  of  Fishes,  he  recognises  the 
Cyclostomata  and  the  Leptocardii  (Amphwxns}. 

Beard  proposes  the  following  classification  of  Ichthyopsida,  insisting 
especially  on  the  separateness  of  Dipnoi  from  Ganoids,  and  on  their 
nearness  to  Amphibians  : — 

fMarsipobranchii  (Cyclostomata). 
Ganodichthyidae.     -!  Ganoidei. 
{ Teleostei. 


CHAPTER    XXIII. 

CLASS    AMPHIBIA. 

Order  I.  LABYRINTHODONTIA  or  STEGOCEPHALA  (extinct). 
II.  GYMNOPHIONA  or  APODA  (a  small  order). 

III.  URODELA  or  CAUDATA,  e.g.,  Newts  and  Salamanders. 

IV.  ANURA  or  ECAUDATA,  e.g.,  Frogs  and  Toads. 

AMPHIBIANS  represent  in  the  evolution  of  Vertebrates  those 
forms  which  made  the  transition  from  aquatic  to  terrestrial 
life,  but  have  lagged  near  the  water.  Certain  acquisitions, 
such  as  lungs  and  a  three-chambered  heart,  gained  by  the 
Dipnoi,  are  here  firmly  established.  The  race  has  dwindled 
in  size  of  body  since  the  early  days  of  its  beginning,  but  it 
seems  to  have  been  progressive,  for  Amphibians  are  not 
awanting  in  affinities  with  Reptiles  or  even  with  Mammals. 

GENERAL  CHARACTERS.  —  Amphibia  are  Vertebrates  in 
which  the  visceral  arches  of  the  larva  almost  always  bear 
gills,  which  may  be  retained  throughout  life,  though  the  adults 
'always  possess  functional  lungs.  When  limbs  are  present 
they  exhibit  distinct  digits,  and  conform  to  the  same  type  as 
those  of  higher  Vertebrates.  Although  unpaired  fins  are 
frequently  present,  both  in  larval  and  adult  life,  there  are  no 
fin  rays.  In  existing  forms  there  is  rarely  any  exoskeleton, 
but  some  extinct  forms  were  furnished  with  an  armour  of 
bony  plates.  The  heart  is  three  chambered,  having  two 
auricles  and  a  ventricle.  The  gut  ends  in  a  cloacal  chamber 
into  which  the  urinogenital  ducts  open.  A  bladder,  which 
grows  out  from  the  hind  region  of  the  gut,  is  probably  homo- 
logous with  *the  allantois  of  the  embryos  of  higher  Vertebrates. 
The  ova  are  small,  numerous,  usually  pigmented,  and  with 
yolk  towards  one  pole.  They  are  almost  always  laid  in 
water ;  the  segmentation  is  holoblastic,  but  unequal.  There 
is  often  a  marked  metamorphosis  in  development. 

34 


530 


AMPHIBIA. 


Huxley  was  the  first  to  recognise  the  affinities  between  Fishes  and 
Amphibians,  and  to  unite  the  two  classes  under  the  title  Ichthyopsida. 

Of  the  common  characters  of  the  two  classes,  we  may  emphasise  the 
following  : — gills  are  always  present,  but  may  be  restricted  to  the  larval 
stages  ;  there  is  no  amnion,  and  at  most  a  homologue  of  the  allantois  ; 
there  are  lateral  sensory  structures,  such  as  the  "  branchial  sense- 
organs"  and  those  of  the  "lateral  line,"  but  these  may  be  diminished 
in  the  adults  ;  unpaired  fins  are  almost  always  represented,  but  may 
not  persist  in  the  adult  life. 

From  the  higher  Vertebrates  or  Amniota  the  Ichthyopsida  are  clearly 
distinguished  by  the  presence  of  gills  (in  youth  at  least)  and  by  the 
absence  of  amnion  and  allantois.  For  though  the  bladder  of  Amphi- 
bians may  be  homologous  with  an  allantoic  outgrowth,  it  does  not  function 
as  such,  /.<?.,  it  does  not  aid  in  the  respiration  or  the  nutrition  of  the 
embryo. 

It  is  more  difficult  to  distinguish  between  Fishes  and  Amphibians, 
more  especially  if  we  include  the  Dipnoi  in  the  former  class.  The  most 
obvious  differences  are  the  absence  of  fin-rays  and  the  development  of 
fingers  and  toes.  In  the  following  table  the  two  classes  are  con- 
trasted : — 


FISHES. 


AMPHIBIANS. 


Gills  persist  throughout  life. 

The  swim  bladder  functions  as  a  lung  in 
Dipnoi  and  less  markedly  in  some 
Ganoids,  but  in  most  cases  its  respira- 
tory significance  is  slight. 

The  heart  is  two-chambered  (incipiently 
three-chambered  in  Dipnoi).  There 
is  no  inferior  vena  cava,  except  in 
Dipnoi. 

The  limbs  are  fins. 

The  unpaired  fins  are  supported  by  fin 
rays. 

The  skull  has,  in  most  cases,  one  occipital 
condyle. 

There  is  usually  an  exoskeleton  of  scales 
or  scutes. 

Except  in  Dipnoi,  the  nasal  sacs  do  not 
open  posteriorly  into  the  mouth. 

There  is  no  certain  homologue  of  the 
allantois. 


Gills  may  disappear  as  the  adult  form  is 

attained. 
Lungs  are  always  developed  in  the  adults. 

It     is    doubtful    whether    they    are 

directly    comparable  with   the  swim 

bladder. 
The  heart  has  three  chambers.  There  is 

an  inferior  vena  cava. 


The  limbs  have  digits. 
There  are  no  fin  rays. 

There  are  two  occipital  condyles. 

There    is    no   exoskeleton,    except   in    a 

few  exceptional  cases,  and  in  extinct 

forms. 
There   are  posterior  nares  opening  into 

the  cavity  of  the  mouth. 
The  bladder  seems  to  be  the  homologue 

of  the  allantois. 


The  Frog  as  a  type  of  Amphibians. 

The  common  British  frog  (Rana  temporarid)  and  the 
frequently  imported  Continental  species  (R.  esculenta)  agree 
in  essential  features.  A  black  patch  on  the  side  of  the 
head  behind  the  ear  distinguishes  our  British  species ;  the 
males  of  the  edible  frogs  have  special  resonating  sacs,  and 
there  are  other  trivial  differences. 


EXTERNAL    FEATURES   OF  FROG.  531 

Though  aquatic  in  youth,  frogs  often  live  in  dry  places, 
hiding  in  great  drought,  reappearing  when  the  rain  returns. 
Everyone  knows  how  they  sit  with  humped  back,  how  they 
leap,  how  they  swim.  They  feed  on  insects  and  slugs. 
These  are  caught  by  the  large  viscid  tongue,  which,  being 
fixed  in  front  of  the  mouth  and  free  behind,  can  be  jerked 
out  to  some  distance,  and  with  even  greater  rapidity 
retracted.  When  we  watch  a  frog,  we  see  that  the  nostrils 
are  alternately  opened  and  closed,  and  that  the  under  side 
of  the  throat  is  rhythmically  expanded  and  compressed,  the 
mouth  remaining  shut  meanwhile;  the  movements  are 
evidently  connected  with  respiration.  That  the  males 
trumpet  in  the  early  spring  to  their  feebly  responsive  mates, 
that  in  our  British  species  the  pairing  takes  place  soon  after, 
that  the  young  are  tadpoles,  that  a  notable  metamorphosis 
takes  place,  are  familiar  facts  of  observation.  In  winter  the 
frogs  hibernate,  buried  in  the  mud  of  the  pond,  and  breath- 
ing through  their  skin. 

Form  and  External  Features. 

We  notice  the  absence  of  neck  and  tail,  the  short  fore- 
limbs  almost  without  thumbs,  the  longer  hind  limbs  with  five 
webbed  nailless  toes  and  with  a  long  ankle  region,  the 
apparent  hump  back  where  the  hip  girdle  is  linked  to  the 
vertebral  column.  There  is  a  very  rudimentary  thumb,  and 
there  is  a  horny  knob  at  the  base  of  the  hallux  or  "  great 
toe."  At  pairing  time,  the  skin  of  the  first  finger  is  modified 
in  the  males  into  a  rough  cushion,  darkly  coloured  in 
R.  temporaria. 

We  see  the  wide  mouth,  the  paired  nostrils,  the  projecting 
eyes,  the  upper  eyelid  thick,  pigmented,  and  almost  im- 
movable, the  lower  semi-transparent  and  moving  very 
freely,  the  circular  drum  of  the  ear,  the  smooth  skin,  with 
patches  of  a  deeper  tint  on  its  yellowish  ground,  and  the 
slightly  dorsal  cloacal  aperture. 

Skin. 

The  smooth,  moist  skin  is  but  loosely  attached  to  some 
parts  of  the  body ;  it  consists  of  an  external  two-layered 
(ectodermic)  epidermis,  and  an  internal  (mesodermic)  dermis. 
The  outer  layer  of  the  epidermis  is  shed  periodically.  The 


532 


AMPHIBIA. 


dermis  differs  markedly  from  that  of  a  fish,  for  there  is  no 
exoskeleton,  but  this  was  present  in  the  extinct  Labyrintho- 
donts  ;  there  are  multicellular  glands,  whose  secretion  makes 
the  skin  moist ;  and  there  is  a  stratum  of  unstriped  muscle 
fibres.  In  the  dermis  there  are  also  branched  pigment 
cells,  usually  in  two  strata.  Through  a  reflex  nervous  action 
they  are  slightly  affected  by  the  colour  of  the  surroundings, 
the  pigment-bearing  internal  cell 
substance  contracting  or  expanding, 
and  thus  producing  colour  change. 
There  are  cutaneous  blood  vessels, 
by  means  of  which  the  frog  can,  to 
a  certain  extent,  breathe  by  its  skin. 
The  tadpole  has  sensory  cells  ar- 
ranged in  distinct  lateral  lines,  but 
of  these  the  adult  retains  no  definite 
trace,  though  there  are  many  nerve- 
endings  and  "touch  spots"  in  the 
skin. 


Skeleton. 


11. 


The  vertebral  column  consists  of 
nine  vertebrae,  and  an  unsegmented 
portion  called  the  urostyle. 

The  first  vertebra  bears  two  facets 
for  the  two  condyles  of  the  skull 
and  an  odontoid  process  which  lies 
between  the  condyles.  Its  arch  is 
incompletely  ossified.  Each  of  the 
next  six  has  an  anteriorly  concave 
or  proccelous  centrum,  a  neural  arch 
surrounding  the  spinal  cord,  a  trans- 
verse process  from  each  side  of  the  base  of  the  arch,  an 
anterior  and  a  posterior  pair  of  articular  processes,  and 
a  short  neural  spine.  The  eighth  vertebra  has  a  biconcave 
or  amphiccelous  centrum.  The  ninth  is  convex  in  front, 
with  two  convex  tubercles  behind,  and  bears  large  trans- 
verse processes  with  which  the  hip  girdle  articulates.  The 
urostyle  has  anteriorly  a  dorsal  arch  enclosing  a  prolonga- 
tion of  the  spinal  cord,  but  both  arch  and  nerve  cord 
disappear  posteriorly.  The  notochord,  around  which  the 


.Fe. 


FIG.  175.  — Vertebral 
column  and  pelvic  girdle 
of  Bull -frog. 

/./,  Transverse  processes  of 
sacral  vertebra  ;  //,  ilium  ;  £/, 
urostyle  ;  Fe^  femur ;  Isck, 
ischiac  region. 


SKELETON. 


533 


Tmx 


vertebral  column  has  developed,  is  finally  represented  only 
by  vestiges  in  the  centra  of  the  vertebrae. 

The  skull  consists  (a)  of  the  persistent  parts  of  the 
original  cartilaginous  brain  box  or  chondrocranium,  de- 
veloped, as  in  the  skate,  from  parachordals  and  trabeculae, 

plus  nasal  and  auditory 
capsules ;  (<£)  of  ossifi- 
cations of  parts  of  the 
chondrocranium,  cartilage 
bones ;  (c)  of  membrane 
or  investing  bones ;  and 
(d)  of  associated  visceral 
arches. 


Part  of  the  chondrocranium 
persists  as  an  encasement  of 
the  brain.  Two  exoccipitals 
bounding  the  foramen  magnum 
and  forming  the  condyles,  two 
pro-otics  or  ossifications  of  the 
original  auditory  capsule,  and 
an  impaired  sphenethmoid 
forming  the  front  of  the  brain- 
case  are  cartilage  bones.  Two 
parieto-frontals  and  two  nasals 
above,  a  paired  vomer  and  an 
unpaired  dagger  shaped  para- 
sphenoid  beneath,  and  two 
lateral  hammer  shaped  squa- 
mosals  are  membrane  bones. 
There  is  no  basisphenoid  ossi- 
fication. 

To  these  are  added  the  small 
pre-maxilkie  in  the  very  front 
of  the  skull,  the  long  maxillae 
on  each  side,  the  quadrato- 
jugals  which  continue  the 
latter  to  the  minute  nodule 
which  represents  the  quadrate 
bone. 

On  the  roof  of  the  mouth, 
extending  from  the  quadrate 
forwards  to  near  the/vomers, 
are  the  triradiate  pterygoids, 
igle 


FIG.  176.— Skull  of  Frog— upper 
and  lower  surface.  (After  W.  K. 
PARKER.) 

Upper  surface — 

Pmx,  premaxilla ;  N,  nasal  ;  M,  max- 
illa ;  Sg,  squamosal ;  Qjt  quadrato-jugal ; 
e.o.,  ex-occipitals  ;  P.f.,  parieto-frontals : 
Sph.E,  sphenethmoid  ;  P.O.,  pro-otic. 

Lower  surface — 

Pmx,  premaxilla  ;  M,  maxilla  ;  Qj,  quad- 
rato-jugal :  Q,  quadrate  ;  Ptt  pterygoid  ; 
Ps,  parasphenoid;  P.O.,  pro-otic;  Sfi.E, 
sphenethmoid  ;  PI,  palatine  ;  V,  vomer ;  c, 
columella. 


while   at   right   angles  to  the 

anterior   end   of    the    parasphenoid   and   behind    the   vomers   are   the 
palatines. 

Each  half  of  the  lower  jaw,  based  on  Meckel's  cartilage,  consists  of 


534  AMPHIBIA. 

three  pieces, — the  largest  an  articular  angulo-splenial,  outside  this  a 
thin  dentary,  and  anteriorly  uniting  with  its  fellow  a  minute  mento- 
meckelian. 

A  delicate  rod — the  columella  auris — extends  from  the  tympanum  to 
the  fenestra  ovalis  in  the  internal  capsule  of  the  ear.  According  to 
Parker,  it  represents  the  upper  part  of  the  hyoid  arch,  the  lower  portion 
of  which  forms  the  cartilaginous  or  partially  ossified  hyoid  plate,  which  lies 
in  the  floor  of  the  mouth  and  is  produced  into  two  anterior  and  two 
posterior  cornua.  According  to  Villy,  however,  the  columella  is  morpho- 
logically connected  with  the  ear  capsule. 

The  teeth  are  borne  by  the  premaxillae,  maxillae,  and  vomers. 
There  is  no  parietal  foramen,  but  in  the  Labyrinthodonts  it  is  always 
distinct,    and   the    pineal    body   is   supposed   to   have   been   well   de- 
veloped.    The  foramen  is  also  very  distinct  in  some  of  the  extinct 
Ganoid  Fishes. 

The  cartilage  which  bears  the  quadrate  at  its  lower  end,  and  runs 
between  pterygoid  and  squamosal,  connecting  the  articulation  of  the 
lower  jaw  with  the  side  of  the  skull  at  the  auditory  capsule,  is  called 
the  suspensorium.  In  Elasmobranchs,  the  hyomandibular  is  the 
suspensorium  ;  in  Teleosteans,  the  name  is  applied  to  the  hyomandibular 
and  symplectic  ;  in  Sauropsida,  the  quadrate  occasionally  gets  the  same 
confusing  title. 

When  the  lower  jaw  is  connected  with  the  skull  wholly  by  elements 
of  the  hyoid  arch,  as  in  most  Elasmobranchs  and  Ganoids,  and  all 
Teleosteans,  the  term  hyostylic  is  used.  When  the  connection  is  due 
to  a  quadrate  element  only,  as  in  Amphibia  and  Sauropsida,  it  is 
called  autostylic.  When  there  is  both  a  hyoid  and  a  quadrate  element, 
as  in  Lepidostetts  among  Ganoids,  or  a  hyoid  and  a  palatoquadrate,  as  in 
Cestracion  among  Elasmobranchs  and  perhaps  also  in  Holocephali,  the 
term  amphistylic  is  used.  Finally,  it  may  be  noted  here  that  in 
Mammals  the  lower  jaw  articulates  with,  the  squamosal. 

The  first  or  mandibular  arch  gives  origin  inferiorly  to  Meckel's 
cartilage,  which  forms  the  basis  and  persistent  core  of  the  lower 
jaw,  and  superiorly  to  the  palato-pterygo-quadrate  cartilage  which 
is  represented  in  the  adult  by  the  minute  quadrate  bone,  by  the 
suspensorial  cartilage,  and  by  other  cartilages  which  are  invested 
by  the  pterygoid  and  palatine  bones. 
The  second  or  hyoid  arch  gives  origin  inferiorly  to  the  hyoid  plate, 

superiorly,  according  to  Parker,  to  the  columella. 
Of  the  four  posterior  branchial  arches,  there  are  in  the  adult  some 
persistent  remnants,  e.g^  in  the  larynx. 

The  Limbs  and  Girdles. 

The  shoulder  girdle  consists  of  a  dorsal  portion — the 
scapula  and  the  partially  cartilaginous  supra-scapula,  and  of 
a  ventral  portion — the  coracoid,  and  the  clavicle,  and  the 
cartilaginous  precoracoid  on  which  the  clavicle  lies.  There 
is  some  uncertainty,  however,  in  regard  to  the  relations  of 
the  last  two ;  according  to  one  view,  the  clavicle  is  unrepre- 


THE   LIMBS  AND    GIRDLES. 


535 


sented.  The  glenoid  cavity  with  which  the  humerus 
articulates  is  formed  as  usual  by  the  junction  of  scapula 
and  coracoid. 

Between  the  median  ends  of  the  coracoids  lie  two 
cartilaginous  epicoracoids,  behind  which  is  a  bony  part  of 
the  sternum,  prolonged  posteriorly  into  a  notched  cartila- 
ginous xiphisternum.  Anteriorly  lies  a  bony  portion  called 
the  omosternum,  which  is  prolonged  forwards  into  an  epi- 
sternum  cartilage.  This  sternum  does  not  arise  like  that  of 
higher  Vertebrates  from  a  fusion  of  the  ventral  ends  of  ribs. 
Indeed,  there  are  no  ribs  in  the  frog,  unless  they  be  minute 
rudiments  at  the  ends  of  the  transverse  processes. 

The  skeleton  of  the  fore  limb  consists  of  an  upper  arm 


FIG.  177. — Pectoral  girdle  of  Rana  esctilenta. 
(After  ECKER.  ) 

The  cartilaginous  parts  are  dotted.  Ep,  episternum  ;  om,  omo- 
sternum ;  Ep.c,  epicoracoids  ;  st,  sternum  ;  x,  xiphisternum  ;  c/, 
clavicle  with  underlying  precoracoid  cartilage  ;  co,  coracoid  ;  Sc, 
scapula  '  S.sc,  supra-scapula  ;  Gl,  glenoid  cavity  for  humerus. 

or  humerus,  a  fore  arm  in  which  the  inner  radius  and  the 
outer  ulna  are  fused,  a  wrist  or  carpus  including  two 
proximal  and  three  distal  elements,  and  a  central  piece 
wedged  in  between  them,  five  metacarpal  bones,  of  which 
the  first— corresponding  to  the  absent  thumb — is  very 
small,  and  four  fingers,  of  which  the  two  innermost 
have  two  joints  or  phalanges,  while  the  two  others  have 
three. 


536  AMPHIBIA. 

The  pelvic  girdle  has  a  long  V-shape,  the  ends  of  which 
are  cartilaginous  and  articulate  with  the  expanded  trans- 
verse processes  of  the  ninth  or  sacral  vertebra.  Each 
limb  of  the  V  is  an  ilium ;  the  united  posterior  part 
consists  of  a  fused  pair  of  ischia,  and  a  ventral  cartila- 
ginous pubic  portion.  Ilia,  ischium,  and  pubes  unite  in 
bounding  the  deep  socket  or  acetabulum  with  which  the 
femur  articulates. 

The  skeleton  of  the  hind  limb  consists  of  a  thigh  bone  or 
femur,  a  lower  leg  formed  from  the  united  tibia  and  fibula, 
an  ankle  region  or  tarsus  including  two  long  proximal 
elements — the  astragalus  or  tibiale  and  the  calcaneum  or 
fibulare — and  two  imperfectly  ossified  distal  elements,  five 
metatarsal  bones,  and  five  toes.  The  first  toe  or  hallux 
has  two  phalanges,  the  second  also  two,  the  third  three, 
the  fourth  four,  the  fifth  three,  and,  finally,  outside  the 


FIG.  178.-— Side  view  of  frog's  Pelvis.     (After  ECKER.) 
//,  ilium  ;  fs,  ischium  ;  Pb,  pubis  ;  Ac,  acetabulum. 

hallux  there  is  a  "calcar,"  which  looks  like  an  extra  toe 
and  consists  of  three  pieces.  The  astragalus  is  in  line  with 
the  first  toe. 

Muscular  System. 

I  shall  not  describe  the  musculature  of  any  of  the 
V  ertebrate  types  With  the  guides  to  practical  work  cited 
in  the  appendix  the  student  will  readily  find  out  what  he 
wishes  to  know. 

The  muscles  are  enswathed  in  connective  tissue.  They 
consist  of  bundles  of  muscle  fibres,  and  at  their  ends  or  at 
one  of  them  they  are  usually  continued  into  strong  tendons, 
which  are  more  or  less  directly  attached  to  parts  of  the 
skeleton. 


NERVOUS  SYSTEM. 


537 


Nervous  System. 

The  brain,  covered  with  a  darkly  pigmented  pia  mater, 
has  the  usual  five  parts. 

The  elongated  cerebral  hemispheres  have  "olfactory 
lobes  "  in  front  of  them,  and  are  connected  by 
anterior  and  posterior  commissures,  and  by  a  hint 
of  a  "  corpus  callosum  "  (?). 

The  thalamencephalon  gives  origin  dorsally  to  a  pineal 
outgrowth  closely  attached  to  the  skull.  On  the 
ventral  side  will  be  seen  the  chiasma  or  interlaced 
crossing  of  the  optic  nerves,  and  a  tongue-shaped  mass 
(the  tuber  cinereum),  to  which  the  pituitary  body  is 
attached,  and  which  is  produced  by  the  depression 


1.V.2 


op.l- 

cb 


V-4 


III 


FIG.  179. — Brain  of  Frog.     (After  WIEDERSHEIM.) 

I.  DORSAL  ASPECT.— o.l.,  olfactory  lobes;  c.h.,  cerebral  hemi- 
spheres ;  />.,  pineal  body,  rising  from  optic  thalami ;  op. I.,  optic  lobes ; 
cb.,  rudimentary  cerebellum  ;  M.O.,  medulla  oblongata. 

II.  VENTRAL  ASPECT.— The  numbers  indicate  the  origins  of  the 
nerves  ;  ch.  optic  chiasma  ;  T.c.,  tuber  cinereum  ;  ff.,  hypophysis. 

III.  LONGITUDINAL  SECTION. — l.v.,  i  and  2,  lateral  ventricles  of 
cerebrum  ;  P.m.,  foramen  of  Monro  ;  K,  3  and  4,  third  and  fourth 
ventricles  ;  Aq.  cavities  of  optic  lobes  and  aqueduct  of  Sylvius  from 
third  to  fourth  ventricle. 

of  the   floor   of  the   third    ventricle   to    form    the 

infundibulum. 
The  optic  lobes,  a  pair  of  oval  bodies  between  and  below 

which  is  the  iter. 
The  cerebellum,  a  very  narrow  transverse  band. 


538 


AMPHIBIA. 


The  medulla  oblongata,  on  the  roof  of  which  the  pia 
mater  forms  a  very  vascular  "choroid  plexus." 

In  the  tadpole  the  pineal  body  lies  outside  the  skull 
under  the  skin  of  the  head ;  it  atrophies  at  the  metamor- 
phosis, so  that  in  the  adult  the  stalk  only  is  represented. 


FIG.  180. — Nervous  system  of  Frog.     (After  ECKER.) 

i-io.  The  cranial  nerves  ;  oc,  eyes  ;  crb,  in  front  of  optic  chiasma  ; 
to,  optic  tract ;  sym,  sympathetic  system :  msp,  spinal  cord  ; 
sp,  spinal  nerves. 

The  cranial  nerves  are,  as  usual,  on  each  side  the  follow- 
ing :— 

(1)  Olfactory,  from  the  olfactory  lobe  to  the  nose  ; 

(2)  Optic,  crossing  and  interlacing  with  its  fellow  ; 

(3)  Oculomotor,  to  four  muscles  of  the  eye  ; 


SENSE   ORGANS.  539 

(4)  Pathetic,  to  the  superior  oblique  eye  muscle  ; 

(5)  Trigeminal,    with     ophthalmic,    maxillary,     and     mandibular 

branches ; 

(6)  Abducens,  to  the  external  rectus  eye  muscle  ; 

(7)  Facial,  arising  along  with  the  auditory,  with  a  ganglion  uniting 

with  the  Gasserian  ganglion  of  the  trigeminal,  with  a  pala- 
tine branch  to  the  roof  of  the  mouth,  and  a  hyoid  branch  to 
the  lower  jaw  ; 

(8)  Auditory,  to  the  ear  ; 

(9)  Glossopharyngeal,  to  the  tongue  and  some  of  its  muscles  ;  with 

a  ganglion  which  unites  with  that  of  the  tenth  ; 
(10)  Vagus,  with  branches  to  lungs,  heart,  stomach,  &c. 
The  student  should  refer  back  to  the  description  of  the  skate,  and  to 
the  chapter  on  the  structure  of  Vertebrates. 

The  spinal  cord  gives  origin  to  ten  pairs  of  spinal  nerves, 
and  is  swollen  at  the  origin  of  those  which  go  to  the  limbs. 
Around  the  union  of  the  anterior  and  posterior  roots  lie  sacs 
with  crystals  of  carbonate  of  lime. 

The  sympathetic  system  consists  of  about  ten  pairs  of 
ganglia — (a)  united  by  branches  to  the  spinal  nerves ;  (^) 
united  to  one  another  by  longitudinal  trunks  which  accom- 
pany the  dorsal  aorta  and  the  systemic  arches,  and  end 
anteriorly  in  the  Gasserian  ganglion  ;  (c)  giving  off  branches 
to  the  heart,  the  aorta,  and  the  viscera  in  the  pelvic  region. 

Sense  Organs. 

The  eyes  project  on  the  top  of  the  head  and  on  the  roof 
of  the  mouth.  There  are  two  lids,  the  upper  thick  and  very 
slightly  movable,  the  lower  transparent  and  movable.  The 
transparent  cornea  in  front,  the  firm  sclerotic  surrounding 
the  eyeball,  and  the  sheath  of  the  optic  nerve,  are  as  usual 
continuous.  The  next  layer  includes  the  vascular  and  pig- 
mented  choroid  and  the  brilliant  iris.  Internally  is  the 
sensitive  retina,  while  vitreous  humour  fills  the  cavity  behind 
the  lens. 

The  internal  ears  have  the  usual  parts  and  lie  within  the 
auditory  capsules,  which  are  in  great  part  bounded  by  the 
pro-otics.  Connecting  the  fenestra  ovalis  of  the  ear  with 
the  tympanic  membrane,  which  is  flush  with  the  skin,  there 
is  a  delicate  bony  rod — the  columella.  This  lies  in  the 
Eustachian  tube,  which  opens  into  the  mouth  at  the  corner 
of  the  gape. 

The  nostrils  open  into  small  nasal  cavities,  with  folded 


540  AMPHIBIA. 

walls  of  sensitive  membrane ;  the  posterior  nares  open  into 
the  front  of  the  mouth. 

There  are  taste  papillae  on  the  tongue,  and  touch  spots 
on  the  skin. 

Alimentary  System. 

The  frog  feeds  in  great  part  on  insects,  which  it  catches 
dexterously  with  its  tongue.  This  is  fixed  in  front  and  loose 
behind.  There  are  teeth  on  the  pre-maxillae,  maxillae,  and 
vomers.  Into  the  cavity  of  the  mouth  the  nasal  sacs  open 
anteriorly,  and  the  Eustachian  tubes  posteriorly.  The  males 
of  Rana  esculenta  have  a  pair  of  sacs  which  open  into  the 
mouth  cavity  at  the  angle  of  the  jaw,  and  are  dilated  during 
croaking.  The  tongue  bears  numerous  taste  papillae.  Be- 
hind the  tongue  on  the  floor  of  the  mouth  is  the  glottis,  the 
opening  of  the  short  larynx  which  leads  to  the  lungs.  The 
larynx  is  supported  by  two  arytenoid  cartilages,  and  also  by 
a  ring;  with  the  arytenoids  the  vocal  cords  are  closely 
associated.  The  lungs  lie  so  near  the  mouth  that  laryngeal, 
tracheal,  and  bronchial  regions  are  hardly  distinguishable. 
On  the  floor  of  the  mouth  is  the  hyoid  cartilage,  which  serves 
for  the  insertion  of  muscles  to  tongue,  &c. 

Of  the  (4)  gill  clefts  which  are  borne  on  the  walls  of  the 
pharynx  in  the  tadpole,  there  are  no  distinct  traces  in  the 
adult.  The  lungs  develop  as  outgrowths  from  the  gullet. 

The  gullet  leads  into  a  tubular  stomach,  which  is  not 
sharply  separated  from  it.  There  is  a  pyloric  constriction 
dividing  the  stomach  from  the  duodenum,  or  first  part  of  the 
small  intestine.  After  several  coils  the  small  intestine  opens 
into  the  wider  large  intestine  or  rectum,  which  enters  the 
cloaca. 

The  liver  has  a  right  and  a  left  lobe,  the  latter  again  sub- 
divided. The  gall  bladder  lies  between  the  right  and  left 
lobes  ;  bile  flows  into  it  from  the  liver  by  a  number  of 
hepatic  ducts,  which  are  continued  onwards  to  the  duodenum 
in  a  common  bile  duct.  The  pancreas  lies  in  the  mesentery 
between  stomach  and  duodenum,  and  its  secretion  enters 
the  distal  portion  of  the  bile  duct.  The  bladder  is  a  ventral 
outgrowth  of  the  cloaca,  has  no  connection  with  the  ureters, 
and  seems  to  be  homologous  with  the  allantois  of  Reptiles, 
Birds,  and  Mammals. 


VASCULAR   SYSTEM.  541 

Vascular  System. 

The  heart,  enclosed  in  a  pericardium,  is  three-chambered, 
consisting  of  a  muscular  conical  ventricle,  which  drives  the 
blood  to  the  body  and  the  lungs,  of  a  thin-walled  right 
auricle  receiving  impure  blood  from  the  body,  and  of  a  thin- 
walled  left  auricle  receiving  purified  blood  from  the  lungs. 
From  each  of  the  auricles  blood  enters  the  ventricles.  The 
two  superior  venae  cavae  which  bring  back  blood  from  the 
anterior  regions  of  the  body,  and  the  inferior  vena  cava 
which  brings  back  blood  from  the  posterior  parts,  unite  on 
the  dorsal  surface  of  the  heart  in  a  thin-walled  sinus  venosus, 
which  serves  as  a  porch  to  the  right  auricle.  From  the 
ventricle  the  blood  is  driven  up  a  truncus  arteriosus,  which 
soon  divides  into  two  branches,  each  of  which  divides  into 
three  aortic  arches. 

Thus  we  may  distinguish  five  regions  in  the  heart, — the  ventricle,  the 
right  auricle,  the  left  auricle,  the  sinus  venosus,  and  the  truncus 
arteriosus.  The  sinus  venosus  is  the  hindmost,  the  truncus  arteriosus 
the  most  anterior  part.  The  two  auricles  are  often  included  in  the  term 
atrium,  the  undivided  part  of  the  truncus  arteriosus  next  the  ventricle  is 
called  the  pylangium,  the  more  anterior  part  from  which  the  arches  arise 
is  known  as  the  synangium.  The  truncus  arteriosus  corresponds,  in 
greater  part  at  least,  to  the  conus  arteriosus  of  many  fishes. 

As  the  heart  continues  to  live  after  the  frog  is  really  dead,  its  contrac- 
tions can  be  readily  observed.  The  sinus  venosus  contracts  first,  then 
the  two  auricles  simultaneously,  and  finally  the  ventricle.  Although  the 
ventricle  receives  both  impure  and  pure  blood,  the  structural  arrange- 
ments are  such  that  most  of  the  impure  blood  is  driven  to  the  lungs,  the 
purest  blood  to  the  head,  and  somewhat  mixed  blood  to  the  body. 

The  blood  contains  in  its  fluid  plasma — (a)  the  oval 
"  red "  corpuscles  with  a  definite  rind,  a  distinct  nucleus, 
and  the  pigment  haemoglobin  ;  (b]  white  corpuscles  or  leuco- 
cytes, like  small  amoebae  in  form  and  movements ;  (c)  very 
minute  bodies,  usually  colourless  and  variable  in  shape. 
When  the  blood  clots,  the  plasma  becomes  a  colourless 
serum,  traversed  by  coagulated  fibrin  filaments,  the  red 
corpuscles  often  arrange  themselves  in  rows,  and  the  white 
corpuscles  are  entangled  in  the  coagulated  shreds.  When 
the  web  of  a  living  frog  is  examined  under  the  microscope, 
it  will  be  seen  that  the  flow  of  blood  is  most  rapid  in  the 
arteries,  more  sluggish  in  the  veins,  most  sluggish  in  the 
capillaries  or  fine  branches  which  connect  the  arteries  and 


542 


AMPHIBIA. 


the  veins.  The  red  corpuscles  are  swept  along  most  rapidly, 
and  are  often  deformed  by  pressure ;  the  leucocytes  tend  to 
cling  to  the  walls  of  the  capillaries,  and  may  indeed  pass 
through  them  (diapedesis). 

The  Arterial  System. 

Each  branch  of  the  truncus  arteriosus  is  triple,  and  divides 
into  the  following  on  each  side  : — 

I.  The  carotid  arch,  the  most  anterior,  corresponding 


FIG.  181. — Arterial  system  of  Frog.     (After  ECKER.) 

/.,  Lingual ;  c.,  carotid  ;  s.,  systemic  ;  cu.,  cutaneous  ;/.,  pulmon- 
ary ;  z/.,  vertebral;  frr.,  brachial  ;  c/m.,  coeliaco-mesenteric  ; 
r.)  renals  ;  zY.,  common  iliacs  ;  A.,  haemorrhoidal. 

to   the   first   efferent  branchial    of    the    tadpole, 
gives  off— 

a  lingual  artery  to  the  tongue, 
a  carotid  artery,  which  bears  near  the  origin  of 
the  lingual  a  spongy  swelling  (the  "  carotid 
gland  "),  and  gives  off  an  external  carotid  to 
the  mouth  and  the  orbit,  and  an  internal 
carotid  to  the  brain. 


THE    VENOUS  SYSTEM.  543 

II.  The  systemic  arch,   the  median  one  of  the  three, 
corresponding  to  the  second  efferent  branchial  in 
the  tadpole,  gives  off—- 
the laryngeal  artery  to  the  larynx, 
the  cesophageal  to  the  oesophagus, 
the  occipito-vertebral  to  the  head  and  vertebral 

column, 

the  subclavian  to  the  fore  limb. 

From  the  left  aortic  arch,  just  as  it  unites  with  its  fellow 
of  the  other  side  to  form  the  dorsal  aorta,  or  from 
the  beginning  of  the  dorsal  aorta,  there  is  given  off 
the  cceliaco-mesenteric  to  the  stomach,  intestine, 
liver,  and  spleen. 
Further  back  the  dorsal  aorta  gives  off— 

the  renal  arteries  to  the  kidneys,  and  the  genital 

arteries  to  the  reproductive  organs  ; 
the  inferior  mesenteric  to  the  large  intestine  ; 
Then  it  divides  into  two  iliacs,  each  of  which  supplies 
the  bladder  (hypogastric),  the  ventral  body  wall 
(epigastric)  and  the  leg  (sciatic). 

III.  The  pulmo-cutaneous  arch,  the  most  posterior, 
corresponding  to  the  fourth  efferent  branchial  in 
the  tadpole,  gives  off— 

the  cutaneous  artery  to  the  skin, 
the  pulmonary  artery  to  the  lungs. 

The  Venous  System. 

I.  Each  superior  vena  cava  is  formed  from  the  union  of 
three  veins,  and  each  of  these  three  is  formed  from  two 
smaller  vessels. 

-p,  ,        i      (  Lingual  from   the    mouth    and 

tongue. 

\  Mandibular  from  the  lower  jaw. 
r  Internal  jugular  from  the  inside 


Superior 
vena  cava. 


Innominate. 


of  the  skull. 

Subscapular   from   the  back  of 
the  arm  and  the  shoulder. 

{Brachial  from  the  arm. 
Musculo-cutaneous     from     the 
skin  and  sides  of  the  body. 


544 


AMPHIBIA. 


II.  The  inferior  vena  cava  begins  between  the  kidneys,  and 
ends  in  the  sinus  venosus.     Its  components  are  as  follows  : — 
jnf    •  (   Efferent  renal  veins  from  the  kidneys. 

Genital  veins  from  the  reproductive  organs. 
[  Efferent  hepatic  veins  from  the  liver. 
The  renal  portal  system,  by  which  venous  blood  from  the 
posterior  region  filters  through  the  kidneys  on  its  way  back 
to  the  heart,  is  as  follows  on  each  side : — 


•sscp 


FIG.  182. — Venous  system  of  Frog.     (After  ECKER.) 

ml.,  Mandibular  and  lingual;   ej.,  external  jugular  ;   ij.,  internal 
jugular;    s.scp.,   subscapular  ;    in.,   innominate;    s.c.,  subclavian 
br.,  brachial ;  mci(.,  musculo-cutaneous  ;  hv.,  hepatic  vein;  h.fiv. 
hepatic  portal ;  G.,  gut  ;  a. a.,  anterior  abdominal  ;  rp.,  renal  portal 

t.,   pelvic;    K.,   kidneys  ;    sc.,   sciatic  ;  f.,  femoral  ;   dL,  dorso 
nbar  ;  ivc.,  inferior  vena  cava  ;  vc.,  cardiac  vein. 

'A  posterior  branch  of  the  femoral  vein  from 

the  hind  limb  forms  the  renal  portal  vein, 

Renal  portal  ^      which  receives  the  sciatic  from  the  back  of 

system.        j      the  leg,  and  the  dorso-lumbar  veins  from 

the  dorsal  wall  of  the  body,  and  oviducal 

w     veins  in  the  female. 

The  anterior  branch  of  the  femoral  vein  is  called  the 
pelvic,  and  unites  with  its  fellow  of  the  opposite  side,  and 


MECHANISM  OF  THE  HEART.  545 

gives  origin  to  a  median  vein  which  runs  to  the  liver — the 
anterior  abdominal.  By  means  of  an  anastomosing  branch, 
the  anterior  branch  of  the  femoral  is  also  connected  to  the 
sciatic. 

The  hepatic  portal  system,  by  which  venous  blood  from 
the  posterior  region  and  from  the  gut  passes  through  the 
liver  on  its  way  back  to  the  heart  is  as  follows : — 

/"Anterior  abdominal  vein,  from  the  union  of 
the  two  pelvics,  receiving  tributaries  from 


Hepatic  portal 
system. 


the  bladder,  ventral  body  wall,  and  trun- 


cus  artenosus. 


Hepatic  portal  vein,  from  the  union  of  veins 
from  the  stomach,  intestine,  and  spleen. 

III.  The  pulmonary  veins  which  bring  back  purified 
blood  from  the  lungs,  unite  just  before  they  enter  the  left 
auricle. 

Lymphatic  System. 

The  lymph  is  a  colourless  fluid,  like  blood  without  red 
corpuscles.  It  is  found  in  the  spaces  between  the  loose  skin 
and  the  subjacent  muscles,  in  the  pleuro-peritoneal  cavity  in 
which  heart,  lungs,  and  other  organs  lie,  in  a  sub-vertebral 
sinus  extending  along  the  backbone,  and  in  special  lympha- 
tic vessels  which  pass  fatty  materials  absorbed  from  the 
intestine  into  the  venous  system.  There  are  two  pairs  of 
contractile  "  lymph  hearts "  at  two  regions  where  the 
lymphatic  system  communicates  with  the  veins.  A  pair  lie 
posteriorly  near  the  end  of  the  urostyle ;  the  other  two  lie 
between  the  transverse  processes  of  the  third  and  fourth 
vertebrae. 

Mechanism  of  the  Heart. 

We  must  now  return  to  the  heart  to  consider  how  it  is  that 
the  blood  is  propelled  from  the  ventricle  along  the  proper 
channels.  The  right  half  of  the  ventricle  being  nearer  the 
right  auricle  contains  more  impure  blood,  and  it  is  from  the 
right  side  of  the  ventricle  that  the  truncus  artenosus  arises. 
Therefore  when  the  ventricle  contracts,  the  blood  which  first 
fills  the  truncus  is  venous.  It  passes  along  the  left  side  of  a 
median  longitudinal  valve  into  the  pulmonary  arteries — along 
the  path  of  least  resistance.  As  the  pulmonary  arteries 
become  distended,  the  next  quantum  of  blood — that  which 

35 


546  AMPHIBIA. 

has  been  mixed  in  the  middle  of  the  ventricle — is  driven 
forwards,  and  passes  on  the  right  side  of  the  longitudinal 
valve  into  the  aortic  arches.  "  And,  as  the  truncus  becomes 
more  and  more  distended,  the  longitudinal  valve,  flapping 
over,  tends  more  and  more  completely  to  shut  off  the  open- 
ings of  the  pulmonary  arteries,  and  to  prevent  any  blood 
from  flowing  into  them.  Finally  the  last  portion  of  blood 
from  the  ventricle,  representing  the  completely  arterialised 
blood  of  the  left  auricle,  which  is  the  last  to  arrive  at  the 
opening  of  the  truncus,  passes  into  the  carotid  trunks,  and 
is  distributed  to  the  head."  (The  last  two  sentences  are 
quoted  from  the  "  Text-book  of  Practical  Biology,"  by 
Professors  Huxley  and  Martin,  Howes  and  Scott.) 

Spleen,  Thyroid,  and  Thymus. — The  spleen,  which  is  probably,  as  in 
some  other  animals,  concerned  with  blood  making,  is  a  small  red  organ 
lying  in  the  mesentery  near  the  beginning  of  the  large  intestine.  The 
thyroid,  which  is  believed  to  have  something  to  do  with  maintaining  the 
health  of  the  blood,  is  represented  by  two  little  bodies  near  the  roots  of 
the  aortic  arches.  The  thymus,  perhaps  originally  associated  with  the 
gill-clefts,  lies  on  each  side  just  behind  the  angle  of  the  lower  jaw. 

Respiratory  System. 

The  larval  frog  breathes  at  first  through  its  skin,  then  by 
external  gills,  and,  finally,  by  internal  gills.  The  adult  frog 
breathes  chiefly  by  its  lungs,  but  some  cutaneous  respiration 
is  still  retained,  for  even  without  its  lungs  a  frog  may  live 
for  some  time,  and  it  does  not  use  them  when  hibernating. 

The  lungs  arise  as  outgrowths  of  the  cesophageal  region 
of  the  gut,  and  are  connected  with  the  back  of  the  mouth  by 
a  short  laryngo-tracheal  tube,  whose  slit-like  aperture  is  the 
glottis.  Each  lung  is  a  transparent  oval  sac,  with  muscle 
fibres  in  its  walls.  The  cavity  is  lessened  by  the  spongy 
nature  of  the  internal  walls,  which  form  numerous  little 
chambers  bearing  the  fine  branches  of  blood  vessels. 

In  respiration,  the  mouth  is  kept  shut,  and  air  passes  in 
and  out  through  the  nostrils.  A  frog  will  die  of  asphyxia 
if  its  mouth  be  artificially  kept  open  for  a  considerable  time. 
When  the  floor  of  the  mouth  is  lowered,  and  the  buccal 
cavity  thus  increased,  air  passes  in.  When  the  elastic  lungs 
and  the  muscles  of  the  sides  of  the  body  contract,  air  passes 
out.  When  the  nostrils  and  also  the  opening  of  the  gullet 


EXCRETORY  AND  REPRODUCTIVE  SYSTEMS.      547 

are  shut,  and  the  floor  of  the  mouth  at  the  same  time  raised, 
air  is  forced  through  the  glottis  into  the  lungs. 

Excretory  System. 

The  paired  kidneys  are  elongated  organs  situated  dorsally 
and  posteriorly  in  the  region  of  the  urostyle.  The  waste 
products  which  they  filter  out  of  the  blood  pass  backward 
by  two  ureters  which  open  separately  on  the  dorsal  wall  of 
the  cloaca,  without  any  communication  with  the  bladder. 
The  ureter  or  Wolffiah  duct  is  seen  as  a  white  line  along  the 
outer  side  of  each  kidney  ;  in  the  male  it  functions  also  as 
the  duct  of  a  testis.  On  the  surface  of  each  kidney  is  a 
longitudinal  yellowish  streak,  which  is  an  adrenal  gland  of 
unknown  significance,  and  little  spots  mark  ciliated  apertures 
or  nephrostomes,  which  remain  as  communications  between 
the  abdominal  cavity  and  the  renal  veins,  though  they  are 
apparently  in  embryonic  life  connected  with  the  urinary 
tubules. 

Reproductive  System. 

The  males  are  readily  distinguished  from  the  females  by 
the  swollen  cushions  on  the  first  fingers,  and  there  are  some 
other  slight  external  differences.  The  breeding  season  begins 
in  spring,  and  then  the  males  trumpet  to  their  mates.  The 
male  clasps  the  female  with  his  fore  limbs,  and  retains  his 
hold  for  several  days,  fertilising  the  ova  as  they  pass  out  into 
the  water. 

The  paired  testes  are  oval  yellowish  bodies  lying  in  front 
of  the  kidneys;  the  spermatozoa  pass  by  vasa  efferentia 
through  the  anterior  part  of  the  kidney  into  the  Wolffian 
duct,  which  functions  both  as  a  ureter  and  as  a  vas  deferens. 
In  the  male  of  R.  esculenta.  the  vas  deferens  is  dilated  for 
some  distance  after  leaving  the  kidney ;  in  R.  temporaries 
it  bears  on  the  outer  side  near  the  cloaca  a  dilated  glandular 
mass  or  "  seminal  vesicle."  In  the  males  rudiments  of  the 
Miillerian  ducts  are  sometimes  seen. 

The  paired  ovaries  when  mature  are  large  plaited  organs, 
bearing  numerous  follicles  or  sacs  containing  the  pigmented 
ova.  The  ripe  ova  are  liberated  into  the  body  cavity,  and 
moved  anteriorly  towards  the  heart  near  which  the  oviducts 
open.  These  oviducts  are  long  convoluted  tubes,  anteriorly 


548 


AMPHIBIA. 


thin-walled  and  straight,  then  glandular  and  coiled,  ter- 
minally thin-walled  and  dilated.  In  the  median  part,  the 
ova  are  surrounded  with  jelly ;  the  terminal  uterine  parts 
open  on  the  dorsal  wall  of  the  cloaca.  In  the  females  the 
Wolffian  ducts  act  solely  as  ureters.  There  are  occasional 
variations  in  the  nature  of  the  reproductive  organs,  and 
sometimes  the  hermaphrodite  stage  through  which  the  tad- 
poles pass,  is  to  some  extent  retained.  Attached  to  the 
anterior  end  of  the  reproductive  organs  are  yellow  lobed 


FIG.  183. — Urinogenital  sys- 
tem of  Male  Frog.  (After 
ECKER.) 

f.b.,  Fatty  bodies  ;  v.c.,  vena  cava  ; 
7\,  testis;  K.,  kidney;  w.d.,  Wolffian 
duct ;  £/.,  cloaca  ;  BL,  bladder. 


FIG.  184. — Urinogenital  sys- 
tem of  Female  Frog.  (After 
ECKER.) 

ovd. ,  Opening  of  oviduct ;  ov. ,  ovary ; 
f.b.,  fatty  body  ;  K.,  Kidney;  Ut., 
uterus;  £/>'.,  opening  of  ureters  into 
cloaca  (cl.) 


"  fatty  bodies,"  largest  in  the  males.  It  has  been  suggested 
that  they  contain  stores  of  reserve  material,  which  is  absorbed 
at  certain  seasons.  They  seem  to  be  fatty  degenerations  of 
the  anterior  part  of  the  genital  ridges.  The  head  kidney  or 


DEVELOPMENT  OF  THE  FROG.  549 

pronephros  persists  for  some  time  in  the  embryo,  but  even- 
tually degenerates.  It  does  not  seem  to  have  anything  to 
do  with  the  fatty  bodies. 

Development  of  the  Frog. 

The  ripe  ovum  exhibits  "  polar  differentiation,"  its 
upper  portion  is  deeply  pigmented,  the  lower  has  no 
pigment  and  contains  much  yolk.  This  yolk-containing 
hemisphere  is  the  heavier,  and  consequently  is  '  always 
the  lower  half  of  the  egg,  whatever  position  this  may 
be  originally  placed  in.  Round  the  ovum  there  is  a  delicate 
vitelline  membrane,  and  this  is  again  surrounded  by  a 
gelatinous  investment  which  swells  up  in  water.  The 
formation  of  polar  bodies  takes  place  before  the  liberation  of 
the  eggs. 

The  spheres  of  jelly  preserve  the  eggs  and  embryos  from 
friction,  prevent  their  being  eaten  by  most  birds,  appear  to 
be  distasteful  to  Gammarids,  and  often  enclose  in  their  inter- 
spaces groups  of  green  Algae,  which  help  in  aeration.  The 
spheres  may  also  be  of  use  in  relation  to  the  absorption  and 
radiation  of  heat. 

Fertilisation  occurs  immediately  after  the  eggs  are  laid. 
The  spermatozoa,  which  exhibit  the  usual  features  of  male 
elements,  work  their  way  through  the  gelatinous  envelopes, 
and  one  fertilises  each  ovum. 

The  first  cleavage  is  vertical,  and  divides  the  ovum  into 
a  right  and  left  half.  If  one  of  these  two  cells  be  punc- 
tured, the  other  will,  according  to  Roux,  form  a  one-sided 
half-embryo.  This  fact,  disputed  by  Hertwig,  suggests  that 
the  very  first  division  of  the  frog's  ovum  is  qualitative.  At 
a  certain  stage,  Roux's  half-embryo  regenerates  the  missing 
half,  usually  by  re-vitalising  the  remains  of  the  cell  which 
was  punctured.  The  second  cleavage  is  also  vertical,  and 
at  right  angles  to  the  first,  dividing  an  anterior  from  a 
posterior  half.  The  third  cleavage  is  equatorial,  at  right 
angles  to  the  first  two,  dividing  the  dorsal  region  from  the 
ventral. 

The  segmentation  is  total  but  unequal,  and  results  in  the 
formation  of  a  ball  of  cells,  those  of  the  upper  hemisphere 
being  smaller  and  more  numerous  than  the  yolk-laden  cells 
below.  Within  there  is  a  small  segmentation  cavity.  Since 


550 


AMPHIBIA. 


the  presence  of  yolk  acts  as  a  check  on  the  activity  of  the  pro- 
toplasm, we  can  understand  why  the  smaller  cells  continue  to 
divide  much  more  rapidly  than  the  large  yolk-containing  cells, 
and  so  how  the  smaller  epiblastic  cells  gradually  spread  over 
the  egg,  covering  in  the  larger  ones.  At  one  point,  where 
upper  and  lower  cells  meet,  a  groove  is  formed.  According 
to  the  older  view,  at  this  point  the  small  cells  are  invaginated, 
and  so  form  a  cavity ;  according  to  recent  research,  the 
cavity  is  simply  formed  by  the  splitting  of  the  large  cells. 
However  this  may  be,  the  cavity,  which  is  the  archenteron 
or  embryonic  gut,  rapidly  enlarges  at  the  expense  of  the 
segmentation  cavity,  which  soon  disappears.  The  groove 
becomes  a  circular  aperture  in  the  epiblast,  which  has  now 


FIG.  185. — Division  of  Frog's  Ovum.     (After  ECKER.) 
The  numbers  indicate  the  number  of  cells  or  blastomeres. 

spread  over  the  whole  egg  except  at  this  spot,  the  blastopore. 
The  embryo  elongates  slightly,  but  the  mass  of  yolk-laden 
cells  which  lie  on  the  floor  of  the  gut  prevents  the  body 
acquiring  at  once  the  fish-like  shape.  The  blastopore  as 
usual  marks  the  posterior  region  of  the  body. 

The  processes  which  follow  are  already  in  outline  familiar 
to  the  student.  Along  the  mid  dorsal  line  an  epiblastic 
neural  plate  is  differentiated.  The  edges  become  raised 
into  the  neural  folds,  these  approach  one  another  and,  fusing 
together,  form  the  medullary  or  neural  canal.  At  the 
posterior  end  this  communicates  with  the  archenteron  for  a 
time  by  the  neurenteric  canal.  Internally,  a  differentiation 


DEVELOPMENT  OF  THE  FROG.  551 

of  hypoblast  forms  the  notochord  along  the  mid  dorsal  line 
of  the  archenteron.  At  each  side  of  this  lie  masses  of 
mesoblast  which  have  been  split  off  from  the  hypoblast. 
Each  of  these  divides  into  the  primitive  segments  (proto- 
vertebrse)  above,  and  the  unsegmented  lateral  plates  below. 
The  lateral  plates  split  into  two  layers,  the  splanchnic  or  inner 
investing  the  gut,  the  somatic  or  outer  layer  being  applied 
to  the  epiblast ;  the  space  between  the  two  layers  is  the 
body  cavity.  The  body  now  becomes  distinctly  divided 
into  regions,  the  eyes  bud  out  from  the  brain,  external  gills 
grow  out  from  the  visceral  arches,  and  the  larva,  still  within 
its  gelatinous  case,  exhibits  peculiar  lashing  movements  of 
the  tail.  Eventually,  about  a  fortnight  after  the  eggs  are 
laid,  it  escapes  from  the  surrounding  jelly  and  swims  freely 
in  the  water.  At  this  time  there  is  a  cloacal  opening,  but 

the  mouth  has  not  yet 
-ep.  appeared.      There   is   a 

large  horse-shoe-shaped 
sucker  on  the  under 
-arch,  surface  of  the  head,  by 
means  of  which  the  tad- 
pole  attaches  itself  to 
foreign  objects. 

In  the  next  stage  of 
development,  which  ex- 
tends from  the  time  of 
FIG.   186 -Gastrula  stage  of  Newt.        hatching  until  the  com- 
(After  HERTWIG.)  mencement  of  the  meta- 

te^'^^yoUc^fi'lfS^s^e^^OT^^tyi     morphosis,     many    and 

important  changes  take 

place.  The  mouth,  which  has  previously  been  merely  a 
blind  pit,  opens  into  the  gut,  the  gut  itself  lengthens  rapidly, 
and  becomes  coiled  like  a  watch  spring ;  the  tadpoles  feed 
eagerly  on  vegetable  matter  and  increase  in  size.  The 
mouth  is  "  bordered  by  a  pair  of  horny  jaws,  and  fringed 
with  fleshy  lips  provided  with  horny  papillae,"  the  sucker 
behind  it  becomes  paired,  and  is  gradually  less  used  as  the 
power  of  locomotion  increases.  About  the  time  when 
the  mouth  is  opened,  four  gill  clefts  open  from  the 
pharynx  to  the  exterior.  The  external  gills  shrivel,  and  are 
replaced  by  an  internal  set ;  these  are  enclosed  on  either 


552 


AMPHIBIA. 


side  by  opercular  folds,  which  form  gill  chambers. 

continued  growth  of  these  folds 

the   gill  chambers   are   closed, 

with  the  exception  of  a  single 

exhalent   aperture   on   the    left 

side.      Through    this    opening 

the  water  which  is  taken  in  by 

the  mouth  in  respiration  passes 

outwards,    having   washed    the 

gills  on  its  way. 

Shortly  afterwards  the  rudi- 
ments of  the  limbs  appear. 
The  fore  limbs  are  concealed 
within  the  gill  chambers,  and 
so  are  not  obvious  until  a  much 
later  stage  ;  but  the  hind  legs 
may  be  watched  in  the  pro- 
gress of  development  from  small 
papillae  to  the  complete  limb. 

The  lungs  are  developed  as 
outgrowths  from  the  oesophagus, 
even  before  hatching,  but  in- 
crease in  size  very  slowly.  After 
the  appearance  of  the  hind  legs, 
the  larvae  come  to  the  surface 
of  the  water  to  breathe,  showing 
that  the  lungs  are  now  to  some 
extent  functional.  At  this  stage 
the  tadpoles,  now  about  two 
months  old,  are  at  the  level  of 
Dipnoi. 

The  changes  in  the  relations 
of  the  blood  vessels,  which 
Accompany  the  successive 
changes  in  the  methods  of 
respiration,  and  render  these 
possible,  are  somewhat  com- 
plicated. 

When  respiration  is  by  the 
gills  only,  the  circulation  is 
essentially  that  of  a  fish.  From  the  two-chambered 


By  the 


FIG.  187. — Dissection  of 
Tadpole.  (After  MILNES 
MARSHALL  and  BLES.) 

DL.,  lower  lip  ;  H.,  ventricle  of  ' 
heart;  DE.,  oesophagus;  ^YA., 
head  kidney;  A.,  aorta;  A'.,  kid- 
ney; KU.,  ureter;  DO.,  cloaca; 
Lff.,  hind  limb;  KV,,  opening  of 
ureter  into  cloaca ;  GR.,  genital 
ridge;  GF.,  fatty  body:  LF.,  fore 
limb  ;  OG.,  internal  gill. 


heart 


DEVELOPMENT  OF  THE  FROG. 


553 


the  blood  is  driven  by  afferent  branchials  to  the  gills,  from 
these  it  collects  in  efferent  vessels  which  unite  on  each  side 
to  form  the  two  aortae.  The  aortae  send  arteries  to  the  head, 
and  passing  backwards  unite  to  form  the  single  dorsal  aorta 
which  supplies  the  body.  For  a  time  there  are  two  dorsal 
aortae.  When  the  external  gills  are  replaced  by  the  internal, 
a  new  set  of  gill  capillaries  are  developed,  but  otherwise  the 
circulation  remains  the  same.  As  in  Ceratodus,  a  pul- 
monary artery  arises  from  the  fourth  efferent  branchial.  At 
the  time  when  the  hind  legs  begin  to  be  developed,  a  direct 
communication  is  established  between  afferent  and  efferent 
branchial  vessels,  so  that  blood  can  pass  from  the  heart  to 
the  dorsal  aorta  without  going  through  the  gills.  As  the 
pulmonary  circulation  becomes  increasingly  important,  the 
single  auricle  of  the  heart  becomes  divided  into  two  by  a 
septum,  and  the  pulmonary  veins  are  established.  At  the 
time  of  the  metamorphosis  an  increasing  quantity  of  blood 
avoids  the  gills  in  the  manner  indicated  above,  and  these, 
being  thrown  out  of  connection  with  the  rest  of  the  body,  soon 
atrophy,  while  the  lungs  become  the  important  respiratory 
organs.  The  fate  of  the  various  branchial  arteries  may  be 
gathered  from  the  table  which  follows. 


ARCHES. 

CLEFTS. 

AORTIC  ARCHES 
IN  THE  EMBRYO. 

AORTIC  ARCHES 
IN  THE  ADULT. 

Mandibular. 

Late    in    develop- 
ment     vessels 

Only  a  trace  persists. 

appear    which 

represent        a 

modification  of 

those      of     a 

branchial  arch. 

Eustachian  tube. 

Hyoid. 

The  arch  is  repre- 
sented in  a  less 

Disappears  entirely. 

modified  form. 

First  cleft. 

First  Branchial. 

First            efferent 

Carotid  arch. 

branchial. 

Second  cleft. 

Second  Branchial. 

Second           ,, 

Systemic  arch. 

Third  cleft. 

Third  Branchial. 

Third             ,,              Atrophies. 

Fourth  cleft. 

Fourth  Branchial. 

Fourth           ,,              Pulmo-cutaneous. 

Before,  however,  all  these  internal  changes  have  taken 
place,  the  external  form  undergoes  a  striking  metamorphosis. 


554 


AMPHIBIA. 


The  tadpole  has  by  this  time  grown  large  and  strong  while 
feeding  eagerly  on  water  weeds.  Now  it  seems  to  fast,  the 
tail  shrinks,  for  from  it  wandering  phagocytes  carry  nourish- 
ment to  other  parts  of  the  body.  The  habit  becomes 
less  active,  the  structural  adaptations  to  the  aquatic  life 
disappear.  "The  horny  jaws  are  thrown  off;  the  large 
frilled  lips  shrink  up ;  the  mouth  loses  its  rounded  suctorial 
form  and  becomes  much  wider;  the  tongue,  previously 
small,  increases  considerably  in  size ;  the  eyes,  which  as  yet 
have  been  beneath  the  skin,  become  exposed ;  the  fore- 
limbs  appear ;  the  left  one  being  pushed  through  the  spout- 


FIG.  188. — Life  history  of  a  Frog.     (After  BREHM.) 

1-3.  Developing  ova  ;  4.  newly  hatched  forms  hanging  to  water 
weeds  ;  5-6.  stages  with  external  gills  ;  7-10.  tadpoles  during  emer- 
gence of  limbs  ;  ii.  tadpoles  with  both  pairs  of  limbs  apparent;  12. 
metamorphosis  to  frog. 

like  opening  of  the  branchial  chamber,  and  the  right  one 
forcing  its  way  through  the  opercular  fold,  in  which  it  leaves 
a  ragged  hole."  (Marshall.) 

While  these  changes  are  in  progress,  and  as  the  supply  of 
food  afforded  by  the  tail  begins  to  be  exhausted,  the  tadpole 
recovers  its  appetite,  but  is  now  carnivorous,  feeding  on  any 
available  animal  matter,  or  even  on  its  fellows.  The  change 


CLASSIFICATION  OF  AMPHIBIA.  555 

is  not,  however,  so  great  as  it  seems,  for  even  at  a  very  early 
stage,  animal  food  is  eagerly  devoured. 

With  the  change  of  diet,  the  abdomen  shrinks,  stomach 
and  liver  enlarge,  the  intestine  becomes  relatively  narrower 
and  shorter.  The  tail  shortens  more  and  more,  and  as  it 
does  so  the  disinclination  for  a  purely  aquatic  life  seems  to 
increase.  Eventually  it  is  completely  absorbed,  the  hind 
limbs  lengthen,  and  the  conversion  into  a  frog  is  completed. 

It  seems  that  for  a  considerable  time  the  tadpole  is 
neither  male  nor  female,  but  hermaphrodite.  Differences 
in  nutrition  and  other  conditions  cause  one  kind  of  sexual 
organ  to  predominate  over  the  other,  and  the  tadpole 
becomes  unisexual.  In  nature  there  are  usually  about  as 
many  males  as  there  are  females,  but  Yung  has  shown  that 
by  increasing  the  quality  of  the  food  given  to  young  tadpoles 
from  fish-flesh  to  beef,  and  from  beef  to  frog-flesh,  he  could 
raise  the  percentage  of  females  to  about  ninety. 

In  many  respects  the  development  of  the  tadpole  is  very 
interesting,  especially  because  it  is  a  modified  recapitulation  of 
that  transition  from  aquatic  to  aerial  respiration,  which  must 
have  marked  one  of  the  most  momentous  epochs  in  the 
evolution  of  Vertebrates. 

CLASSIFICATION  OF  AMPHIBIA. 
Order  ANURA  or  ECAUDATA. 

The  attainment  of  the  adult  form  is  associated  with  the  loss  of  tail 
and  gills.  The  body  is  broad.  The  long  and  very  muscular  hind-legs 
are  powerful  in  leaping. 

(a)  The  frog  and  its  allies  : — 

The  British  frog  ( Rana  temporaria),  brown  in  colour,  with  a  black 

patch  on  the  side  of  the  head  : 

the  Edible  frog  (R.  esculenta}^  not  indigenous  in  Britain,  common 
on  the  Continent,  greenish  in  colour,   without   the   black 
patch  : 
the  North  American  bull-frog  (R.  catesbiand),  sometimes  eight 

inches  in  length,  with  a  very  sonorous  croak  : 
some  Asiatic  and  African  "tree-frogs,"  such  as  Rhacophorus  and 

Hyperolius : 

some  toothless  frogs,  such  as  the  American  Dendrobates. 
(/>)  Those  allied  to  the  toad,  all  toothless  : — 

the  toads  in  the  strict  sense  (Bufo),  with  poisonous  skin  : 
the  crimson -bellied  Bombinator  igneus,  the  Feuerkrote  of  Ger- 
many : 


556  AMPHIBIA. 

the  obstetric  toad — Alytes  obstetmcans,  the  male  of  which  carries 
the  eggs  on  his  back  and  legs  :  Hy lodes  in  tropical  America, 
with  rapid  development  without  metamorphosis  : 

the  South  American  Ceratophrys,  of  which  some  species  have 
bony  plates  in  the  skin  of  the  back :  Pelobates,  which  like 
many  others  lives  for  the  most  part  underground  :  the 
brightly  coloured  tree-toads,  such  as  Hyla>  with  adhesive 
discs  at  the  ends  of  the  digits  ;  Nototrema,  with  a  dorsal 
egg-pouch  in  the  females :  Liopelma  hochstetteri,  the  only 
Amphibian  in  New  Zealand. 
(c)  The  tongueless  Surinam  Toad  (Pipa  americana),  in  which  the 

eggs   develop  in  pouches  on  the  back  of  the  female  ;  and  the 

allied  Ethiopian  genus  Xenopus,  with  a  ''tentacle"  extending 

backwards  on  each  side  of  the  head. 

Order  URODELA  or  CAUDATA. 

The  tail  persists  in  adult  life  ;  the  body  is  elongated  ;  the  limbs  are 
weak  when  compared  with  those  of  Anura. 

(a]  Forms  like  Proteus : — Two  extant  genera  Proteus  and  Necturus 

both  with  persistent  gills.  Several  species  of  Proteus  inhabit  the 
water  in  the  caves  of  Carinthia  and  Dalmatia  in  Austria.  The 
gills  persist,  there  are  two  pairs  of  limbs.  The  eyes  are  de- 
generate, the  colours  are  pale,  as  we  should  expect  in  cave- 
animals.  Two  species  of  Necturus  (or  Menobranchus]  occur 
in  North  America,  in  rivers  and  lakes,  such  as  those  of  the 
Mississippi  and  Ohio  basins.  The  pigment  of  the  skin  is  well- 
developed. 

(b]  Forms  like  Siren: — Two  extant  genera,  St'renand  Pseudobranchus^ 

both  North  American,  both  with  persistent  gills,  with  only  the 
anterior  limbs. 

(c]  Forms  like  the  newts  and  salamanders: — The  North  American 

Amphiuma,  with  two  pairs  of  rudimentary  legs,  with  a  slit  per- 
sisting in  adult  life  as  a  remnant  of  the  gilled  state  :  Megalo- 
batrachus  or  Cryptobranchus  maximus,  the  largest  living  Amphi- 
bian, found  in  Japan  and  Thibet,  attains  a  length  of  three 
feet;  Amblystoma  and  its  gilled  form  the  Axolotl;  DesmognatJuis 
ftisca,  the  common  water  salamander  of  the  United  States,  lays 
its  eggs  in  a  wreath  which  one  of  the  sexes  twines  round  its 
body ;  Salamandra  maculosa  and  S.  atra,  both  European,  both 
viviparous  ;  the  newts  —  Triton  or  Molge  —  of  which  Triton 
alpestris  becomes  sexually  mature  while  still  larval. 

Order  GYMNOPHIONA  or  APODA. 

Worm-like  or  snake-like  forms,  subterranean  in  habit ;  without  limbs 
or  girdles  or  tail ;  with  dermic  scales  concealed  in  the  skin  ;  in  at  least 
some  forms,  gills  occur  in  early  life  ;  the  eyes  are  rudimentary-;  peculiar 
"  tentacles  "  are  connected  with  the  orbit,  and  are  perhaps  equivalent 
to  the  "balancers,"  which  occur  in  larval  Urodela  in  front  of  the  first 
gill  cleft.  Cczcilia  in  West  Africa,  Malabar,  South  America;  Siphonops 


CLASSIFICATION  OF  AMPHIBIA. 


557 


in  Brazil  and  Mexico  ;  Epicrium  in  Ceylon  ;  Rhinatrema  in  Cayenne  ; 
C cecilia  compressicauda  is  viviparous. 

Order  LABYRINTHODONTIA  or  STEGOCEPIIALA. 

Extinct  forms,  occurring  from  Carboniferous  to  Triassic  strata. 
Dermal  armour  is  present,  the  teeth  are  frequently  folded  in  a  com- 
plex manner.     Mastodonsaurus,  Dendrerpeton,  Archegosaurus. 

Life  of  Amphibians. 

Most  Amphibians  live  in  or  near  fresh  water  ponds,  swamps,  and 
marshes.  Even  those  adults  which  have  lost  all  trace  of  gills  are  usually 
fond  of  water.  The  tree-toads,  such  as  Hyla>  are  usually  arboreal  in 
habit,  while  the  Gymnophiona  and  some  toads  are  subterranean. 

The  black  Salamander  (Salamandra  atra]  of  the  Alps  lives  where 
pools  of  water  are  scarce,  and  instead  of  bringing  forth  gilled  young,  as 
its  relative  the  spotted  salamander  (S.  maculosa)  does,  bears  them  as 
lung-breathers,  and  only  a  pair  at  a  time.  But  if  the  unborn  young  are 

removed  from  the  body  of  the 
mother  and  placed  in  water,  they 
form  gills  like  other  tadpoles. 
Within  the  mother,  the  respira- 
tion and  nutrition  of  the  young 
seems  to  be  effected  by  crowds 
of  red  blood  corpuscles  which 
are  discharged  from  the  walls  of 
the  uterus. 

Species    of  Hylodes,  such  as 
H.  martinicensis   of   the  West 
Indian   Islands,   live   in  regions 
where  there  are  few  pools.     In 
such  cases  the    development  is 
completed  within  the  egg-case, 
FIG.  189.— Ccecilian  (Icthyophis)         and  a  lung-breathing  tailed  larva 
with  eggs.     (After  SARASIN.)  is    hatched    in   about    fourteen 

days.     It  is  likely  that  the  tail 

helps  in  respiration  before  hatching,   but  one    observer    reports    the 
presence  of  small  gills. 

In  some  Mexican  and  N.  American  lakes,  there  is  an  interesting 
amphibian  known  as  Amblystoma  or  Siredon.  It  has  two  forms,  one 
losing  its  gills  (Amblystoma),  the  other  retaining  them  (Axolotl}.  Both 
these  forms  reproduce,  and  both  may  occur  in  the  same  lake.  Formerly 
they  were  referred  to  different  genera.  But  the  fact  that  some  Axolotls, 
kept  in  the  Jardin  des  Plantes  in  Paris,  lost  their  gills  when  their 
surroundings  were  allowed  to  become  less  moist  than  usual,  led 
naturalists  to  recognise  that  the  two  forms  were  but  different  phases  of 
one  species.  It  has  been  shown  repeatedly,  that  a  gilled  Axolotl  may 
be  transformed  into  a  form  without  gills,  and  this  metamorphosis  seems 
to  occur  constantly  in  one  of  the  Rocky  Mountain  lakes.  The  facts  do 
not,  however,  justify  the  hasty  conclusion  that  the  change  from  the 
gilled  to  the  gill-less  form  is  determined  only  by  differences  in  amount 


558  AMPHIBIA. 

of  moisture.  The  transformation  may  indeed  take  place  in  water, 
and  both  Axolotl  and  Amblystoma  have  been  observed  in  the  same 
lake.  Further,  the  absence  or  presence  of  gills  is  not  the  only  difference 
between  the  two  forms. 

Amphibians  are  very  defenceless,  but  their  colours  often  conceal  them. 
Not  a  few  have  considerable  power  of  colour-change. 

Many  Amphibians  live  alone,  but  they  usually  congregate  at  the 
breeding  seasons  when  the  amorous  males  often  croak  noisily.  Alike 
in  their  love  and  their  hunger  they  are  most  active  in  the  twilight. 

Their  food  usually  consists  of  worms,  insects,  slugs,  and  other  small 
animals,  but  some  of  the  larval  forms  are  for  a  time  vegetarian  in  diet. 
They  are  able  to  survive  prolonged  fasting,  and  many  hibernate  in  the 
mud.  Though  the  familiar  tales  of  "  toads  within  stones"  are  for  the 
most  part  inaccurate,  there  is  no  doubt  that  both  frogs  and  toads  can 
survive  prolonged  imprisonment.  Besides  having  great  vital  tenacity, 
Amphibians  have  considerable  power  of  repairing  injuries  to  the  tail 
or  limbs. 

Although  the  life  of  Amphibians  seems  to  have  on  an  average  a  low 
potential,  even  the  most  sluggish  wake  up  in  connection  with  reproduc- 
tion. The  males  often  differ  from  their  mates  in  size  and  colour.  Some 
of  their  parental  habits  seem  like  strange  experiments. 

Thus  in  the  Surinam  toad  (Pipa  awericana),*the  large  eggs  are  placed 
by  the  male  on  the  back  of  the  female  and  fertilised  there.  The  skin 
becomes  much  changed — doubtless  in  response  to  the  strange  irritation 
— and  each  fertilised  ovum  sinks  into  a  little  pocket,  which  is  closed  by 
a  gelatinous  lid.  In  these  pockets  the  embryos  develop,  perhaps  absorb- 
ing some  nutritive  material  from  the  skin.  They  are  hatched  as 
miniature  adults.  In  Nototrema  and  Opisthodelphis^  the  female  has  a 
dorsal  pouch  of  skin  opening  posteriorly,  and  within  this  tadpoles  are 
hatched.  In  Rhinoderma  darwinii,  the  male  carries  the  ova  in  his 
capacious  croaking-sacs.  In  the  case  of  the  obstetric  toad  (Alytes 
obstetricans],  not  uncommon  in  some  parts  of  the  Continent,  the  male 
carries  the  strings  of  ova  on  his  back  and  about  his  hind  legs,  buries 
himself  in  damp  earth  until  the  development  of  the  embryos  is  ap- 
proaching completion,  then  plunges  into  a  pool,  where  he  is  freed  from 
his  living  burden.  Thus  among  Amphibians,  as  among  Fishes,  the 
males  sometimes  take  upon  themselves  the  task  of  hatching  the  eggs. 

In  the  Anura  the  ova  are  fertilised  by  the  male  as  they  leave  the 
oviduct ;  in  the  newt  the  male  deposits  a  spermatophore  in  the  water 
close  to  the  female ;  in  Salamandra  atra,  S.  maculosa,  and  Ccecilia 
conipressicatida  the  fertilisation  must  take  place  internally,  for  the  young 
are  hatched  within  the  mother. 

The  eggs  of  the  frog  are  laid  in  masses,  each  being  surrounded  by  a 
globe  of  jelly ;  those  of  the  toad  are  laid  in  long  strings ;  those  of 
newts  are  fixed  singly  to  water  plants ;  those  of  some  tree-toads,  such 
as  Hy lodes,  are  laid  on  or  under  leaves  in  moist  places. 

In  Salamandra  atra,  Pipa  americana,  Hylodes^  and  Cizcilia  com- 
pressicauda,  the  young  are  hatched  as  miniature  adults ;  and  marked 
metamorphosis  can  hardly  be  said  to  occur  in  any  Urodela. 

There  are  about  900  living  species  of  Amphibia,  most  of  them  tail- 
less. All  are  averse  to  salt-water,  hence  their  absence  from  almost  all 


HISTORY.  559 

oceanic  islands.     The  Anura  are  well-nigh  cosmopolitan ;  the  Urodela 
are  limited  to  the  temperate  parts  of  the  northern  hemisphere. 

History. 

It  is  likely  that  Amphibians  were  derived  from  a  stock 
from  which  the  Dipnoi  and  perhaps  also  the  Elasmobranchs 
sprang.  The  order  Labyrinthodontia  or  Stegocephala  does 
not  seem  very  homogeneous ;  it  perhaps  includes  two  or 
more  distinct  orders.  Of  extant  forms,  the  Gymnophiona 
are  more  old-fashioned  than  the  others.  The  modern  types 
gradually  appear  in  Tertiary  times.  Some  of  the  extinct 
forms  were  gigantic. 

Huxley  has  emphasised  the  following  affinities  between 
Amphibians  and  Mammals  : — the  Amphibia,  like  Mammals, 
have  two  condyles  on  the  skull ;  the  pectoral  girdle  of 
Mammals  is  as  much  amphibian  as  it  is  sauropsidian ; 
the  mammalian  carpus  is  directly  reducible  to  that  of 
Amphibians.  In  Amphibians  only  does  the  articular 
element  of  the  mandibular  arch  remain  cartilaginous  ;  the 
quadrate  ossification  is  small,  and  the  squamosal  extends 
down  over  it  to  the  osseous  elements  of  the  mandible,  thus 
affording  easy  transition  to  the  mammalian  condition  of 
these  parts. 

There  are  many  remarkable  affinities  between  the  Labyrin- 
thodont  Amphibians  and  a  class  of  extinct  Reptiles  known 
as  Anomodontia,  and  as  the  latter  have  also  many  affinities 
with  Mammals,  it  is  possible  that  both  Mammals  and  Ano- 
modonts  diverged  from  an  Amphibian  stock.  The  strange 
extinct  Eotetrapoda  of  Credner  seem  to  unite  the  Stego- 
cephala to  the  Rhychocephalia,  a  class  of  Reptiles  now 
represented  by  the  New  Zealand  "  lizard  "  Sphenodon. 

eUn  e  ^-^ 


CHAPTER    XXIV. 


REPTILES. 

Classes — CHELONIA.     RHYNCHOCEPHALIA.    LACERTILIA. 
OPHIDIA.     CHELONIA,  and  EXTINCT  CLASSES. 

THE  diverse  animals — Tortoises,  Lizards,  Snakes,  Croco- 
dilians, &c. — which  are  classed  together  as  Reptiles,  are  the 
modern  representatives  of  those  Vertebrates  which  first 
became  independent  of  the  water  and  began  to  possess  the 
dry  land.  While  almost  all  Amphibians  spend  at  least  their 
youth  in  the  water,  breathing  by  gills,  this  is  not  necessary 
for  Reptiles,  in  which  embryonic  respiration  is  secured  by  a 
vascular  foetal  membrane  known  as  the  allantois.  As  in 
still  higher  Vertebrates,  gill  slits  are  present  in  the  em- 
bryos, but  they  are  not  functional,  and  are  without  gills. 
Reptiles  are  essentially  creatures  of  the  earth,  but  many 
lizards,  snakes,  and  turtles,  and  all  the  crocodilians,  are 
aquatic.  Partially  marine  forms  are  represented  by  the 
Galapagos  lizard,  which  swims  out  among  the  seaweed,  by 
some  crocodilians  which  venture  down  the  estuaries ;  some 
turtles  live  far  out  to  sea  and  only  seek  the  shores  to 
lay  their  eggs  ;  the  Hydrophidae,  or  sea-snakes,  never  leave 
the  water. 

Reptiles,  Birds,  and  Mammals  are  often  distinguished  as 
Amniota  from  Amphibians  and  Fishes,  which  are  called 
Anamnia,  the  terms  referring  to  the  presence  or  absence  of 
a  characteristic  protective  foetal  membrane — the  amnion — 
with  which  the  allantois  is  always  associated. 

Of  these  three  highest  classes  of  Vertebrates,  the  Reptiles 
and  Birds,  so  different  in  form  and  habit,  are  united  by  deep 
structural  resemblances.  These  were  first  clearly  recognised 


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562 


REPTILES. 


by  Professor  Huxley,  when  he  united  the  two  classes  as 
Sauropsida,  in  contrast  to  Mammalia  on  the  one  hand,  and 
Ichthyopsida  (Amphibians  and  Fishes)  on  the  other.  Let  us 
state  some  of  the  contrasts  which  he  recognised,  noting  at  the 
same  time  that  Reptiles  form  among  Vertebrates  a  great 
central  assemblage,  like  "  worms "  among  Invertebrates, 
rather  a  number  of  classes  than  a  class,  exhibiting  affinities 
not  only  with  Birds,  but  with  Mammals  and  Amphibians  as 
well. 

Again  we  shall  virtually  quote  from  Huxley  in  noting  some  of  the 
distinctions  between  Reptiles  and  Birds  : — 


REPTILES. 


The  exoskeleton  consists  of  horny  epi- 
dermal scales,  or  of  bony  dermal  scutes, 
or  of  both. 

The  centra  of  the  vertebrae  are  rarely 
like  those  of  birds. 

When  there  is  a  sacrum,  its  vertebrae 
(usually  two  in  number)  have  large  ex- 
panded ribs,  with  the  ends  of  which  the 
ilia  articulate. 

The  cartilaginous  sternum  may  become 
bony,  but  is  not  replaced  by  membrane 
bones,  unless  perhaps  in  Pterodactyls. 

When  there  is  an  interclavicle,  it  remains 
distinct  from  the  clavicle  and  sternum. 

The  hand  has  more  than  three  digits, 
and  at  least  the  three  radials  are  clawed. 


In  living  reptiles  the  ilia  are  prolonged 
further  behind  than  in  front  of  the  aceta- 
bulum  ;  the  pubes  slope  downward  and 
forward  ;  there  are  pubic  and  ischiac 
symphyses. 

There  are  often  five  toes  ;  the  tarsals 
and  the  metatarsals  remain  distinct. 


At  least  two  aortic  arches  persist  ;  only 
the  Crocodilia  have  a  structurally  four- 
chambered  heart  ;  more  or  less  mixed 
blood  always  goes  to  the  posterior  body. 

The  body  has  approximately  the  tem- 
perature of  the  surrounding  medium. 

The  optic  lobes  lie  on  the  upper  surface 
of  the  brain. 


BIRDS. 


There  is  an  outer  covering  of  feathers, 
and  though  there  may  be  a  few  scales, 
there  are  never  scutes. 

The  centra  of  the  vertebrae  have  usually 
a  peculiar  terminal  curvature. 

The  two  sacral  vertebrae  have  no  ex- 
panded ribs,  they  fuse  with  others  to  form 
a  long  composite  sacrum. 

The  cartilaginous  sternum  is  replaced 
by  membrane  bones  from  several  centres. 

When  there  is  an  interclavicle,  it  is 
confluent  with  the  clavicles. 

The  hand  has  not  more  than  three 
digits,  and  at  most  two  radials  are  clawed. 
The  fore-limbs  are  modified  as  wings ; 
some  carpals  fuse  with  the  metacarpals. 

The  ilia  are  greatly  prolonged  in  front 
of  the  acetabulum,  the  inner  wall  of  which 
is  membranous.  The  pubes  slope  back- 
wards, parallel  with  the  ischia  ;  only  in 
Struthio  is  there  a  pubic  symphysis,  only 
in  Rhea  is  there  is  an  ischiac  one. 

There  are  not  more  than  four  toes  ;  the 
proximal  tarsals  unite  with  the  tibia, 
forming  a  tibio-tarsus ;  the  first  meta- 
tarsal  if  present  is  free,  but  the  three  others 
are  fused  to  one  another  and  to  the  distal 
tarsals,  forming  a  tarso-metatarsus. 

There  is  but  one  aortic  arch,  to  the 
right  ;  the  heart  is  four-chambered  ;  the 
blood  sent  to  the  body  is  purely  arterial. 

The  body  temperature  is  very  high. 

The  optic  lobes  lie  on  the  side  of  the 
brain. 

The  lungs  have  associated  air  sacs. 

The  sutures  between  the  bones  of  the 
skull  are  usually  obliterated  at  an  early 
stage. 

The  right  ovary  atrophies. 


TORTOISES  AND    TURTLES.  563 

Class  CHELONIA.     Tortoises  and  Turtles. 

GENERAL  CHARACTERS. — The  body  is  compact  and  broad 
in  the  region  of  the  trunk.  There  is  a  dorsal  and  a  ventral 
shield^  within  the  shelter  of  ivhich  the  head  and  neck,  tail 
and  limbs,  can  be  more  or  less  retracted. 

The  dorsal  shield  or  carapace  is  formed  from  the  neural 
spines  of  the  vertebra,  from  the  expanded  ribs,  and  from  a 
series  of  marginal  plates  around  the  outer  edge. 

The  ventral  shield  or  plastron  consists  of  nine  or  so 
dermal  plates.  There  is  no  sternum. 

Overlapping,  but  in  no  way  corresponding  to  the  bony 
plates,  are  epidermic  horny  plates  of  " tortoise  shell"  which, 


FIG.  190. — External  appearance  of  Tortoise, 

though  very  hard,  are  not  without  sensitiveness,  numerous 
nerves  ending  upon  them. 

The  quadrate  is  immovably  fixed. 

The  jaws  are  covered  by  a  horny  sheath,  and  are  without 
teeth,  though  rudiments  of  these  have  been  seen  in  some 
embryos. 

The  average  life  of  Chelonians  is  sluggish.  Perhaps  this  is 
in  part  due  to  the  way  in  which  the  ribs  are  lost  in  the 
carapace,  for  this  must  tend  to  make  respiration  less  active. 

All  are  oviparous.  The  eggs  have  firm  usually  calcareous 
shells. 

Some  Peculiarities  in  the  Skeleton  of  Chelonia. 

The  dorsal  vertebrae  seem  to  be  without  transverse  processes,  and 
along  with  the  ribs  are  for  the  most  part  immovably  fused  in  the 
carapace.  The  tail  and  the  neck  are  the  only  flexible  regions. 


REPTILES. 


Professor  Berry  Haycraft  gives  the  following  account  of  the  develop- 
ment of  the  dorsal  shield  : — 

If  we  compare  a  very  early  embryo  turtle  with  that  of  a  crocodile,  we 
notice  the  following  difference  : — In  the  crocodile,  each  cartilaginous 
rib  is  completely  invested  by  a  tubular  sheath  of  young  connective  tissue, 
and  in  the  intercostal  spaces  are  distinct  muscle  plates.  In  the  turtle  the 
cartilaginous  ribs  are  simply  embedded  in  young  osteogenetic  tissue, 
which  forms  the  whole  of  the  body  wall,  extending  superficially  up  to 
the  skin.  As  development  proceeds  in  the  crocodile,  the  tubular  sheath 
of  connective  tissue  (periosteum)  investing  each  cartilaginous  rib,  grows 
in  size,  and  forms  bone  (the  rib)  anteriorly,  the  cartilage  being  absorbed. 
Thus  we  get  the  adult  cylindrical  rib,  separated  from  its  neighbours  by 
the  intercostal  muscles,  developed  from  the  muscle  plates.  In  the 
green  turtle  bone  begins  to  form  upon  the  rib  cartilage,  the  latter  sub- 
sequently being  absorbed,  but  as  there  is  no  investing  periosteal  sheath, 
this  formation  of  bone  spreads  out  on  all  sides,  right  up  to  the  skin 
superficially,  and  as  far  as  the  neigh- 
bouring growths  laterally,  to  form  the 
solid  bone  of  the  carapace.  In  the 
mud  turtles,  the  growth  of  bone  which 
is  extending  laterally  from  each  carti- 
laginous rib,  does  not  meet  its  neigh- 
bour, for  already  the  intercostal  tissue 
has  partly  become  differentiated  into 
fibrous  tissue,  and  a  fibro-osseous  cara- 
pace results.  In  the  green  turtle,  the 
rib  cartilage,  at  both  its  distal  and 
proximal  ends,  is  invested  by  true 
periosteum,  which  causes  in  these  parts 
the  formation  of  cylindrical  bone. 

What  then  is  a  costal  plate  ? 

It  is  more  than  a  rib  ;  it  is  a  rib, 
which,  in  its  development,  has  spread 
into  and  involved  the  surrounding  inter- 
costal tissue. 

Is  it  an  intramembranous  or  intra- 
cartilaginous  bone?  We  now  know 
that  all  bones  are  developed  through 
the  agency  of  membranes,  and  that  the 
humerus,  for  example,  an  intracarti- 
laginous  bone,  is  eventually  formed  en- 
tirely from  its  membranous  periosteum. 
A  membrane  bone  is  therefore  not  a  bone  developed  from  a  membrane, 
for  every  bone  in  the  body  is  now  known  to  be  so  formed,  it  is  a  bone 
whose  place  was  never  represented  by  cartilage. 

If  we  accept  this  view  of  an  intramembranous  and  intracartilaginous 
bone,  a  view  forced  upon  us  by  modern  inquiry,  then  the  costal  plate  is 
an  intracartilaginous  bone,  and  comes  out  in  its  proper  contrast  from  the 
marginal  and  plastron  plates  which  are  not  preformed  in  cartilage. 
The  neural  plates  may  be  looked  upon  as  similar  in  their  origin  to  the 
costal  plates,  bone  encrusting  the  cartilaginous  vertebrae,  and  then  ex- 


F I G.  191.  —  Carapace  of 
Tortoise.  (From  Edin- 
burgh Museum  of  Science 
and  Art.) 

The  dark  contours  are  those  of 
the  bony  pieces  ;  the  lighter  con- 
tours are  those  of  the  scales  which 
have  been  removed. 


TORTOISES  AND    TURTLES.  565 

tending  into  the  tissue  between  neighbouring  spinous  processes,  and 
superficially  up  to  the  tissue  which  has  differentiated  into  the  thin 
layer  of  connective  tissue  below  the  scales. 

The  ventral  shield  or  plastron  consists  of  dermal  bones;  according 
to  some,  three  anterior  pieces  represent  clavicles  and  interclavicle. 

The  cervical  vertebrae  have  at  most  little  rudiments  of  ribs,  are 
remarkably  varied  as  regards  their  articular  faces,  and  give  the  neck 
many  possibilities  of  motion.  There  are  no  lumbar  vertebrce. 

The  bones  of  the  skull  are  immovably  united  ;  there  are  no  ossified 
alisphenoids,  but  downward  prolongations  of  the  large  parietals  take 
their  place  ;  neither  presphenoid  nor  orbitosphenoids  are  ossified  ;  there 
are  no  distinct  nasal  bones  in  modern  Chelonians  ;  the  premaxilloe  are 
very  small ;  there  are  no  teeth  ;  there  is  a  complete  bony  palate  formed 


FIG.  192. — Internal  view  of  skeleton  of  Turtle.     (From 
Edinburgh  Museum  of  Science  and  Art.) 

H,  humerus  ;  Sc,  scapula,  running  dorsally  ;  c,  coracoid  ;  e.c, 
epi-coracoid ;  p.c,,  pre-coracoid  ;  P,  pubes ;  z7,  ilium,  running 
dorsally  ;  Is,  ischium  ;  F,  femur. 

in  great  part  from  the  junction  of  the  pterygoids  with  the  basisphenoid 
and  with  one  another. 

There  is  no  sternum. 

The  pectoral  girdle  on  each  side  consists  of  a  dorsal  scapula  attached 
to  the  carapace,  a  ventral  coracoid  bearing  terminally  a  small  epicora- 
coid,  and  anterior  to  the  coracoid  a  "  precoracoid." 

The  pelvic  girdle  consists  of  dorsal  ilia  attached  to  the  carapace,  an- 
terior pubes  fused  in  a  symphysis,  and  posterior  ischia  similarly  fused. 

The  girdles  originally  lie  in  front  of,  or  behind  the  ribs,  but  are  over- 
arched by  the  carapace  in  the  course  of  its  development. 


566 


REPTILES. 


Some  Peculiarities  in  the  Organs  of  Chelonia. 

The  brain  of  the  adult  shows  a  slight  curvature.  In  Chelonians  and 
in  all  higher  animals  except  serpents,  there  are  twelve  cranial  nerves, 
for  in  addition  to  the  usual  ten,  a  hypoglossal  to  the  tongue,  and  a 
spinal  accessory  to  cervical  muscles  are  ranked  as  the  eleventh  and 
twelfth. 

The  gullet  often  bears  internally  pointed  horny  papillae  directed 
downwards.  There  are  blind  pockets  or  anal  bursae  connected  with 
the  cloaca. 

The  heart  is  three  chambered,  but  an  incomplete  septum  divides  the 
ventricle  into  a  right  portion 
from  which  the  pulmonary 
arteries  and  the  left  aortic 
arch  arise,  and  a  left  portion 
from  which  the  right  aortic 
arch  issues.  From  the  right 
aortic  arch,  which  contains 
more  pure  blood  than  the 
left,  the  carotid  and  sub- 
clavian  arteries  are  given  off. 
The  left  aortic  arch  gives  off 
the  cceliac  artery  before  it 
unites  with  the  right. 

Unlike  other  Reptiles,  the          FIG.  193. — Dissection  of  Chelonian 
Chelonians  are  said  to  have  heart.     (After  HUXLEY.) 

no  renal  portal  system. 

The  lungs  are  attached  to 
the  dorsal  wall  of  the  thorax, 
and  have  only  a  ventral  in- 
vestment of  peritoneum  ; 
each  is  divided  into  a  series  of  compartments  into  which  branches  of 
the  bronchus  open.  There  is  a  slight  muscular  diaphragm. 

In  the  males,  the  kidney,  the  epididymis,  and  the  testes,  lie  adjacent 
to  one  another  on  each  side.  The  males  have  a  grooved  penis  attached 
to  the  anterior  wall  of  the  cloaca.  There  is  a  urinary  bladder. 

Classification  of  Chelonia. 

I.  ATHEC^E.     Vertebrae  and  ribs  free  from  bony  shield.     Skull 

without  descending  processes  from  parietals. 

Sphargidse,  leather  turtles,  with  flexible  carapace.  Dermochelys 
coriacea,  the  only  living  species,  the  largest  modern  Chelonian,  some- 
times measuring  six  feet  in  length.  It  is  widely,  but  now  sparsely,  dis- 
tributed in  tropical  and  temperate  seas,  and  is  said  to  be  herbivorous. 

II.  THECOPHORA.     Dorsal  vertebrae  and  ribs  fused  in  the  carapace. 

Parietals    prolonged    downwards.      Including    the   following    and 

other  families. 

Chelonicke,  marine  turtles,  with  fin-like  feet,  and  partially  ossified 
carapace.  They  occur  in  intertropical  seas,  and  bury  their  soft-shelled 
eggs  on  sandy  shores.  The  green  turtle  (Chelone  viridis}  is  much 


r.v.,  Right  half  of  ventricle  ;  s,  septum  ;  /.?'., 
left  half  of  ventricle  ;  ra,  right  auricle  ;  l.a. , 
left  auricle  ;  Lao,  left  aortic  arch  ;  r.ao,  right 
aortic  arch  ;  p. a.,  pulmonary  arch. 


TORTOISES  AND    TURTLES. 


567 


esteemed  as  food;  the  hawk's-bill  turtle  (Caretta  imbricata)  furnishes 
much  of  the  commercial  tortoise  shell. 

Testudinidae,  land  tortoises,  with  convex  perfectly  ossified  carapace 
and  feet  adapted  for  walking.  They  are  found  in  the  warmer  regions 
of  both  the  old  and  the  new  world,  but  not  in  Australia.  In  diet  they 
are  vegetarian.  The  common  tortoise  ( Testudo  graca],  and  the  extermi- 
nated giant  tortoises  of  the  Mascarene  and  Galapagos  Islands  are  good 
representatives. 

Chelydidse,  fresh  water  tortoises,  more  or  less  aquatic,  with  per- 
fectly ossified  carapace,  and  feet  with  sharp  claws.  Examples — Chelys 
fivibriata,  from  Brazil  and  the  Guianas,  with  warty  growths  of  decep- 


FIG,  194. — Heart,  and  associated  vessels,  of  Tortoise. 

(After  NUHN.) 

r.a,  Right  auricle  ;  superior  venae  cavae  (s.v.c.)  and  inferior  vena 
cavafz'.z/.rr.)  enter  it.  r.v.  Right  half  of  ventricle  ;  pulmonary  arteries 
(p. a)  and  left  aortic  arch  (Lew)  leave  it ;  coel  coeliac  ;  d.ao,  dorsal 
aorta.  /. a,  Left  auricle;  p.v.  pulmonary  veins  enter  it.  l.u,  Left 
half  of  ventricle  ;  right  aortic  arch  (r.ao),  giving  off  carotids  (c)  and 
subclavians  (s.cf),  leaves  it. 

tive  appearance  ;    Emys  orbicularis  common  in  S.  Europe  ;    Chelydra 
and  MacrodemmyS)  the  aquatic  terrapins  of  N.  America. 

Trionychidoe,  fresh  water  turtles,  with  depressed  carapace  covered 
with  soft  skin,  with  webbed  digits.  Each  foot  has  sharp  claws  on  the 
three  inner  digits.  They  are  carnivorous  in  habit.  Examples — Trionyx^ 
javanicus,  gangeticus,  niloticus,  from  Java,  the  Ganges,  and  the  Nile 
respectively. 


568  REPTILES. 

Class  RHYNCHOCEPHALIA. 

The  only  living  representative  of  this  class  is  the  New 
Zealand  "  Lizard " — Hatteria  or  Sphenodon  punctatus^  the 
Tuatara  of  the  Maoris.  Lizard-like  in  appearance,  it  mea- 
sures from  one  to  two  feet  in  length,  has  a  compressed 
crested  tail,  is  dull  olive-green  spotted  with  yellow  above 
and  whitish  below.  It  is  now  rare,  but  is  being  preserved 
in  some  small  islands  off  the  New  Zealand  coast,  It  lives 
in  holes  among  the  rocks  or  in  small  burrows,  feeds  on 
small  animals,  and  is  nocturnal  in  habit. 

The  skull,  unlike  that  of  any  lizard,  has  an  ossified 
quadrato-jugal,  and  therefore  a  complete  infra-temporal 
arcade  ;  the  quadrate  is  firmly  united  to  pterygoid,  squa- 
mosal,  and  quadrato-jugal ;  the  pterygoids  meet  the  vomer 
and  separate  the  palatines ;  there  are  teeth  on  the  palatine 


FIG.  195. — Hatteria  or  Sphenodon.     (After  HAYEK.) 

in  a  single  longitudinal  row,  parallel  with  those  on  maxilla 
and  mandible,  and  the  three  sets  seem  to  wear  one  another 
away;  there  is  also  a  single  tooth  on  each  side  of  a 
kind  of  beak  formed  by  the  premaxillae ;  the  nares  are 
divided. 

The  vertebrae  are  biconcave,  as  in  geckos  among  lizards 
and  in  many  extinct  Reptiles.  Some  of  the  ribs  bear 
uncinate  processes,  as  in  Birds ;  as  in  crocodiles,  there  are 
numerous  "abdominal  ribs,"  ossifications  in  the  sub-cutane- 
ous fibrous  tissue  of  the  abdomen.  The  anterior  end  of  the 
"  plastron  "  thus  formed  overlaps  the  posterior  end  of  the 
sternum.  Clavicles  and  interclavicle  present. 

The  pineal  or  parietal  eye,  which  reaches  the  skin  on  the 
top  of  the  head,  is  less  degenerate  than  in  other  animals, 
retaining,  for  instance,  distinct  traces  of  a  complex  retina. 


LIZARDS.  569 

Near  the  living  Sphenodon,  the  Permian  Palceohatteria,  the  Triassic 
Hyperodapedon,  and  some  other  important  types  may  be  ranked.  Along 
with  these  may  be  included  the  remarkable  Proterosaurus  from  the  Per- 
mian, though  Seeley  establishes  for  it  a  special  order  Proterosauria  as 
distingushed  from  Rhynchocephalia.  According  to  Baur,  quoted  by 
Nicholson  and  Lydekker,  "  the  Rhynchocephalia,  together  with  the 
Proterosauria,  to  which  they  are  closely  allied,  are  certainly  the  most 
generalised  group  of  all  Reptiles,  and  come  nearest,  in  many  respects,  to 
that  order  of  Reptiles  from  which  all  others  took  their  origin."  We 
have  already  noted  how  they  are  linked  to  the  Amphibia. 

Class  LACERTILIA — Lizards. 

This  class  occupies  a  somewhat  central  position  among 
Reptiles. 

GENERAL  CHARACTERS. — The  body  is  usually  well  covered 
with  scales. 

In  most,  both  fore  and  hind  limbs  are  developed  and  bear 
clawed  digits,  but  either  pair  or  both  pairs  may  be  absent. 
The  shoulder  and  hip  girdles  are  always  present,  in  rudiment 
at  least. 

Unlike  snakes,  lizards  have  non-expansible  mouths,  and 
almost  always  movable  eyelids  and  external  ear  openings. 

The  teeth  are  fused  to  the  edge  or  to  the  ridge  of  the  jaws, 
never  planted  in  sockets. 

The  tongue,  broad  and  short  in  some,  e.g.,  Geckos  and 
Iguanas,  long  and  terminally  clubbed  in  Chamceleons,  is 
oftenest  a  narrow  bifid  organ  of  touch. 

The  opening  of  the  cloaca  is  transverse. 

There  is  a  urinary  bladder  and  a  double  penis. 

Most  are  oviparous,  but  in  a  few  the  eggs  are  hatched 
within  the  body. 

They  are  usually  active  agile  animals,  beautifully  aud  often 
protectively  coloured. 

The  caudal  region  is  often  very  brittle  ;  lost  tails  and  even 
legs  may  be  regenerated. 

The  food  generally  consists  of  insects,  worms,  and  other 
small  animals,  but  some  prey  upon  larger  animals,  and 
others  are  vegetarian. 

Most  are  terrestrial,  some  arboreal,  a  few  semi-aquatic,  and 
there  is  one  marine  form. 

Lizards  are  most  abundant  in  the  tropics,  and  are  absent 
from  very  cold  regions. 


570  REPTILES. 

Some  Peculiarities  in  the  Skeleton  of  Lizards  (mostly  quoted 
from  Huxley). 

The  epidermic  exoskeleton  of  scales  is  sometimes,  as  in  Cyclodus, 
associated  with  scutes  or  ossifications  in  the  dermis.  In  Geckos  and 
Amphisbsenas  there  is  hardly  any  exoskeleton. 

Except  the  Geckos,  all  living  Lizards  have  procoelous  vertebrae.  The 
sacral  vertebrae,  two  or  rarely  three  in  number,  are  not  fused.  Under- 
neath the  vertebrae,  in  the  anterior  part  of  the  tail,  there  are  usually 
special  "  chevron  "  bones.  In  many  cases  there  is  an  unossified  septum 
across  the  middle  of  each  caudal  vertebra,  and  it  is  across  this  that  the 
tail  so  readily  breaks. 

•  In  the  skull,  there  is  an  interorbital  septum  except  in  Amphisbsenas, 
there  are  no  alisphenoids  nor  completely  ossified  presphenoid  or  orbito- 
sphenoids,  there  is  usually  an  unossified  parietal  foramen  on  the  roof  of 
the  skull,  in  most  an  epipterygoid  (or  "  columella  ")  runs  from  the  parietal 
to  the  pterygoicl,  in  most  there  are  prominent  parotic  processes  formed 
from  prolongations  of  the  opisthotics,  pro-otics  and  ex-occipitals,  with 
the  outer  end  of  one  of  these  processes  the  quadrate  articulates,  and  is 
usually  movable,  the  fronto-parietal  region  is  often  slightly  movable  on 
the  occipito-sphenoidal.  part,  the  quadrato-jugal  is  usually  represented 
by  ligament  only,  from  the  union  of  the  palatine  and  pterygoid  a  trans- 
verse bone  extends  to  the  maxilla,  the  two  rami  of  the  lower  jaw  are  in 
most  cases  firmly  connected. 

Teeth  occur  on  the  premaxillas,  maxilke  and  dentaries,  and  sometimes 
also  on  palatines  and  pterygoids.  They  generally  become  fused  to  the 
bones  which  bear  them.  When  they  are  attached  by  their  bases  to  the 
ridge  of  the  jaw,  the  dentition  is  described  as  acrodont  ;  when  they  are 
attached  by  their  sides  to  the  side  of  the  jaw,  the  dentition  is  described 
as  pleurodont. 

DESCRIPTION  OF  A  LIZARD. 

The  following  description  applies  especially  to  the  long- 
tailed  green  lizard  (Lacerta  viridis\  found  abundantly  in 
Jersey,  but,  except  in  minor  points,  it  will  be  found  to  apply 
equally  to  the  small  British  grey  lizard  (Lacerta  agilis)  and 
to  the  viviparous  lizard  (Zootoca  vivipara). 

Form  and  External  Features. 

The  depressed  head  is  separated  from  the  body  by  a  dis- 
tinct neck,  but  the  posterior  region  of  the  body  passes 
gradually  into  the  long  tail,  which  is  often  mutilated  in 
captured  specimens.  Both  fore  and  hind  limbs  are  present, 
and  both  are  furnished  with  five  clawed  digits.  Of  the 
apertures  of  the  body,  the  large  mouth  is  terminal,  the 
external  nares  are  close  to  the  end  of  the  snout,  and  the 


LIZARDS.  571 

cloacal  aperture  is  a  considerable  transverse  opening  placed 
at  the  root  of  the  tail.  There  is  no  external  ear,  but  the 
tympanic  membrane  at  either  side  is  slightly  depressed 
below  the  level  of  the  skin  of  the  head.  The  eyes  are  fur- 
nished with  both  upper  and  lower  eyelids,  and  also  with  a 
nictitating  membrane. 

Skin. 

As  contrasted  with  that  of  the  frog,  the  skin  is  remarkable 
as  possessing  a  distinct  exoskeleton  of  epidermic  scales.  In 
the  head  region  these  exhibit  a  definite  arrangement  char- 
acteristic of  the  species.  With  the  presence  of  an  exo- 
skeleton we  must  associate  the  absence  of  the  numerous 
cutaneous  glands  of  the  frog ;  these  are  here  represented 
only  by  a  row  of  "femoral  glands,"  which  open  by  pores  on 
the  ventral  surface  of  the  thigh.  Their  secretion  is  most 
obvious  in  the  male  at  pairing  time.  The  histological  com- 
position of  the  skin  is  very  similar  to  that  of  the  frog's  skin. 
Pigment  is  deposited  here  also  in  two  layers,  of  which  the 
outer  is  greenish,  the  inner  black.  It  is  of  special  interest 
to  notice  that  over  the  parietal  foramen  (see  Skull)  the  black 
pigment  is  absent,  the  green  only  feebly  represented ;  in  this 
region,  therefore,  the  skin  is  almost  transparent. 

Many  Lizards,  such  as  the  Chameleons,  exhibit  in  a  remarkable  degree 
the  power  of  rapidly  changing  the  colour  of  their  skin.  This  is  due  to  the 
fact  that  the  protoplasm  of  the  pigment  cells  contracts  or  expands  under 
nervous  control.  The  change  of  colour  is  sometimes  advantageously  pro- 
tective, but  it  seems  often  to  be  merely  a  reflex  symptom  of  the  nervous 
condition  of  the  animals. 

In  a  few  cases,  e.g.,  some  of  the  skinks,  there  are  minute  dermal 
ossifications  beneath  the  scales. 

Skeleton. 

The  backbone  consists  of  a  variable  number  of  vertebrae, 
and  is  divisible  into  cervical,  dorsal,  lumbar,  sacral,  and 
caudal  regions.  Except  the  atlas  and  the  last  caudal,  all  the 
vertebrae  are  procoelous. 

The  atlas  consists  of  three  separate  pieces,  its  centrum  ossifies  as  usual 
as  the  odontoid  process  of  the  axis.  There  are  two  sacral  vertebrre  with 
large  expanded  sacral  ribs.  To  the  ventral  surfaces  of  many  of  the 
caudal  vertebrae  Y-shaped  "chevron"  bones  are  attached.  Across  the 
centre  of  the  caudal  vertebrae  there  extends  a  median  unossified  zone,  it 
is  in  this  region  that  separation  takes  place  when  a  startled  lizard  loses 
its  tail. 


572 


REPTILES. 


The  ribs  are  numerous,  but  only  five  reach  the  sternum. 

The  skull  is  well  ossified,  but  in  the  region  of  the  nares, 
in  the  interorbital  septum,  &c.,  the  primitive  cartilaginous 
brain-box  persists.  On  the  dorsal  surface  the  bones  exhibit 
numerous  impressions  made  by  the  epidermic  scales,  which 
render  it  difficult  to  distinguish  the  true  sutures  of  the 
bones. 

Two  fused  parietals  with  the  rounded  median  "  parietal  foramen,"  two 
frontals,  and  the  two  nasals,  are  the  most  important  constituents  of  the 
roof  of  the  skull.  Anteriorly  the  premaxillae  appear  between  the  nasals, 
while  posteriorly  the  sickle-shaped  squamosal  is  attached  by  a  suture  to 
the  parietal,  and  is  overlapped  by  one  of  the  two  small  supra-temporal 


FIG.  196.— Side  view  of  skull  of  Lacerta.     (After  W.  K.  PARKER.) 

/jr.,  Premaxilla ;  e.n.,  external  nostril;  vix.,  maxilla;  /.,  lach- 
rymal ;  j.,  jugal ;  II.,  oplic  nerve  ;  t.pa.,  trans -palatine  ;  ep-g-,  epi- 
pterygoid ;  pg.,  pterygpid  ;  b.pg,,  basi-pterygoid  ;  b.o.,  basi-occipital ; 
^.,  quadrate;  oc.c.,  occipital  condyle;  s<?.,  squamosal ;  pr.o.,  pro-otic; 
pt.o.,  post-orbital ;  st\.  ste.,  supra-temporals  ;  ps.,  presphenoid  ;  p.e., 
mesethmoid  ;  s.o.b.,  supra-orbitals ;  z.o.n.,  inter-orbital  notch  ;  pf., 

Crefrontal ;  «.,  nasal ;  <?./.,  olfactory  sac  ;  ar.,  articular  ;  ag.,  angu- 
tr  ;  sag.,  surangular  ;  cf-.,  coronary  ;  d.,  dentarj-. 

bones.     The  orbit  is  roofed  by  a  series  of  small  bones,  of  which  the 
anterior  and  posterior  are  respectively  known  as  pre-  and  post-frontal. 

On  the  floor  of  the  adult  skull  there  is  a  large  basal  bone,  composed 
of  fused  occipital  and  sphenoidal  elements,  and  continued  forward  as  a 
slender  bar  (parasphenoid).  This  bone  gives  off  two  stout  processes, 
the  basipterygoid  processes,  which  articulate  with  the  pterygoids.  Each 
pterygoid  is  connected  posteriorly  to  the  quadrate  bone  of  the  corre- 
sponding side,  and  anteriorly  with  the  palatine.  From  the  union  of 


LIZARDS.  573 

pterygoid  and  palatine,  a  stout  os  transversum  or  trans-palatine  extends 
outwards  to  the  maxilla.  In  front  of  the  palatines  lie  the  small  vomers, 
which,  in  their  turn,  articulate  with  the  premaxilla  and  maxilla,  both  of 
which  are  furnished  with  small  pointed  teeth.  In  the  posterior  region 
of  the  skull  we  have  still  to  notice  the  large  ex-occipitals  with  which  the 
opisthotics  are  fused,  and  which  are  continued  into  the  conspicuous 
parotic  processes.  The  lateral  walls  of  the  brain  case  are  largely  formed 
by  the  paired  pro-otics.  Internally,  an  important  bone,  the  epipterygoid 
or  "columella"  (not  to  be  confounded  with  the  columella  or  stapes  of 
the  ear),  extends  from  the  prootic  to  the  pterygoid.  The  orbit  is  bounded 
posteriorly  and  inferiorly  by  the  jugals.  There  is  no  ossified  quadrato- 
jugal,  and  thus  the  lateral  temporal  fossa  is  open  below  in  the  dried 
skull  (contrast  Hatteria}.  The  other  fossae  of  the  dried  skull  are  the 
supra-temporal  on  the  upper  surface,  and  the  posterior-temporal  on  the 
posterior  face. 

Each  half  of  the  lower  jaw  is  composed  of  six  bones,  which  do  not 
fuse  in  the  adult. 

The  Limbs  and  Girdles. 

In  the  shoulder-girdle,  the  flat  coracoids,  with  an  anterior 
precoracoidal  region,  articulate  with  the  sternum,  which  is 
represented  by  a  cartilaginous  plate  of  rhomboidal  shape. 
Over  it  projects  the  long  limb  of  the  T-shaped  interclavicle, 
which,  at  the  sides,  is  continued  backwards  by  the  curved 
clavicles.  The  remaining  elements  are  the  scapulae,  which 
are  continuous  with  the  cartilaginous  supra-scapulae. 

The  fore  limbs  have  the  usual  parts.  In  the  carpus  all 
the  typical  nine  bones  are  represented,  and  there  is  in 
addition  an  accessory  "  pisiform  "  bone. 

In  the  pelvic  girdle,  ilium,  pubis,  and  ischium  are  repre- 
sented as  usual ;  there  are  both  pubic  and  ischiac  sym- 
physes. 

In  the  tarsus  the  fibulare  and  tibiale  are  united,  and  the 
distal  row  consists  of  only  two  bones. 

Nervous  System. 

The  brain  consists  of  the  usual  parts.  The  cerebellum 
is  small  and  only  partially  overlaps  the  fourth  ventricle. 
In  the  region  of  the  thalamus  the  epiphysis  is  distinct  and 
conspicuous,  but  in  the  adult  the  pineal  body  is  quite 
separated  from  it,  and  lies  in  its  connective  tissue  capsule 
below  the  skin. 

Alimentary  System. 

Small  pointed  teeth  are  present  on  the  maxillae,  pre- 
maxillae,  palatines,  and  on  the  lower  jaw  ;  they  are  attached 


574  REPTILES. 

by  their  sides  (pleurodont).  Salivary  glands  occur  on  the 
floor  of  the  mouth  cavity.  The  narrow  gullet  passes 
gradually  into  the  muscular  stomach,  which  again  passes 
into  the  coiled  small  intestine.  Near  the  commencement 
of  the  large  intestine  there  is  a  small  caecum.  A  volumi- 
nous liver,  with  a  gall  bladder  embedded  in  it,  and  a  pancreas 
are  present  as  usual. 

Embedded  in  the  mesentery  below  the  stomach  lies  the 
rounded  spleen.  A  whitish  thyroid  gland  lies  on  the  ventral 
surface  of  the  trachea  a  short  distance  in  front  of  the  heart. 


J 


FIG.  197. — Heart,  and  associated  vessels,  oi  Lizard. 
(After  NUHN.) 

A.,  Right  auricle  ;  jugulars  (/.)  subclavians  (Sc.v.\  and  inferior 
vena  cava  (I.V.C.)  enter  it.  V.,  ventricle  ;  tr.,  truncus  arteriosus  ; 
i,  first  aortic  arch  giving  off  carotids  ;  2,  second  aortic  arch  ;  /.«., 
pulmonary  artery  ;  Sc.a..,  subclavian  ;  Ao.,  dorsal  aorta.  A},  left 
auricle  ;  pulmonary  veins  (/.?'.)  enter  it.  In  the  lizard  described, 
the  left  jugular  is  not  developed. 

Vascular  System. 

The  heart  is  completely  enveloped  by  the  pericardium, 
and  is  three  chambered,  consisting  of  two  thin-walled 
auricles  and  a  muscular  ventricle.  From  the  ventral  surface 
of  the  ventricle  arises  the  conspicuous  truncus  arteriosus, 


LIZARDS.  575 

which  is  formed  by  the  bases  of  the  aortic  arches,  and 
exhibits  a  division  into  two  parts.  From  the  more  ventral 
part  arises  the  left  aortic  arch,  which  curves  round  to  the 
left  side,  first  giving  off  a  short  connecting  vessel  (ductus 
Botallii}  to  the  carotid  arch.  From  the  other  division  of 
the  truncus  arteriosus,  a  great  arterial  trunk  arises,  and  this 
gives  off  the  right  aortic  arch  and  the  right  and  left  carotid 
arches.  The  right  aortic  arch  sends  a  ductus  Botallii  to  the 
carotid  arch  of  the  right  side,  and  then  curves  round  the 
heart  to  join  the  left  arch,  when  the  two  form  the  dorsal 
aorta.  The  carotid  arches  supply  the  head  region  with 
blood.  From  the  base  of  the  truncus  arteriosus,  the  right 
and  left  pulmonary  arteries  also  arise. 

From  the  right  aortic  arch  as  it  curves  round,  arise  the  right  and  left 
subclavian  arteries,  which  carry  blood  to  the  fore  limbs.  A  coeliaco- 
mesenteric  artery  arises  from  the  dorsal  aorta  and  supplies  the  viscera. 
Smaller  vessels  are  also  given  off  to  the  genital  organs,  &c.,  and  then  at 
the  anterior  end  of  the  kidneys,  the  aorta  divides  into  two  femoral 
arteries  which  break  up  into  a  network  of  small  vessels,  supplying  hind 
limbs  and  kidneys,  and  finally,  at  the  posterior  end  of  the  kidneys,  re- 
unite to  form  the  caudal  artery,  which  runs  down  the  tail. 

The  blood  from  the  anterior  region  of  the  body  is  returned  to  the 
heart  by  the  right  and  left  precaval  veins  or  superior  venae  cavse.  The 
right  precaval  is  formed  by  the  junction  of  external  and  internal 
jugulars  with  the  subclavian  vein  ;  on  the  left  side  the  jugular  is  absent. 
From  the  posterior  region  of  the  body,  blood  is  brought  back  by  the 
postcaval  vein  or  inferior  vena  cava.  The  three  great  veins  open  into 
a  thin-walled  sinus  venosus,  which  opens  into  the  right  auricle. 

The  postcaval  is  formed  by  the  union  of  two  veins  which  run  along 
the  genital  organs,  and  receive  renal  veins  from  the  kidneys.  In  passing 
through  the  liver  the  postcaval  receives  important  hepatic  veins. 

From  the  tail  region  the  blood  is  brought  back  by  a  caudal  which 
bifurcates  in  the  region  of  the  kidneys  into  two  pelvics.  The  pelvic 
veins  give  off  renal  portals  to  the  kidneys,  and  receive  the  femoral  and 
sciatic  veins  from  the  hind  limbs.  They  then  unite  to  form  the 
epigastric  or  anterior  abdominal,  which,  carries  blood  to  the  liver. 
Except  through  the  medium  of  the  renal-portal  system,  there  is  no 
connection  between  the  anterior  abdominal  and  the  postcaval.  To  the 
liver  blood  is  carried  as  usual  from  the  stomach,  £c.,  by  the  portal 
vein. 

From  the  lungs  blood  is  brought  to  the  left  auricle  by  the  pulmonary 
veins. 

A  lymphatic  system  including  a  pair  of  lymph  hearts  is  present. 

Respiratory  System. 

The  lungs  are  elongated  oval  structures  which  taper  away 
posteriorly.  The  mouth  does  not,  as  in  the  frog,  play  any 


576  REPTILES. 

part  in  the  respiratory  movements.  In  some  lizards 
(Chamseleon  and  Geckos)  the  lungs  are  prolonged  in  air- 
sacs,  suggesting  those  of  Birds  (Fig.  198). 

Excretory  System. 

The  paired  kidneys  lie  in  the  extreme  posterior  region 
of  the  abdominal  cavity,  and  extend  a  little  further  back 
than  the  level  of  the  cloaca.  Each  is  furnished  with  a  very 
short  ureter.  In  the  male  the  ureters  unite  with  the  vasa 
deferentia;  in  the  female  they  open  separately  into  the 
cloaca.  Into  the  cloaca  opens  also  a  large  thin-walled 
"  urinary  bladder  "  ;  this  is  a  remnant  of  the  fcetal  allantois, 
and  has  no  functional  connection  with  excretion.  The 
urine  is  semi-solid,  and  consists  largely  of  uric  acid. 


FIG.  198. — Lung  of  Chamceleo  vulgaris,  showing 
air  sacs.     (After  WIEDERSHEIM.) 

Reproductive  System. 

In  the  male  the  testes  are  two  white  oval  bodies  sus- 
pended in  a  dorsal  fold  of  mesentery.  Along  the  inner 
surface  of  each  runs  the  epididymis,  which  receives  the  vasa 
efferentia,  and  is  continuous  posteriorly  with  the  vas 
deferens.  The  two  vasa  deferentia,  after  receiving  the 
ureters,  open  by  small  papillae  into  the  cloaca.  In  con- 
nection with  the  cloaca  there  is  a  pair  of  eversible  copulatory 
organs. 

In  the  female  the  ovaries  occupy  a  similar  position  to 
that  of  the  testes  in  the  male.  The  oviducts  open  far  for- 
ward by  wide  ciliated  funnels  ;  as  they  pass  backward  they 
show  a  gradual  increase  in  cross  section,  but  there  is  no 
line  of  demarcation  between  an  oviducal  and  a  uterine 
portion.  Posteriorly,  the  oviducts  open  into  the  cloaca. 


LIZARDS.  577 

The  right  reproductive  organ  tends  to  be  larger  and  in  front  of  the 
left.  In  many  of  the  males,  the  Wolffian  body  is  well  developed. 
Viviparous,  or  what  is  clumsily  called  ovo-viviparous,  parturition  is 
well  illustrated  by  Zootoca  viviparus,  Angztis  fragilis,  Seps,  &c.,  but 
most  lay  eggs  with  more  or  less  calcareous  shells.  In  Trachydosaurus 
and  Cyclodus  the  embryo  seems  to  absorb  food  from  the  wall  of  the 
uterus.  It  is  likely  that  Lacertilians  existed  in  Permian  ages,  but  their 
remains  are  not  numerous  before  the  Tertiary  strata. 

Many  instructive  illustrations  of  evolutionary  change  are  afforded  by 
lizards.  Thus  there  are  numerous  gradations  in  the  reduction  of  the 
limbs,  from  a  decrease  in  the  toes  to  entire  absence  of  limbs.  The 
diverse  forms  of  tongue  and  the  varied  positions  of  the  teeth  are  also 
connected  by  gradations.  From  the  variations  of  the  wall  lizard  (Lacerta 
muralis],  Eimer  elaborated  most  of  his  theory  of  evolution. 

Some  Families  of  Lacertilia* 

The  class  includes  great  variety  of  form. 

In  the  Geckos  (Geckonidre)  the  vertebrae  are  biconcave  or  amphi- 
coelous,  the  tongue  is  short  and  fleshy,  the  eyelids  are  rudimentary,  the 
teeth  are  pleurodont,  the  toes  bear  numerous  plaits,  by  means  of  which 
they  adhere  to  smooth  surfaces.  The  Geckos  have  been  observed  to  eat 
their  own  young  and  even  their  own  tails.  The  name  Gecko  indicates 
their  call.  Examples  : — Platydactylus  mauritantcus  (S.  Europe),  Hemi- 
dactylus  in  most  warm  countries,  Ptychozoon,  with  lateral  webs  of  skin 
which  serve  as  parachutes. 

The  Agamas  (Agamidse)  are  acrodont  lizards  common  in  the  eastern 
hemisphere.  Examples  : — Agama;  Draco,  with  the  skin  extended  on 
long  prolongations  of  five  or  six  posterior  ribs;  Chlamydosaurus,  an 
Australian  lizard,  with  a  large  scaled  frill  around  the  neck  ;  Moloch, 
another  Australian  form  bristling  with  sharp  spikes. 

The  Iguanas  (Iguanidae)  are  pleurodont  lizards,  represented  in  the 
warmer  parts  of  the  New  World.  Examples  : — Iguana,  an  arboreal 
lizard,  with  a  large  distensible  dewlap  ;  Amblyrhynchus  or  Oreocephalus 
cristatus,  a  marine  lizard  confined  to  the  Galapagos  Islands  ;  Basiliscus, 
in  S.  Mexico,  with  none  of  the  marvellous  qualities  of  the  mytho- 
logical basilisk ;  Anolis,  the  American  chamseleon,  with  powers  of 
rapid  colour  change;  Phrynosoma,  the  American  "horned  toad,"  with 
numerous  horny  scales,  and  a  collar  of  sharp  spines  suggesting  in 
miniature  that  of  some  of  the  extinct  Reptiles. 

The  slow  worms  (Anguidse),  are  limbless  lizards,  with  serpentine 
body,  long  tail,  rudimentary  girdles  and  sternum.  The  British  species, 
Anguis  fragilis,  is  neither  blind  nor  poisonous  ;  the  tail  breaks  very 
readily  ;  the  young  are  hatched  within  the  mother.  The  American 
"  glass  snake  " — Opheosaurus  ventralis — is  in  many  ways  like  our  slow 
worm. 

The  poisonous  Mexican  lizard  (Heloderma  suspectuiii)  measures  over 
a  foot  in  length,  and  is  covered  with  bead-like  scales.  Its  bite  is 
poisonous,  and  rapidly  fatal  to  small  Mammals.  It  is  interesting  to 
find  poisonous  powers  like  those  of  many  serpents  exhibited  by  this  ex- 
ceptional lizard. 

The  water  lizards  (VaranidcE)  are  large  semi-aquatic  forms  of  carni- 
37 


578  REPTILES. 

vorous  habit,  most  at  home  in  Africa,  but  represented  also  in  Asia  and 
Australia.  The  Monitor  of  the  Nile,  Varanus  niloticus,  may  attain 
a  length  of  five  or  six  feet,  and  is  noteworthy  because  of  its  fondness  for 
the  eggs  and  young  of  Crocodiles. 

The  family  Teiidae  includes  many  New  World  pleurodont  lizards, 
mostly  terrestrial  in  habit,  for  example,  Teius  tegnexim,  the  variegated 
lizard  of  tropical  Brazil,  sometimes  measuring  five  feet  in  length  ;  Ameiva 
dorsalis,  the  common  ground  lizard  of  Jamaica. 

The  Amphisbaenidae  are  degenerate  subterranean  lizards,  without 
limbs,  with  rudimentary  girdles,  with  no  sternum,  with  small  covered 
eyes,  with  hardly  any  scales.  The  sooty  Amphisbaena  (A.  fuliginosa), 
at  home  in  the  warmer  parts  of  S.  America,  is  the  commonest  species. 

The  Lacertidae  are  Old  World  acrodont  lizards,  such  as  Pseudopus 
(Europe  and  S.  Asia),  Lacerta  viridis,  the  green  lizard  of  Jersey  and 
S.  Europe,  L.  agilis,  the  British  grey  lizard,  L.  muralis,  abundant  about 
ruins  in  S.  Europe,  L.  or  Zootoca  vivipara,  the  British  scaly  lizard. 

The  Scincidae  are  common  in  tropical  countries,  e.g.>  Scincus, 
CycloduS)  Seps,  Acontias  (without  limbs),  Oligosoma  (abundant  in  the 
Southern  States  of  America),  Eumeces  (common  in  America  and  else- 
where). 

The  Chamseleons  (Chamseleontidae)  are  very  divergent  lizards,  mostly 
African.  There  is  one  genus  Chanuzlco.  The  head  and  the  body  are 
compressed  ;  the  scales  are  minute  ;  the  eyes  are  very  large  and  mov- 
able, with  circular  eyelids  pierced  by  a  hole  ;  the  tympanum  is  hidden  ; 
the  tongue  is  club  shaped  and  viscid  ;  the  digits  are  divided  into  two 
sets,  and  well  adapted  for  prehension  ;  the  tail  is  prehensile  ;  the  power 
of  colour  change  is  remarkably  developed. 

The  Chamseleons  exhibit  numerous  anatomical  peculiarities.  As  in  the 
Amphisbsenas,  there  is  no  epipterygoid  nor  interorbital  septum.  The 
pterygoid  does  not  directly  articulate  with  the  quadrate  which  is  ankylosed 
to  the  adjacent  bones  of  the  skull. 

Class  OPHIDIA.     Serpents  or  Snakes. 

The  elongated  limbless  form  of  snakes  seems  at  first  sight 
almost  enough  to  define  this  order  from  other  Reptiles,  but 
it  must  be  carefully  noticed  that  there  are  limbless  lizards, 
limbless  amphibians,  and  limbless  fishes,  which  resemble 
serpents  in  shape  though  very  different  in  internal  structure. 
For  the  external  shape  seems  in  great  part  an  adaptation  to 
the  mode  of  life,  to  the  habit  of  creeping  through  crevices 
or  among  obstacles.  Even  in  the  thin-bodied  weasels  is 
there  not  some  suggestion  of  the  serpent  ?  Yet  the  limbless- 
ness  of  serpents  is  not  a  merely  superficial  abortion,  for  there 
is  no  pectoral  girdle  nor  sternum,  and  never  more  than  a 
hint  of  a  pelvis. 

The  skin  is  covered  with  scales,  which  being  simply  folds 


SNAKES.  579 

of  the  epidermis  have  much  coherence,  and  are  periodically 
shed  in  a  continuous  slough.  The  scales  on  the  head  form 
large  plates,  and  those  on  the  ventral  surface  are  transverse 
shields.  There  are  no  separate  eyelids,  but  the  thin  trans- 
parent epidermis  extends  over  the  staring  eyes.  The  nostrils 
lie  near  the  tip  of  the  head ;  there  are  no  external  ear  open- 
ings. In  many  cases  there  are  odoriferous  glands  near  the 
cloacal  aperture. 

The  muscular  system  is  very  highly  developed,  and  the 
limbless  serpent,  Owen  says,  "can  outclimb  the  monkey, 
outswim  the  fish,  outleap  the  zebra,  outwrestle  the  athlete, 
and  crush  the  tiger." 

There  are  many  remarkable  peculiarities  in  the  skeleton. 


dv-< 


FIG.   199.  —  Snake's  head.     (After  NUHN. 


civ.,  Poison  fangs  ;    b.,  sheath  of  fang  ;  /.  tongue  ;   rt.,  muscles  ot 
tongue. 

The  vertebrae  are  very  numerous,  some  pythons  having 
four  hundred  ;  they  are  proccelous,  and  are  distinguishable 
only  into  a  pre-caudal  and  caudal  series. 

All  the  pre-caudal  vertebrae  except  the  first  —  the  atlas  — 
have  associated  ribs,  which  are  movably  articulated  and 
used  as  limbs  in  locomotion.  In  the  caudal  region,  the 
transverse  processes,  which  are  elsewhere  very  small,  take 
the  place  of  ribs. 

The  serpent  "  literally  rows  on  the  earth,  with  every 
scale  for  an  oar;  it  bites  the  dust  with  the  ridges  of  its 
body."  On  a  perfectly  smooth  surface  it  can  make  no 
headway,  but  in  normal  conditions  the  edges  of  the 
anterior  ventral  scales  are  fixed  against  the  roughnesses 


580  REPTILES. 

of  the  ground,  the  ribs  are  drawn  together  first  on  one 
side  then  on  another,  the  body  is  thus  wriggled  forward 
to  the  place  of  attachment,  the  front  part  shoots  out  as 
the  hind  part  fixes  itself,  an  anterior  attachment  is  again 
effected,  and  thus  the  serpent  flows  onward.  But  this 
account  of  the  mechanism  of  movement  does  not 
suggest  the  swiftness  or  the  beauty  of  what  Ruskin 
calls  "  one  soundless,  causeless  march  of  sequent  rings, 
and  spectral  procession  of  spotted  dust,  with  dissolu- 
tion in  its  fangs,  dislocation  in  its  coils."  "Startle  it; 
the  winding  stream  will  become  a  twisted  arrow ; — the 
wave  of  poisoned  life  will  lash  through  the  grass  like  a 
cast  lance." 

One  of  the  most  distinctive  characteristics  of  the  skull,  is 
the  mobility  of  some  of  the  bones.  Many  of  the  Ophidians 
swallow  animals  which  are  larger  than  the  normal  size  of  the 
mouth  and  throat.  The  mobility  of  the  skull  bones  is  an 
adaptation  to  this  habit.  Thus,  the  rami  of  the  mandible 
are  united  by  an  elastic  ligament ;  the  quadrates  and  the 
squamosals  are  also  movable,  forming  "a  kind  of  jointed 
lever,  the  straightening  of  which  permits  of  the  separation 
of  the  mandibles  from  the  base  of  the  skull."  The  nasal 
region  may  also  be  movable.  On  the  other  hand,  the 
bones  of  the  brain  case  proper  are  firmly  united.  The 
premaxillae  are  very  small  and  rarely  bear  teeth ;  the 
palatines  are  usually  connected  with  the  maxillae  by  trans- 
verse bones,  and  through  the  pterygoids  with  the  movable 
quadrates. 

Teeth,  fused  to  the  bones  which  bear  them,  occur  on  the 
dentaries  beneath,  and  above  on  the  maxillae,  palatines,  and 
pterygoids,  and  very  rarely  on  the  premaxillae.  The  fang- 
like  teeth  of  venomous  serpents  are  borne  by  the  maxillae, 
and  are  few  in  number.  Each  fang  has  a  groove  or  canal 
down  which  the  poison  flows.  When  the  functional  fangs  are 
broken,  they  are  replaced  by  reserve  fangs  which  lie  behind 
them.  In  the  egg  eating  African  Rachiodon  the  teeth  are 
rudimentary,  but  the  inferior  spines  of  some  of  the  anterior 
vertebrae  project  on  the  dorsal  wall  of  the  gullet,  and  serve  to 
break  the  egg  shells. 

When  a  venomous  snake  strikes,  the  mandible  is  lowered, 
the  distal  end  of  the  quadrate  is  thrust  forward,  this 


5*1 


B 


FIG.  200.— Skull  of  Grass  Snake.     (From  W.  K.  PARKER). 

A.  Dorsal  surface— px. ,  premaxilla  ;  mx.,  maxilla  ;  an.,  external 
nostril;  n.,  nasal;  ol.,  nasal  cartilages;^/!,  prefronto-lachrymal  ;  p., 
parietal  ;  /.,  frontal  \pa.,  palatine  ;  t.fia.,  trans-palatine  ; pg. ,  pterygoid  ; 
Pro.,  pro-otic;  ep.,  epiotic :  op.,  opisthotic  ;  so.,  supra-occipital;  eo., 
ex-occipital;  ar.,  articular;  s.ag.,  sur-angular ;  ag.,  angular;  d., 
dentary  ;  q.,  quadrate  ;  sg.,  squamosal  ;  B.  Ventral  surface — px.,  pre- 
maxilla ;  ol.,  nasal  cartilage;  m.r.,  maxilla;  v.,  vomer  ;  pa.,  palatine; 
p.,  parasphenoid  ;  f.,  frontal  ;  pg. ,  pterygoid  ;  frs.,  basisphenoid  ; 
als.,  alisphenoid  ;  b.o.,  has  -occipital  ;  oc.c.,  occipital  condyle  ;  eo.,  ex- 
occipital  ;  q. ,  quadrate  ;  ar.  articular  ;  ag.,  angular  ;  s.ag'.,  sur-angular  ; 
cr.,  coronary  ;  sp.,  splenial  d.,  dentary  ;  op.,  opisthotic  region. 


582  REPTILES. 

pushes  forward  the  pterygoid,  the  pterygo-palatine  joint 
is  bent,  the  maxilla  is  rotated  on  its  lachrymal  joint,  the 
fangs  borne  by  the  maxilla  are  erected  into  a  vertical  posi- 
tion, the  poison  gland  is  compressed  by  a  muscle,  and  the 
venom  is  forced  through  the  fang. 

While  there  are  no  hints  of  anterior  appendages,  pythons, 
boas,  and  some  other  snakes,  have  rudiments  of  a  pelvis 
and  even  small  clawed  structures  which  represent  hind 
legs. 

Some  of  the  peculiarities  in  the  internal  organs  of  Ophidia 
may  be  connected  with  the  elongated  and  narrow  shape  of 
the  body.  Thus  one  lung,  usually  the  left,  is  always  smaller 
than  its  neighbour,  or  only  one  is  developed ;  the  liver 
is  much  elongated ;  the  kidneys  are  not  opposite  one 
another. 

The  brain  presents  no  remarkable  peculiarities  :  there  are 
only  ten  cranial  nerves ;  the  sense  of  hearing  is  often  slightly 
developed,  and  there  is  no  tympanic  cavity ;  the  eyelids  are 
fused  and  transparent;  the  bifid,  mobile,  retractile  tongue 
is  a  specialised  organ  of  touch. 

The  poison  gland  is  a  specialised  salivary  gland;  the 
venom  is  useful  in  defence,  and  in  killing  the  prey,  which  is 
always  swallowed  whole. 

The  heart  is  three  chambered,  the  ventricular  septum 
being  incomplete,  as  in  all  other  Reptiles  except  Croco- 
dilians. 

There  is  a  transverse  cloacal  aperture.  In  the  males, 
a  double  saccular  and  spiny  copulatory  organ  is  eversible 
from  the  cloaca. 

The  British  adder  (Pelias  bents)  is  viviparous,  and  so  are 
a  few  others.  The  great  majority  are  oviparous,  but  confine- 
ment and  abnormal  conditions  may  make  oviparous  forms, 
like  the  Boa  constrictor  and  the  British  grass  snake  (Tropi- 
donotus  natrix\  viviparous.  The  female  python  incubates 
the  eggs. 

Many  Ophidians  become  lethargic  during  extremes  of 
temperature,  or  after  a  heavy  meal. 

Though  most  abundant  in  the  Tropics,  snakes  occur  in 
most  parts  of  the  world.  They  are  absent  from  many 
islands  ;  thus  there  are  none  in  New  Zealand,  and  we  all 
know  that  there  are  no  snakes  in  Iceland.  Most  are  ter- 


CROCODILES,   ALLIGATORS,    G A  VIALS.  583 

restrial,  but  not  a  few  readily  take  to  the  water,  and  there 
are  many  habitual  sea  serpents. 

The  serpent  still  bites  the  heel  of  progressive  man,  the 
number  of  deaths  from  snake  bite  in  India  alone  amounting 
to  many  thousands  yearly,  though  there  can  be  little  doubt 
that  the  snakes  are  often  innocent  scape  goats. 

True  Ophidians  first  occur  in  Tertiary  strata. 

Classification  of  Ophidia. 

Sub-order  i.  Typhlopicke.  The  lowest  and  most  divergent  Ophidians, 
occurring  in  most  of  the  warmer  parts  of  the  earth,  generally 
smaller  than  earthworms,  usually  subterranean  burrowers,  with 
eyes  hidden  under  scales,  with  a  non-distensible  mouth,  with 
teeth  restricted  either  to  the  upper  or  to  the  lower  jaw.  "  The 
palatine  bones  meet,  or  nearly  meet,  in  the  base  of  the  skull, 
and  their  long  axes  are  transverse  ;  there  is  no  transverse 
bone ;  the  pterygoids  are  not  connected  with  the  quadrates  " 
(Huxley). 

Example  : — Typhlops,  British  India. 

In  all  other  Ophidians,  the  palatines  are  widely  separated,  and  their 
long  axes  are  longitudinal ;  there  are  transverse  bones  con- 
necting palatines  and  maxilloe  ;  the  pterygoids  are  connected 
with  the  quadrates. 

Sub-order  2.  Colubriformes  (innocuous  Snakes).     The  poison  gland 
is  not  developed  as  such  ;  the  maxillary  teeth  are  not  grooved. 
Examples  : — The    British   smooth  snake  (Coronella   Itevts], 
the  British  grass  snake  (Tropidonotus  natrix],  the  Pythons, 
the    Boas.      The   Anaconda   (Boa   murind),   which   may 
attain  a  length  of  almost  thirty  feet,  is  the  largest  living 
Ophidian. 
Sub-order  3.   Colubriformes  Venenosi. 

Examples  : — Cobras,   Naja  tripudians  (Indian),  Naja  haje 
(African);  the  Hamadryad  (Ophiophagus  elaps\  eating  other 
snakes ;  Coral  snakes  (Elaps,  &c.) ;  Sea  snakes  (Hydrophis, 
&c.),  with  paddle-shaped  tails. 
Sub-order  4.   Viperiformes. 

Examples  : — The  British  adder  (Pelias  or  Viperaberus} ;  the 
Rattlesnake  ( Crotalus},  with  a  rattle  formed  chiefly  from 
epidermic  remnants  of  successive  sloughings  ;  the  African 
Puff  adder  ( Clotho  arietans}. 

CROCODILIA.     Crocodiles,  Alligators,  Gavials. 

GENERAL  CHARACTERS. — The  Crocodilians  are  carnivor- 
ous fresh  water  reptiles  of  large  size,  now  represented  by  three 
genera — Crocodilus,  Alligator,  and  Gavialis. 

The  skin  bears  epidermic  scales,  underneath  some  of  which 
there  are  dermic  bones  or 


584 


REPTILES. 


The  tail  is  laterally  compressed  and  assists  in  swimming. 

Teeth  occur  in  distinct  sockets 
in  the  premaxillcz,  maxilla, 
and  dentaries. 

In  modern  Crocodilians,  al- 
most all  the  vertebra  are  pro- 
ccslous. 

The  skull  has  many  char- 
acteristic features,  such  as  the 
union  of  maxillce,  palatines, 
and  pterygoids  in  the  middle 
line  on  the  roof  of  the  mouth, 
and  the  consequent  shunting  of 
the  posterior  nares  to  the  very 
back  of  the  mouth. 

Some  of  the  ribs  have  double 
articulating  heads,  and  bear 
small  uncinate  processes  some- 
what like  those  of  birds  ;  trans- 
verse ossifications  associated 
with  the  subcutaneous  fibrous 
tissue  of  the  abdomen  from  so- 
called  abdominal  ribs. 

The  heart  is  four  chambered; 
a  muscular  diaphragm  par- 
tially separates  the  thoracic 
from  the  abdominal  cavity. 

The  cloaca  has  a  longitudinal  opening. 
a  grooved  penis. 

The  Crocodilians  are  oviparous.  The  eggs  have  firm  cal- 
careous shells,  and  are  laid  in  holes  in  the  ground. 

Some  of  the  Characteristic  Features  in  the  Skeletal  System  of 
Crocodilians. 

(These  notes  on  the  skeleton  are  in  great  part  taken  from  Huxley's 
Manual.) 

Numerous  transverse  rows  of  sculptured  bony  plates  or  scutes,  ossified 
in  the  dermis,  form  a  dorsal  shield.  On  the  ventral  surface  the  scutes 
are  absent,  except  in  some  alligators,  in  which  they  are  partially  ossified. 
But  besides  and  above  the  scutes,  there  are  horny  epidermic  scales  like 
those  in  other  Reptiles.  The  hide  is  often  used  as  leather. 

The  vertebral  column  consists  of  distinct  cervical,  dorsal,  lumbar, 


FIG.  201. — Lower  surface  of 
skull  of  a  young  Crocodile. 

P.mx.,  Premaxilla  ;  mx.,  maxilla  ; 
pal.,  palatine;  o.t.,  os  transversum ; 
pt.,  pterygoid  ;  /.,  jugal ;  Q.j.,  quad- 
rato-jugal ;  Q.,  quadrate;  p.n.,  pos- 
terior nares  ;  c.,  condyle. 


The  males  have 


CROCODILES,   ALLIGATORS,    G A  VIALS.  585 

sacral,  and  caudal  vertebrae,  all  procoelous  except  the  first  two  cervicals, 
the  two  sacrals,  and  the  first  caudal.  In  most  of  the  pre-cretaceous 
Crocodilians,  however,  the  vertebrae  were  amphicoelous.  The  centra  of 
the  vertebrae  are  united  by  fibro-cartilages,  and  the  sutures  between  the 
neural  arch  and  the  centrum  persist  at  least  for  a  long  time.  Chevron 
bones  are  formed  beneath  the  centra  of  many  of  the  caudal  vertebrae. 

Many  of  the  ribs  have  two  heads — capitulum  and  tubercle — by  which 
they  articulate  with  the  vertebrce.     From  seven  to  nine  of  the  anterior 


at 


"  y       ; 

FIG.  202. — Crocodile's  skull  from  dorsal  surface. 

^p.mx.,  Pre-maxilla ;  mx.,  maxilla;  /.,  lachrymal ;  pr.f.,  pre-frontal ; 
/•>  jugal ;  p.f.,  post-frontal;  q.j.,  quadrato-jugal  ;  g.,  quadrate;  sq., 
squamosal ;  pa.,  parietal  ;  e.pt.,  epi-pterygoid  ;  /.,  frontal  ;  pt.,  ptery- 
goid  (on  lower  surface);  o.t.,  os  transversum  (on  lower  surface);  »., 
nasal. 

dorsal  ribs  are  connected  with  the  sternum  by  sternal  ribs,  and  from 
several  of  these  anterior  ribs  cartilaginous  or  partially  ossified  uncinate 
processes  project  backwards.  The  so-cal  led  abdominal  ribs  have  nothing 
to  do  with  ribs,  but  are  ossifications  in  the  fibrous  tissue  which  lies 


586 


REPTILES. 


They  form  seven  transverse 


under  the  skin  and  above  the  muscles, 
series,  each  composed  of  several  ossicles. 

As  to  the  skull,  there  is  an  interorbital  septum  with  large  alisphenoids  ; 
the  presphenoid  and  orbitosphenoids  are  at  best  incompletely  ossified  ; 
all  the  bones  are  firmly  united  by 
persistent  sutures  ;  both  upper 
and  lower  temporal  arcades  are 
completely  ossified ;  the  maxillae, 
the  palatines,  and  the  pterygoid, 
meet  in  the  middle  line  of  the 
roof  of  the  mouth,  covering  the 
vomers,  and  determining  the 
position  of  the  posterior  nares — 
at  the  very  back  of  the  mouth  ; 
an  os  transversum  extends  be- 
tween the  maxilla  and  the  junc- 
tion of  palatine  and  pterygoid  ; 
an  epi-pterygoid  runs  down  from 
post-frontal  to  os  transversum  ; 
the  quadrate  is  large  and  immov- 
able ;  there  are  large  parotic 
processes  ;  the  tympanic  cavity 
is  completely  bounded  by  bone  ; 
the  teeth,  which  are  borne  by 


FIG.    203.— Half  of   the 
girdle  of  a  young  Crocodile. 


pelvic 


//.,  Ilium  ;  a.f.,  acetabulum  ;  7s,, 

ischium ;  /*.,  pubis. 
premaxillae,    maxillae,   and    den- 
taries,  are  lodged  in  distinct  cavities ;   beside  and  eventually  beneath 
the  teeth  lie  reserve  "germs"  of  others. 

Each  ramus  of  the  mandible  consists,  as  in  most  Reptiles,  of  a  cartilage- 
bone — the  articular — working  on  the  quadrate,  and  five  membrane  bones 
— dentary,  splenial,  coronoid,  angular,  and  surangular. 

The  hyoid  region  is  very  simple. 

In  the  pectoral  arch  there  are  no  clavicles  nor  epicoracoids,  but 
there  is  a  so-called  interclavicle  or  episternum  ;  the  fore  limb  is  well 
though  not  strongly  developed  ;  there  are  five  digits,  webbed  and 
clawed. 

In  the  pelvic  arch,  large  ilia  are  united  to  the  strong  ribs  of  the  two 
sacral  vertebrae  ;  the  pubes  slope  forward  and  inward  and  have  a  cartila- 
ginous symphysis  ;  the  ischia  slope  backward  and  have  a  symphysis  ; 
ilia  and  ischia  form  almost  the  whole  of  the  acetabulum.  The  hind- 
limbs  bear  four  digits,  webbed  and  clawed. 

Some  of  the  Characteristics  of  the  various  organs  of  Crocodilians. 

The  Crocodilians  are  seen  to  best  advantage  in  the  water,  swimming 
by  powerful  tail  strokes.  The  limbs  are  too  weak  for  very  effective 
locomotion  on  land,  the  body  drags  on  the  ground,  and  the  animals  are 
stiff  necked.  Although  many,  especially  in  their  youth,  feed  on  fishes 
and  small  animals,  the  larger  forms  lurk  by  the  edge  of  the  water,  lying 
in  wait  for  mammals  of  considerable  size.  These  they  grasp  in  their 
extremely  powerful  jaws,  and  drown  by  holding  them  under  water.  If 
the  dead  booty  cannot  be  readily  torn,  it  is  often  buried  and  left  until  it 


CROCODILES,   ALLIGATORS,    G A  VIALS.  587 

begins  to  rot.  In  connection  with  their  way  of  feeding,  we  should  notice 
several  peculiarities  of  structure  ;  as  the  nostrils  are  at  the  upper  end  of 
the  snout,  and  the  eyes  and  ears  also  near  the  upper  surface,  the  Croco- 
dilians  can  breathe,  see,  and  hear,  while  the  body  is  altogether  immersed 
except  the  upper  surface  of  the  head  ;  as  the  nostrils  can  be  closed  by 
valves,  and  the  eyes  by  transparent  third  eyelids,  and  the  ears  by  movable 
flaps,  the  head  can  be  comfortably  immersed  ;  a  flat  tongue  is  fixed  to 
the  floor  of  the  mouth,  and  the  cavity  of  the  mouth  is  bounded  behind 
by  two  soft  transverse  membranes  which,  meeting  when  the  reptile 
is  drowning  its  prey,  prevent  water  rushing  down  the  gullet ;  the 
posterior  opening  of  the  nostrils  is  situated  at  the  very  back  of  the 
mouth,  and  when  the  booty  is  being  drowned,  the  Crocodilian  keeps 
the  tip  of  its  snout  above  water,  the  glottis  is  pushed  forward  to  meet 
the  posterior  nares,  a  complete  channel  for  the  passage  of  air  is  thus 
established,  and  respiration  can  go  on  unimpeded.  For  their  shore 
work  the  Crocodilians  prefer  the  darkness,  but  they  often  float  basking 
in  the  sun,  with  only  the  tip  of  the  snout  and  the  ridge  of  the  back 
exposed. 

Glands  with  a  secretion  which  smells  like  musk  are  usually  developed 
on  the  margin  of  the  lower  jaw,  at  the  side  of  the  cloacal  aperture,  and 
on  the  posterior  margins  of  the  dorsal  scutes.  The  musky  odour  is  very 
strong  during  the  pairing  season,  and  when  the  animals  are  attacked. 

In  connection  with  the  muscular  system,  the  presence  of  what  is  often 
called  an  incipient  diaphragm  between  the  thoracic  and  the  abdominal 
cavity  is  of  interest. 

The  brain  seems  very  small  in  relation  to  the  size  of  the  skull. 

The  eyes  are  provided  with  a  third  eyelid,  as  in  most  Reptiles,  Birds, 
and  Mammals  ;  there  are  large  lachrymal  glands,  but  there  is  no  special 
deceitfulness  about  "  crocodile's  tears." 

The  ears  open  by  horizontal  slits,  over  which  lies  a  flap  of  skin ;  three 
Eustachian  tubes — one  median  and  one  on  each  side — open  into  the 
mouth  behind  the  posterior  nares. 

The  nostrils  also  can  be  closed,  and,  as  we  have  already  noticed,  their 
internal  openings  lie  at  the  back  of  the  mouth. 

The  stomach  suggests  a  bird's  gizzard,  for  it  has  strong  muscular  walls, 
and  its  pyloric  end  is  twisted  upward  so  as  to  lie  near  the  cardiac  part. 

The  heart  is  four  chambered,  the  septum  between  the  ventricles  being 
complete  as  in  Birds  and  Mammals.  But  as  the  dorsal  aorta  is  formed 
from  the  union  of  a  left  aortic  arch  containing  venous  blood,  and  a  right 
aortic  containing  arterial  blood,  the  blood  which  is  driven  to  many  parts 
of  the  body  is  "mixed  blood,"  i.e.,  blood  partly  venous,  partly  arterial, 
with  some  of  its  red  blood  corpuscles  carrying  hcemoglobin  and  others 
oxy-hoemoglobin.  At  the  roots  of  the  two  aortic  arches  there  is  a  minute 
communication  between  them — the  foramen  Panizzse. 

Into  the  right  auricle  venous  blood  is  brought  by  the  two  superior 
venae  cavse  and  by  the  inferior  vena  cava.  The  blood  passes  through  a 
valved  aperture  into  the  right  ventricle,  and  is  driven  thence  (a)  by  the 
pulmonary  artery  to  either  lung,  or  (b)  by  the  left  aortic  arch  to  the  body. 
From  this  left  aortic  arch,  before  it  unites  with  its  fellow  on  the  right  to 
form  the  dorsal  aorta,  is  given  off  the  great  cceliac  artery.  The  anterior 
viscera  thus  receive  wholly  venous  blood  from  the  heart. 


588  REPTILES. 

The  blood  driven  to  the  lungs  is  purified  there,  and  returns  by  pul- 
monary veins  to  the  left  auricle.  Thence  it  passes  through  a  valved 
aperture  into  the  left  ventricle.  Thence  it  is  driven  into  the  right  aortic 
arch.  From  this  the  carotids  to  the  head  and  the  subclavians  to  the 
fore  limbs  are  given  off.  These  parts  of  the  body  thus  receive  wholly 
arterial  blood  from  the  heart. 

The  venous  blood  returning  from  the  posterior  regions  may  pass 
through  the  kidneys  in  a  renal  portal  system,  and  thence  into  the  inferior 
vena  cava ;  or  it  may  pass  through  the  liver  in  a  hepatic  portal  system, 
and  thence  by  hepatic  veins  into  the  inferior  vena  cava ;  or  some  of  it 
may  pass  directly  into  the  inferior  vena  cava.  The  renal  portal  veins 
arise  from  a  transverse  vessel  uniting  the  two  branches  of  the  caudal,  but 
the  latter  are  also  continued  forward  as  lateral  epigastrics  which  enter 
the  liver. 

The  temperature  of  the  blood  is  not  above  that  of  the  surrounding 
medium. 

In  regard  to  the  respiratory  system,  we  should  notice  that  the  lungs 
are  invested  by  pleural  sacs  as  is  the  case  in  Mammals. 

The  ureters  of  the  kidneys,  the  vasa  deferentia  from  the  testes  in  the 
male,  the  oviducts  from  the  ovaries  in  the  female  open  into  the  cloaca, 
which  has  a  longitudinal  opening. 

The  eggs,  which  in  size  are  like  those  of  geese,  have  a  thin  calcareous 
shell,  are  buried  in  excavated  hollows,  and,  warmed  by  the  sun,  hatch 
without  incubation. 

Of  one  species  of  crocodile  it  is  known  that  the  mother  opens  up  the 
nest  when  the  young,  ready  to  be  hatched,  are  heard  to  cry  from  within 
the  eggs.  The  mothers  take  some  care  of  the  young,  which  require  to 
be  defended  even  from  the  appetite  of  the  males. 

Crocodiles  are  relatively  sluggish,  and  fond  of  basking  passively, 
sometimes  hiding  in  the  mud  during  the  hot  season.  They  are  remark- 
able for  the  long  continuance  of  growth,  which  does  not  seem  to  have 
so  definite  a  limit  as  in  most  other  animals. 

Classification  of  Crocodilia. 

(a)  The  true  crocodiles,   of  the  genus   Crocodilus,  occur  in  Africa, 
Southern   Asia,    tropical   Australia,    Central   America,    and    the   West 
Indies. 

The  Indian  Crocodile  ( C.  porosus]  may  measure  about  eighteen  feet 
in  length,  and  even  larger  forms  have  been  recorded.  The  sacred  African 
crocodile  (C.  vulgaris)  is  still  formidably  common  in  some  of  the  fresh 
waters  of  tropical  Africa. 

The  eggs  and  the  young  are  often  eaten  by  a  mammal  called  the 
Ichneumon,  and  by  a  species  of  lizard.  The  adults  have  few  enemies 
except  man.  They  seem  to  live  in  friendly  partnership  with  little  birds 
(Phivianus  czgypticus),  which  remove  parasites  from  the  body,  and  in 
their  familiarity  almost  justify  the  account  which  Herodotus  gives  of 
their  cleaning  the  reptile's  teeth. 

(b]  The  Alligators,  of  the  genus  Alligator,  are,  with  the  exception  of 
one  Chinese  species,  confined  to  North  and  South  America.     In  North 
America  A.  mississippiensis,  in  South  America  A.  sclerops,  are  common. 


CROCODILES,    ALLIGATORS,    G A  VIALS. 


589 


(c]  The  gavials  or  gharials,  of  the  genus  Gavialis,  are  distinguished  by 
their  long  narrow  snout.  In  the  Ganges  and  its  tributaries,  G.  gangeticus, 
said  to  attain  a  length  of  twenty  feet,  is  common.  They  feed  chiefly  on 
fishes.  "Old  males  have  a  large  cartilaginous  hump  on  the  extremity 
of  the  snout,  containing  a  small  cavity  for  the  retention  of  air,  by  which 
means  these  individuals  are  enabled  to  remain  under  water  for  a  longer 
time  than  females  or  young." 


DIFFERENCES  BETWEEN  CROCODILES,  ALLIGATORS, 
AND  GAVIALS. 


ALLIGATORS. 

CROCODILES. 

GAVIALS. 

The    head    is    short    and 

Longer. 

The  snout  is  very  long. 

broad. 

First    and     fourth     lower 

The  first  bites  into  a 

First    and     fourth     lower 

teeth   bite   into  pits    in   the 

pit  ;  the  fourth  into  a 

teeth  bite  into  grooves  in  the 

upper  jaw. 
The  union  of  the  two  rami 

groove. 
Not      beyond      the 

upper  jaw. 
The  union  extends  at  least 

of  the  lower  jaw  does  not  ex- 

eighth. 

to  the  fourteenth. 

tend  beyond  the  fifth  tooth. 

The  nasal  bones  form  part 

As  in  the  alligator. 

The  nasal  bones  do  not  form 

of  the  nasal  aperture. 

part  of  the  nasal  aperture. 

The    teeth    are    very   un- 

Unequal. 

Almost  equal. 

equal. 

The  scutes  on  the  neck  are 

Sometimes  distinct, 

Continuous. 

distinct  from    those  on   the 

sometimes  continuous. 

back. 

All  American,  except  one 

Living     in     Africa, 

Living   in   India,   Borneo, 

Chinese  species. 

India,            Australia, 

N.  Australia. 

Cuba,  S.  America. 

History  of  Crocodilian*. — These  giant  reptiles  form  a  decadent  order. 
Fossil  forms  are  found  in  Triassic  strata  (e.g.,  Belodon,  Parasuchus, 
and  Stagonolepis] ;  their  remains  are  abundant  in  Jurassic  rocks.  In 
Cretaceous  strata,  crocodilians  with  proccelous  vertebras  first  occur,  the 
pre-Cretaceous  forms  having  centra  of  the  amphiccelous  type.  Huxley 
has  worked  out  an  "almost  unbroken"  series  from  the  ancient  Triassic 
crocodilians  down  to  those  of  to-day. 

Development  of  Reptiles. 

As  the  development  of  Birds  will  be  discussed  in  the  next  chapter,  a 
few  notes  on  that  of  Reptiles,  which  is  in  many  respects  similar,  will 
be  sufficient. 

The  ovum  contains  much  yolk,  at  one  pole  of  which  there  is  a  small 
quantity  of  formative  protoplasm  surrounding  the  germinal  vesicle. 
Formation  of  polar  globules  has  not  been  observed.  The  segmentation 
is  necessarily  meroblastic  and  discoidal,  as  in  Birds. 

The  segmented  area  or  blastoderm,  originally  at  one  pole,  gradually 
grows  round  the  yolk.  The  central  region  of  the  dorsal  blastoderm 
is  separated  from  the  yolk  by  a  shallow  space  filled  with  fluid,  and 
is  clearer  than  the  rest  of  the  blastoderm.  In  this  central  region  or  area 
pellucida,  the  germinal  layers  and  subsequently  the  parts  of  the  embryo 
are  established,  while  the  rest  of  the  blastoderm — the  area  opaca — 


590 


REPTILES. 


simply  forms  a  sac  round  the  yolk.  One  of  the  first  signs  of  develop- 
ment is  the  appearance  of  a  thickened  band  of  cells  extending  forward 
in  the  middle  line  from  the  posterior  margin  of  the  area  pellucida.  This 
band  is  called  the  primitive  streak,  and  seems  to  represent  a  fusion  of 
the  two  edges  of  the  blastoderm  behind  the  future  embryonic  region. 
The  embryo  develops  in  front  of  the  primitive  streak,  and  one  of  the 
first  signs  of  its  development  is  the  formation  of  a  primitive  or  medullary 
groove  in  a  line  with  the  primitive  streak.  As  development  proceeds, 
folds  appear  around  the  embryo,  con- 
stricting it  off  from  the  subjacent  yolk 
or  yolk  sac. 

Fatal  Membranes. — It  is  with  Rep- 
tiles that  the  series  of  higher  Verte- 
brates or  Amniota  begins.  It  is  here 
that  the  fcetal  membranes  known  as 
amnion  and  allantois  are  first  formed. 
Let  us  consider  their  development. 

(a)  The  Amnion. — At  an  early  stage 
in  development,  the  head  end  of  the 
embryo  seems  to  sink  into  the  subjacent 
yolk.  A  semilunar  fold  of  the  blasto- 
derm, including  epiblast  and  mesoblast, 
rises  up  in  front.  Similar  folds  appear 
laterally.  All  the  folds  increase  in  size, 
arch  upwards,  and  unite  above,  forming 
a  dome  over  the  embryo.  Each  of 
these  folds  is  double ;  the  inner  limbs 
unite  to  form  "the  true  amnion;"  the 
outer  limbs  unite  to  form  "  the  false 
amnion,"  "  serous  membrane,"  or  sub- 
zonal  membrane.  The  cavity  bounded 
by  the  true  amnion  contains  an  amniotic 
fluid  bathing  the  outer  surface  of  the 
embryo ;  the  cavity  between  the  true 
and  the  false  amnion  is  lined  by  meso- 
blast, and  is  continuous  with  the  pleuro- 

FIG.  204. — Origin  of  Amnion  and 
Allantois.     (After  BALFOUR.) 

1.  Rise  of  amniotic  folds  (a.f.)  ;  around  em- 
bryo (e)  ;   p-p- 1    pleuro-peritoneal   cavity ;   y. , 
yolk. 

2.  Further  growth   of  amniotic  folds  (ft.f^) 
over  embryo  and  around  yolk. 

3.  Fusion  of  amniotic  folds  above  embryo 
«./.,   amnion  proper;   s.z.m.,  sub-zonal  mem- 
brane ;  JJ/.A-.,  yojk  sac. 

4.  Outgrowth    of  allantois  («/.)  ;    amniotic 
cavity  (a.c.)  ',  h.,  head  end  ;  /.,  tail  end. 

5.  Complete  enclosure  and  reduction  of  yolk 

sac  (y~s.) ;  s.z.m.,  sub-zonal  membrane;  ct.p.,  \~T~~' 

amnion    proper ;    «/.,    allantois :    g.>    gut    of  dl    5 

embryo. 


z.m 


CROCODILES,    ALLIGATORS,    G A  VIALS.  591 

peritoneal  or  body  cavity  of  the  embryo.  The  amniotic  folds  extend 
not  only  over  the  embryo,  but  ventrally  around  the  yolk  sac  which  they 
completely  invest. 

(b)  The  Allantois. — While  the  amnion  is  being  formed,  a  sac  grows 
out  from  the  hind  end  of  the  embryonic  gut.  This  is  the  ajlantois,  lined 
internally  by  hypoblast,  externally  by  mesoblast.  It  rapidly  insinuates 
itself  between  the  two  limbs  of  the  amnion,  eventually  surrounding  both 
embryo  and  yolk  sac. 

The  amnion  is  a  protective  membrane,  forming  a  kind  of  water  bag 
around  the  embryo.  It  may  be  due  in  part  to  the  embryo  sinking 
into  the  yolk  sac  by  its  own  weight. 

The  allantoic  sac  is  vascular,  and  has  respiratory  and  perhaps  also 
some  yolk  absorbing  functions.  It  seems  to  be  homologous  with  the 
outgrowth  which  forms  the  cloacal  bladder  of  Amphibians  ;  it  has  been 
called  "a  precociously  developed  urinary  bladder." 

Before  the  amnion  is  developed,  the  heavy  head  end  of  the  embryo 
has  already  sunk  into  a  depression  (in  Lizards,  Chelonians,  Birds  (?) 
and  Mammals),  and  is  surrounded  by  a  modification  of  the  head  fold 
termed  the  pro-amnion.  This  does  not  include  any  mesoblast,  and  is 
afterwards  replaced  by  the  amnion. 

Some  Peculiarities  in  Chelonians.  —  Mitsukuri  has  recently  investigated 
the  development  of  the  foetal  membranes  in  Chelonians  (Clemmys  and 
Trionyx],  and  has  demonstrated  some  interesting  peculiarities. 

The  amnion  has  at  first  the  nature  of  a  pro-amnion,  consisting  in  the 
region  of  the  sunken  head  of  epiblast  and  hypoblast,  and  in  the  dorsal 
region  of  epiblast  alone,  being  as  yet  non-mesoblastic.  The  ccelomic 
cavities  of  the  amniotic  folds  are  not  united  with  each  other  dorsally  in  the 
usual  fashion  ;  a  connection  between  the  "  true  amnion  "  and  the  "  serous 
membrane"  separates  the  cavities  to  the  very  end  of  the  development. 
The  anterior  and  lateral  amniotic  folds  are  continued  backward  beyond 
the  posterior  end  of  the  embryo,  as  a  long  tube  connecting  the  amniotic 
sac  with  the  exterior.  This  tube  perhaps  conveys  nutritive  matter  from 
the  albumen  into  the  amniotic  cavity.  In  Clemmys,  a  process  from 
the  foetal  membranes  projects  into  a  small  persistent  mass  of  albumen, 
and  seems  to  absorb  nutritive  particles. 

Hints  of  a  Placenta  before  Mammals. — As  will  be  explained  after- 
wards, the  placenta,  which  characterises  most  Mammals,  is  an  organic 
connection  between  mother  and  unborn  young.  Its  embryonic  part  is 
chiefly  formed  from  a  union  of  the  serous  or  sub-zonal  membrane  and 
the  allantois,  but  in  some  cases  the  yolk  sac  and  the  sub-zonal  membrane 
form  a  provisional  placenta.  The  placenta  establishes  a  vital  union 
between  the  embryo  and  the  mother. 

Now  it  is  interesting  to  notice,  that  there  are  some  hints  of  placenta 
connection  in  animals  which  are  much  lower  than  Mammals.  In 
some  species  of  Mustelus  and  Carcharids,  there  is  a  connection 
between  the  yolk  sac  and  the  wall  of  the  uterus ;  in  the  Teleostean 
Anabteps,  the  yolk  sac  has  small  absorbing  outgrowths  or  villi ;  in 
Trachydosaurus  and  Cyclodus  among  Lizards,  the  vascular  yolk  sac  is 
separated  from  the  wall  of  the  uterus  "  only  by  the  porous  and  friable 
rudiment  of  the  egg  shell ; "  in  Clemmys  among  Chelonians,  there  is, 
as  above  described,  an  absorbing  protrusion  of  the  foetal  membranes. 


592  REPTILES. 

In  Birds  also  small  villi  of  the  yolk  sac  absorb  yolk,  and  others  on  the 
allantois  absorb  albumen.     (See  A.  C.  Haddon's  Embryology.} 

Extinct  Reptiles. 

The  first  known  occurrence  of  fossil  Reptiles  is  in 
Permian  strata ;  in  the  Trias  most  of  the  orders  or  classes 
are  represented ;  while  the  "  golden  age  "  of  the  group  was 
undoubtedly  during  Jurassic  and  Cretaceous  times. 

Some  of  the  modern  Reptiles  are  linked  by  a  series  of 
fine  gradations  to  very  ancient  progenitors,  the  Crocodiles 
of  to-day  lead  back  to  those  of  the  Trias,  the  New  Zealand 
Hatteria  to  the  Triassic  Rhynchocephalia,  but  we  have  no 
example  of  a  Reptilian  genus  which  has  persisted  from  age 
to  age  as  Ceratodus  has  done  among  fishes.  It  follows 
naturally  from  this  linking  of  the  present  with  the  past,  that 
among  the  fossil  forms  we  find  "  generalised  "  types,  types 
which  exhibit  affinities  with  groups,  which  in  our  classifica- 
tion of  recent  forms  may  be  very  widely  separated.  It  is 
indeed,  as  has  been  said,  only  because  of  our  ignorance  of 
their  past  history  that  we  are  able  to  classify  living  genera 
into  separate  orders  at  all. 

The  following  types  of  extinct  reptiles  seem  to  have  en- 
tirely disappeared : — 

Anontodontia. — Lizard-like  animals  with  limbs  adapted  for  walking, 
found  in  the  Permian  and  Trias.  The  most  generalised  forms  approach 
the  Labyrinthodont  Amphibians  very  closely,  especially  in  the  characters 
of  the  skull  and  pelvis.  They,  however,  also  exhibit  affinities  with  the 
Monotreme  Mammals.  In  the  more  specialised  types  the  nature  of  the 
dentition  is  the  most  interesting  feature.  In  Galcsaurus,  for  example, 
which  is  a  carnivorous  form,  the  teeth  are  arranged  in  three  series,  the 
anterior  series  (incisors)  are  separated  by  a  tusk-like  tooth  (canine)  from 
a  lateral  series  of  cheek  teeth  (molars).  It  is  hardly  necessary  to  insist 
upon  the  close  affinity  between  such  a  dentition  and  that  of  carnivorous 
Marsupials,  and  we  cannot  doubt  that  the  Anomodohtia  are  in  some  way 
related  to  Mammals. 

Examples  : — Pariasatirus,  Galesatirus,  Dicynodon. 

Sauropterygia. — Reptiles  represented  from  the  Trias  to  the  Chalk, 
without  exoskeleton,  usually  with  a  long  neck  and  short  tail.  The 
limbs  vary  ;  in  the  earlier,  more  generalised,  forms  they  are  adapted  for 
walking  on  land,  but  in  the  more  specialised  types  they  are  modified 
into  powerful  paddles,  like  those  of  Chelonia.  The  nearest  affinities  are 
with  the  Chelonia.  Notosaurtis  had  limbs  adapted  for  progression  on 
land  ;  Plesiosaurus  and  Pliosaurus  were  carnivorous  forms  adapted  to 
an  aquatic  life.  Plesiosaurus  had  a  very  long  neck,  and  sometimes 
attained  a  length  of  40  feet.  In  Pliosaurus  the  neck  was  much  shorter, 


CROCODILES,   ALLIGATORS,    G A  VIALS.  593 

while  the  head  was  very  large.  In  both,  the  limbs  form  powerful 
elongated  paddles,  with  apparently  no  trace  of  nails. 

Ichthyopterygia. — Large  marine  carnivorous  Reptiles,  represented 
from  the  Trias  to  the  Chalk,  with  tapering  body  like  that  of  a  shark, 
large  dorsal  and  caudal  fins,  and  two  pairs  of  paddle-like  limbs.  There 
is  no  exoskeleton.  The  length  of  the  body  is  sometimes  30  to  40  feet. 
In  the  paddle  the  number  of  digits  is  often  more  than  five,  and  the 
phalanges  of  each  are  often  very  numerous.  The  skull  has  a  large 
parietal  foramen,  and  shows  other  affinities  with  that  of  Sphenodon. 
Some  species  were  apparently  viviparous. 

Examples  : — Ichthyosaurus,  Ophthalmosaurus. 

Mosasauria. — These  strange  Cretaceous  Reptiles  should  probably  be 
placed  between  the  Lacertilia  and  the  Rhynchocephalia.  They  are 
specially  characterised  by  the  enormous  elongation  of  the  body,  which 
sometimes  reached  a  length  of  75  to  80  feet.  The  skull  is  like  that  of 
the  Monitor  among  the  lizards,  but  according  to  Cope  it  also  presents 
affinities  with  snakes.  The  body  is  snake-like,  but  there  are  two  well- 


FIG.  205. — Comparison  of  pelvic  girdles  of  Cassowary 
(to  left)  ;  Iguanodon,  an  extinct  Reptile  (in  centre)  ; 
Crocodile  (to  right). 

//.,  Ilium  ;  Is.,  ischium  ;  P.,  pubis. 

developed  pairs  of  limbs,  forming  swimming  paddles.  All  were  car- 
nivorous and  marine  ;  the  distribution  was  cosmopolitan. 

Mosasatirus,  Clidastes,  Liodon. 

Dinosauria. — Terrestrial  Reptiles,  ranging  from  the  Trias  to  the 
Chalk,  often  very  large,  and,  like  Marsupials,  specialised  in  various 
directions.  They  exhibit  many  points  of  resemblance  to  Crocodiles  on 
the  one  side  and  to  Birds  on  the  other.  Brontosaurus,  a  gigantic, 
herbivorous  form,  nearly  sixty  feet  in  length  was  probably  amphibious. 
Atlantosaurus  was  even  larger,  the  femur  measuring  over  six  feet  in 
length.  Compsognathus,  Iguanodon,  and  Camptosaurus  are  examples 
of  the  "  bird-footed  "  herbivorous  Dinosaurs.  In  all  these  the  form 
of  the  pelvis  and  of  the  hind  limbs  presents  very  strong  affinities 
with  the  conditions  seen  in  Birds.  Compsognathtts  only  reached  a 
length  of  two  feet,  and  hopped  on  its  hind  legs  like  a  bird.  Iguanodon 
habitually  walked  on  its  hind  limbs,  and,  like  several  others,  had  hollow 
bones  ;  it  reached  a  height  of  fifteen  feet.  Of  the  carnivorous  Dinosaurs, 
38 


594  REPTILES. 

Megalosaurus  is  a  good  type.  The  pelvis  has  a  Crocodilian  aspect, 
for  the  pubes  slope  forwards  instead  of  backwards  as  in  Birds  and 
Iguanodon,  &c.  The  limbs  were  furnished  with  powerful  claws,  and 
the  teeth  show  much  specialisation.  Stcgosaurus  was  furnished  with 
heavy  armour  of  plates  and  spines.  Triceratops  had  three  horns  on  its 
enormous  head.  The  point  of  greatest  interest  about  the  Dinosaurs 
is  the  resemblance  to  Birds.  This  was  first  insisted  on  by  Huxley, 
and  since  then  it  has  been  generally  held  that  Birds  have  diverged 
from  a  Dinosaur  stock.  It  is,  however,  fair  to  notice  that  by  some 
these  resemblances  have  been  declared  to  be  unimportant,  while  the 
points  of  resemblance  between  Birds  and  the  next  order  of  Reptiles 
are  much  dwelt  upon. 

Ornithosauria. — Flying  Reptiles,  represented  from  the  lower  Jurassic 
to  the  Upper  Chalk,  exhibiting  many  points  of  resemblance  to  Carinate 
Birds,  but  still  distinctly  Reptilian  in  type.  An  expansion  of  the  skin 
seems  to  have  been  stretched  on  the  much  elongated  outermost  ringer, 
and  to  have  extended  backwards  to  the  hind  legs  and  the  tail.  The 
long  bones  contained  air-sacs  as  in  many  Birds.  The  sternum  is  keeled, 
and  teeth  are  often  present  on  both  jaws.  Some  are  said  to  have  had 
an  expanse  of  wing  of  nearly  twenty-five  feet,  but  others  were  no 
larger  than  sparrows.  It  is  a  question  how  far  the  resemblances  ot 
these  forms  to  Birds  are  a  consequence  of  similar  habits,  and  how  far 
they  can  be  regarded  as  indicating  true  affinities. 

Examples  : — Pterodactylus,  Rhamphorhynchus,  Pteranodon . 

Relationships. 

While  it  is  still  rash  to  venture  on  general  conclusions, 
this  much  seems  clear  that  the  Reptiles,  in  their  widest 
sense,  form  a  central  assemblage  among  Vertebrates.  As 
we  have  noted  above,  some  of  the  extinct  forms  exhibit 
affinities  with  Amphibians,  others  with  Birds,  others  again 
with  Mammals.  Though  we  cannot  with  certainty  point  to 
any  of  the  extinct  types  as  directly  ancestral  to  either  Birds 
or  Mammals,  it  seems  likely  that  the  ancestors  of  both 
were  derived  from  the  plastic  Saurian  stock. 


CHAPTER     XXV 

CLASS    AVES.       BIRDS. 

I.  Sub-class.     ARCH^ORNITHES  (or  Saururae)  extinct  Archaopteryx. 
II.  Sub-class.     NEORNITHES. 

1.  Division,  Ratitse.     "  Running  Birds. "    Ostrich,  &c. 

2.  Division,  Odontolcse.     Extinct  toothed  birds,  ^T^- 

perorniS)  &c. 

3.  Division,  Carinatae.     "Flying   birds"   with  keeled 

sternum. 

BIRDS  share  with  Mammals  the  rank  of  the  highest  Verte- 
brates. Their  muscles  and  skeleton,  heart  and  lungs — 
indeed,  most  of  their  structural  arrangements — are  not  less 
differentiated  than  those  of  Mammals.  Nor  are  they  in- 
ferior in  integration.  The  body  temperature,  exceeding 
that  of  all  other  animals,  is  a  physiological  index  to  their 
rapid  metabolism,  to  their  intense  activity.  Most  Mammals 
show  a  higher  degree  of  brain  development,  and  a  closer 
organic  connection  between  mother  and  offspring,  but  the 
truth  is  that  the  two  classes  represent  markedly  divergent 
lines  of  progress. 

Life  having  begun  in  the  waters,  in  all  likelihood  not  far 
from  the  sea-shore,  slowly  gained  possession  of  the  dry  land 
and  then  of  the  air.  Insects  among  the  lower  animals,  and 
Birds  among  the  higher,  are  pre-eminently  the  creatures  of 
the  air ;  intensely  vivacious,  typically  beautiful  in  form  and 
colour,  lovely  and  delightful  in  their  ways. 

In  the  Birds  we  observe  a  marked  increase  of  emotional 
life,  so  that  their  affection  for  their  mates,  their  care  of  their 
young,  the  joyousness  of  their  mood,  often  bursting  forth  in 
song,  have  become  proverbial  among  us.  With  their  power 
of  flight  they  are  emblems  of  freedom. 


596  BIRDS. 

GENERAL  CHARACTERS.  —  The  fore-limbs  are  generally 
modified  as  wings  capable  of  flight ;  the  neck  is  long,  and  the 
tail  is  short  except  in  the  extinct  Saurura. 

The  epidermic  exoskeleton  is  represented  by  feathers,  with 
sometimes  a  few  scales  ;  there  are  no  scutes. 

Almost  the  only  skin  gland  is  an  oil  or  preen  gland  at  the 
root  of  the  tail. 

The  pectoral  muscles  used  in  flight  are  generally  large  ;  in 
many  there  is  a  muscular  gizzard ;  the  diaphragm  is  only 
hinted  at. 

In  the  brain,  the  predominance  of  the  basal  parts  of  cerebrum 
and  cerebellum  has  resulted  in  displacing  the  optic  lobes  to  the 
sides. 

The  nostrils  are  often  overhung  by  a  sensitive  cere  ;  there  is 
no  external  ear ;  the  connection  between  tympanum  and 
inner  ear  is  by  means  of  a  columella;  the  eyeball  is  strengthened 
by  sclerotic  ossicles,  there  is  a  well-developed  third  eyelid  and 
a  large  nutritive  pecten. 

There  are  no  epiphyses  in  connection  with  the  bones,  many 
of  which  contain  prolongations  of  the  air  sacs  connected  with 
the  lungs,  and  are  in  the  adult  without  marrow.  The  curva- 
ture of  the  vertebral  centra,  viewed  from  in  front,  is  concave 
from  side  to  side,  and  convex  from  above  downwards.  The 
cervical  vertebra  have  small  ribs.  A  large  number  of  vertebra 
(one  to  three  dorsals,  all  the  lumbars,  and  some  caudals)  fuse 
with  the  two  or  three  true  sacrals.  The  terminal  vertebra 
fuse  in  a  ploughshare  bone. 

Most  of  the  bones  of  the  skull  fuse,  the  sutures  being  obliter- 
ated. Only  the  lower  jaw,  the  quadrate,  the  columella,  and 
hyoid  are  ahvays  movable,  but  the  pterygoids  usually  articulate 
freely  with  the  basi-sphenoid,  the  lachrymals  may  remain  free, 
and  there  may  be  a  joint  in  the  beak  at  the  end  of  the  pre- 
maxilla.  There  is  but  one  condyle.  A  membrane  bone  called 
the  bast-temporal  covers  the  basi-sphenoid.  There  is  an  inter- 
orbital  septum  formed  from  presphenoid  and  mesethmoid.  The 
otic  bones  fuse  with  adjacent  bones  and  with  one  another 
about  the  same  time.  In  modern  birds  there  are  no  teeth,  but 
the  jaws  are  covered  by  horny  sheaths.  The  premaxillce  are 
large,  and  form  most  of  the  beak.  The  lower  jaw  consists  on 
each  side  of  five  membrane  bones  and  a  cartilage  bone — the 
articular — which  works  on  the  quadrate.  Many  of  the  skull 


GENERAL    CHARACTERS.  597 

bones  have  a  spongy  texture,  due  to  cavities  filled  with  air  from 
the  nasal  and  Eustachian  tubes. 

There  is  a  well-developed  sternum,  generally  with  a  keel,  to 
which  the  pectoral  muscles  are  in  part  attached.  The  strong 
coracoids  reach  and  articulate  ivith  the  sternum.  In  flying 
birds,  the  clavicles  are  well  developed,  and  are  usually  con- 
nected by  an  inter  clavicle,  which  is  often  fused  to  the  top  of  the 
breast  bone.  The  fore-limb  has  not  more  than  three  digits, 
the  three  metacarpals  are  fused  (except  in  Archseopteryx), 


P'iG.  206. — Position  of  Organs  in  a  Bird.     (After 
SELENKA.) 

«.,  Nostrils ;  tr.,  trachea;  cr.,  crop  ;  ^.,  heart ;  st.,  sternum  ;  pr., 
proventriculus  ;  £-.,  gizzard  ;  c.,  caeca  ;  /.,  pygostyle  ;  pv.,  pelvis ;  k., 
kidney ;  /.,  lung. 

and  there  are  only  two  separate  carpals,  the  others  fusing 
with  the  metacarpals,  and  thus  forming  a  carpo-metacarpus. 
The  metacarpals  and  digits  bear  the  primary  feathers  or 
quills. 

The  ilia  of  the  pelvis  are  firmly  fused  to  the  complex  sacrum  ; 
the  acetabulum  is  incompletely  ossified  ;  the  pub es  (or  post-pubic 


598 


BIRDS. 


processes)  are  directed  backwards  parallel  to  the  ischia.  There 
is  no  pubic  symphysis  except  in  the  African  ostrich  (Struthio), 
and  no  ischiac  symphysis  except  in  the  American  ostrich  (Rhea). 
In  the  hind-limb,  the  fibula  is  incomplete  and  united  to  the 
tibia  ;  there  are  no  free  tar  sal bones,  half  of  them  being  united 
to  the  distal  end  of  the  tibia  (which  is  therefore  called  a  tibio- 
tarsus),  the  others  being  united  to  the  proximal  end  of  three 
united  metatarsals  (which  thus  form  a  tarso-metatarsus).  In 
other  words,  the  ankle  joint  is  intertarsaL  The  maximum 
number  of  toes  is  four,  of  which  the  first  is  the  hallux,  and  if 
there  be  four,  the  metatarsal  of  the  hallux  is  free  from  the 
other  three. 

In  regard  to  the  alimentary  system,  the  absence  of  teeth,  the 
frequent  occurrence  of  a  crop  and  a  gizzard,  the  usual  short- 
ness of  the  large  intestine,  the 
presence  of  a  cloaca,  may  be  noted. 

The  heart  is  four-chambered ; 
the  single  aortic  arch  curves  to  the 
right  side ;  only  the  pulmonary 
artery  rises  from  the  right  ven- 
tricle ;  the  two  valves  between  the 
right  auricle  and  the  right  ven- 
tricle are  in  part  muscular ;  the 
red  blood  corpuscles  are  oval  and 
nucleated  ;  the  temperature  of  the 
body  is  from  2° -14  F.  higher  than 
that  of  Mammals. 

The  lungs  are  fixed  to  the 
dorsal  wall  of  the  thorax ;  the 
bronchial  tubes  expand  in  irre- 
gular branches  in  the  lungs ;  the 
ends  of  some  of  these  branches  are 
continued  into  surrounding  air 
sacs  ;  and  these  are  continued  into  air  spaces  in  the  bones. 
The  trachea  is  supported  by  bony  rings,  and  has  a  voiceless 
larynx  at  its  upper  end,  and  a  syrinx  or  song-box  (with  vocal 
cords)  at  the  origin  of  the  bronchi. 

The  kidneys  are  three-lobed,  and  lie  embedded  in  the  pelvis  ; 
the  ureters  open  into  the  cloaca  ;  there  is  no  bladder  ;  the  urine 
is  semi-solid,  and  consists  chiefly  of  urates. 

The  testes  lie  beside  the  kidneys ;  the  vasa  deferentia  run 


FlG.  207. — Diagrammatic 
section  of  young  Bird.  (After 
GADOW.) 

n.,  Spinal  cord;  ?/.,  vertebra 
r.,  rib;  L.,  liver;  G.,  gut',som 
(dotted),  somatic  layer  of  meso 
blast  ;  spl.  (dotted),  splanchnic 
layer  of  mesoblast  ;  ao. ,  aorta 
7?.,  reproductive  organ  ;  K. 
kidney. 


THE  PIGEON  AS  A    TYPE   OF  BIRDS.  599 

beside  the  ureters,  and  open  into  the  middle  region  of  the 
cloaca.  The  right  ovary  atrophies,  the  right  oviduct  is  rudi- 
mentary. 

The  eggs  have  much  yolk  and  hard  calcareous  shells.  The 
segmentation  is  meroblastic  and  discoidal.  The  allantois  is 
chiefly  respiratory,  though  it  may  also  help  in  absorbing  the 
nutritive  substance  of  the  egg. 


The  Pigeon  (Columba)  considered  as  a  type  of  Birds. 

The  varieties  of  domesticated  pigeon  with  which  we  are 
familiar,  are  all  descended  from  the  rock -dove,  Columba 
livia,  and  afford  vivid  illustrations  of  variation,  and  of  the 
results  of  artificial  selection.  Certain  variations,  e.g.,  in 
beak  or  tail,  crop  up,  we  know  not  how ;  and  similar  forms 
are  bred  together  until  a  new  breed  is  established.  The 
power  of  rapid  flight,  the  diet  of  seeds,  the  wooing  of  mates, 
the  feeding  of  the  young  by  both  parents,  are  well  known. 


Form  and  External  Characters. 

The  body,  well  suited  for  rapid  flight,  ceases  to  be  grace- 
ful when  stripped  of  its  feathers.  The  cere  above  the 
nostrils,  the  third  eyelid  hidden  in  the  anterior  upper  corner 
of  the  eyeball,  the  external  opening  of  the  ear  concealed  by 
the  feathers,  the  preen  gland  on  the  dorsal  surface  at  the 
root  of  the  tail,  the  cloacal  aperture,  are  external  features 
easily  recognised. 

Feathers. 

The  feathers  most  important  in  flight  are  the  remiges  of 
the  wing,  divided  into  primaries  borne  by  the  metacarpals 
and  phalanges  of  the  two  fingers,  and  secondaries  by  the 
ulna.  The  feathers  of  the  tail  help  to  guide  the  flight,  and 
are  called  rectrices.  A  distinct  tuft  of  feathers  borne  by  the 
thumb  is  called  the  bastard  wing.  Covering  the  bases  of 
the  large  feathers  are  the  coverts, — wing-coverts  and  tail- 
coverts, — while  the  contour  feathers  give  shape  to  the  whole 
body.  In  the  pigeon  there  are  no  true  down  feathers  or 
plumules,  but  among  the  ordinary  contour  feathers  or  pennse, 


6oo  BIRDS. 

there  are  little  hair-like  feathers  (filoplumes)  with  only  a  few 
terminal  barbs. 

Any  one  of  the  large  feathers  consists  of  an  axis  or  scapus  divided 
into  a  lower  hollow  portion — the  calamus  or  quill,  and  an  upper  solid 
portion— the  rachis,  which  forms  the  axis  of  the  vane.  This  vane  con- 
sists of  parallel  rows  of  lateral  barbs,  linked  to  one  another  by  barbules, 
which  may  be  joined  to  one  another  by  microscopic  booklets.  The 
quill  is  fixed  in  a  pit  or  follicle  of  the  skin,  with  which  muscle  fibres 
are  connected.  At  the  base  of  the  quill  there  is  a  little  hole — the  in- 
ferior umbilicus — through  which  a  nutritive  papilla  of  dermis  is  continued 
into  the  growing  feather.  At  the  base  of  the  vane  there  is  a  little  chink 
— the  superior  umbilicus — but  this  has  no  importance,  except  that  para- 
sites sometimes  enter  by  it.  Close  to  this  region,  however,  in  many 
birds,  a  tuft  or  branch  arises,  which  is  called  the  aftershaft.  In  the 
Emu  and  Cassowary,  the  aftershaft  is  so  long  that  each  feather  seems 
double. 

A  feather  grows  from  a  papilla  of  skin,  but  the  whole  of  the  feather 
is  really  formed  from  the  cornification  of  the  inner  layer  of  the  epidermis. 
The  papillae  rarely  occur  diffusely  on  the  skin,  but  are  usually  disposed 
along  definite  feather-tracts.  Each  papilla  consists  externally  of  epider- 
mis and  internally  of  dermis,  and  becomes  surrounded  by  a  depression 
or  moat,  which  deepens  to  form  the  feather-follicle  or  the  sac  in  which 
the  base  of  the  quill  is  sunk.  The  epidermis  has  two  layers — (a)  an 
outer  stratum  corneum,  which  in  the  developing  feather  forms  merely  a 
protective  external  sheath,  and  (b]  an  inner  stratum  Malpighii,  which 
becomes  cornified  and  forms  the  whole  feather.  The  process  by  which 
this  cylinder  of  cells  becomes  horny  is  remarkable  ;  in  the  upper  part 
ridges  are  formed,  which  separate  from  one  another  as  a  set  of  barbs,  the 
lower  part  remains  intact  as  the  quill.  When  we  pull  the  horny  sheath 
off  a  young  feather,  we  disclose  a  set  of  barbs  lying  almost  parallel  with 
one  another,  yet  slightly  divergent.  The  central  one  predominates  as 
the  rachis,  and  its  neighbours  gradually  become  the  lateral  barbs.  The 
external  sheath  falls  off;  the  core  of  dermis  is  wholly  nutritive,  and  dis- 
appears as  the  feather  ceases  to  grow. 

On  the  toes  and  on  the  base  of  the  legs  small  epidermic 
scales  occur.  The  toes  are  clawed,  and  in  some  birds  the 
same  is  true  of  the  thumb  and  first  finger.  Only  in  the 
embryos  of  the  hoatzin  (Opisthocomus)  and  of  the  ostriches 
(Struthio  and  Rhed]  is  the  second  finger  clawed.  The  beak 
is  covered  by  a  horny  sheath,  which  is  annually  moulted  in 
the  puffin.  The  dermis  is  very  thin  and  vascular,  and  is 
rich  in  tactile  nerve  endings  or  Paccinian  corpuscles,  which 
are  especially  abundant  in  the  cere.  The  only  skin  gland 
— the  preen  gland — secretes  an  oily  fluid,  with  which  the 
bird  anoints  its  feathers.  It  is  absent  in  the  ostrich,  emu, 
cassowary,  and  kiwi,  and  in  a  few  Carinate  birds. 


MUSCULAR  SYSTEM— SKELETON.  601 

Muscular  System. 

The  largest  pectoral  muscle  (pectoralis  major)  arises  from 
the  sternum  and  its  keel,  and  from  the  clavicle ;  is  inserted 
on  the  humerus;  and  depresses  the  wing.  The. smaller  but 
longer  muscle  (pectoralis  minor),  exposed  when  the  large 
one  is  reflected,  elevates  the  wing.  It  arises  from  the  keel 
and  sides  of  the  sternum,  and  is  continued  over  the  shoulder 
to  its  insertion  on  the  dorsal  surface  of  the  humerus.  Aris- 
ing chiefly  from  the  coracoid,  but  in  part  from  the  sternum, 
and  inserted  on  the  humerus  is  a  small  coraco-brachialis 
which  helps  a  little  in  raising  the  wing.  There  are  several 
yet  smaller  muscles. 

Interesting  also  is  the  mechanism  of  perching.  When  the  bird  sits 
on  its  perch,  the  toes  clasp  this  tightly.  The  flexor  tendons  of  the 
toes  are  continued  upwards  in  flexor  muscles  over  the  metatarsal  joint 
to  the  tibia,  and  are  flexed  automatically  when  the  leg  is  bent  dur- 
ing perching.  Furthermore,  an  ambiens  muscle,  inserted  on  the  front 
of  the  pubis,  is  continued  down  the  anterior  side  of  the  femur,  and  its 
tendon  bending  round  the  knee  to  the  opposite  side  of  the  tibia,  is 
inferiorly  connected  with  the  flexors  of  two  digits.  When  the  leg  is 
bent  in  sitting,  the  ambiens  tendon  is  stretched,  and  the  digits  clasp  the 
branch.  Thus  the  bird,  when  asleep,  does  not  fall  off  its  perch. 

In  connection  with  the  muscular  system,  we  may  also 
notice  that  the  walls  of  the  gizzard  consist  of  thick  muscles 
radiating  around  tendinous  discs.  Two  small  sterno- 
tracheal  muscles  ascend  from  sternum  to  trachea.  Complex 
muscles  are  associated  with  the  song-box. 

Skeleton. 

In  Birds  there  is  a  marked  tendency  to  fusion  of  bones, 
as  seen  in  the  skull,  vertebral  column,  pelvis,  and  limbs. 
In  the  pigeon  most  of  the  bones,  except  those  of  the  tail, 
fore-arm,  hand,  and  hind-limb,  contain  air  spaces. 

The  vertebral  column  is  divided  into  five  regions — cer- 
vical, thoracic,  lumbar,  sacral,  and  caudal.  In  the  pigeon  the 
mobile  neck  consists  of  fourteen  cervical  vertebrae  with 
cervical  ribs,  short  except  in  the  last  two,  which  have 
them  well-developed.  Of  the  thoracic  vertebrae,  namely 
those  whose  ribs  reach  the  sternum,  the  anterior  four  are 
fused  to  one  another,  while  the  fifth  is  fused  to  the  sacral 


6O2 


BIRDS. 


region.  The  complex  sacral  region  consists  of  the  last 
thoracic  (with  ribs),  two  or  three  lumbars,  three  or  four 
sacrals,  and  six  caudals  all  fused.  Lastly,  there  are  about 
six  free  caudals,  ending  in  a  pygostyle  or  ploughshare  bone, 
which  represents  a  fusion  of  several  vertebrae. 

When  we  examine  one  of  the  cervical  vertebrae,  we  notice 
that  the  anterior  surface  of  the  centrum  has  a  complex  and 
distinctive  curvature,  often  described  as  saddle-shaped.  It 
is  concave  from  side  to  side,  convex  from  above  downwards. 
Posteriorly  the  curvatures  are,  of  course,  the  reverse.  The 
vertebra  also  bears  expanded  transverse  processes,  perforated 
on  each  side  by  an  aperture  for  the  vertebral  artery,  anterior 
articular  processes  or  zygapophyses,  posterior  articular  pro- 


\£.0c. 


FIG.  208. — Disarticulation  of  Bird's  Skull.     (After  GADOW.) 
Membrane  bones  shaded. 

B.  Oc.,  basi-occipital ;  E.  Oc.,  ex-occipital;  S.  Oc.,  supra- 
occipital  ;  Pa.,  parietal ;  Fr.,  frontal ;  Na..,  nasal  ;  #m.,  premaxilla  ; 
M.,  maxilla;  /«.,  jugal ;  (7;.,  quadrato-jugal  ;  Qu.,  quadrate  ;  /£., 
periotic  ;  Sq.,  squampsal ;  BS.,  basi-sphenoid  ;  OS.,  orbito-sphenoid  ; 
Pr.Sph.,  pre-sphenoid  ;  vo.t  vomer  ;  z'os.,  interorbital  septum;  E., 
ethmoid;  Se.,  nasal  septum;  De.,  dentary;  Sp.,  splenial ;  An., 
angular;  Sa.,  surangular ;  Ar.,  articular;  MK.,  Meckel's 
cartilage. 

cesses,  and  a  large   neural   arch  culminating   in  a  neural 
spine. 

The  ribs,  borne  by  five  vertebrae,  have  two  heads — a 
capitulum  articulating  with  a  centrum,  a  tubercle  articulating 
with  a  transverse  process.  The  ventral  part  of  the  rib 
which  reaches  the  sternum  is  called  the  sternal  rib,  and  is 
joined  at  an  angle  to  the  dorsal  part,  which  articulates  with 
a  vertebra.  On  the  posterior  surface  of  each  of  the  first 


SKELETON. 


603 


four  ribs  there  is  an  uncinate  process,  absent  only  in  the 

horned  screamers  (Palamedeae). 

The   skull  has  a   rounded   cranial  cavity  and  a  narrow 

beak,  which  is  mostly  composed  of  the  premaxillae.     All  the 

bones  are  fixed  except  the  quadrate,  lower  jaw,  columella, 

and  hyoid.  The  surface  is 
polished,  the  sutures  are 
obliterated  very  early  in  life. 
The  back  part  of  the  skull 
is  formed  by  the  basi-occipi- 
tal,  the  two  ex-occipitals,  and 
the  supra-occipital.  These 
bound  the  foramen  magnum 
through  which  the  spinal 
cord  passes.  The  basi-occipi- 
tal  forms  most  of  the  single 
condyle  on  which  the  skull 
rotates. 

The  top  of  the  skull  is 
formed  from  the  paired  parie- 
tals,  frontals,  and  nasals,  the 
last  being  small  and  in  part 
superseded  by  the  upward 
extension  of  the  premaxillae. 
The  line  of  the  upper  jaw 
consists  of  premaxilla,  small 
maxilla,  jugal,  and  quadrato- 
jugal,  the  last  abutting  on 
the  movable  quadrate. 

Of  the  membrane  bones 
on  the  side  of  the  skull,  the 


FIG.  209.  —  Under  surface  of 
Gull's  Skull.  (From  Edinburgh 
Museum  of  Science  and  Art. ) 


lachrymal  in  front  of  the 
orbit,  and  the  squamosal 
above  the  quadrate,  are  the 
most  important. 

On  the  roof  of  the  mouth 
the  basisphenoid,  which  lies 
just  in  front  of  the  basi-occipital,  is  covered  over  by  a 
membrane  bone — the  basi-temporal.  In  front  of  this  is  a 
sharp  "  basisphenoidal  rostrum,"  or  parasphenoid,  also  a 
membrane  bone.  Articulating  with  the  quadrate  and  with 


c.,  Condyle  ;  3./.,  basi-temporal;  b.s., 
basi-sphenpidal  rostrum  ;  //.,  pterygoid  ; 
•bet.,  palatine  ;  7>. ,  vomer  ;  p^mx.,  pre- 
maxilla ;  mx.,  maxilla;  /.,  jugal;  q.j., 
quadrato-jugal  ;  q.,  quadrate. 


6o4 


BIRDS. 


the  rostrum  are  the  pterygoids,  in  front  of  these  lie  the  pala- 
tines, between  which  a  part  of  the  vomer  may  be  seen. 
The  bony  front  of  the  palate  is  formed  from  inward  exten- 


sions of  the  premaxillae  and  maxillae.  The  inter-orbital 
septum  is  formed  chiefly  from  the  mesethmoid  but  also  from 
the  presphenoid.  From  the  tympanum  to  the  inner  ear 


SKELETON.  605 

extends  the  rod-like  columella.  The  lower  jaw  originally 
consists  of  four  membrane  bones — dentary,  splenial, 
angular,  and  surangular ;  and  one  cartilage  bone — the 
articular.  The  hyoid  consists  of  a  flat  "body,"  with 
anterior  and  posterior  "  horns." 

The  pectoral  girdle  consists  of  sabre-like  scapulae  ex- 
tending dorsally  over  the  ribs,  of  stout  coracoids  sloping 
ventrally  and  articulating  with  the  sternum,  of  the  clavicles 
which  are  united  by  the  interclavicle  to  form  the  merry- 
thought. 

The  sternum  bears  a  conspicuous  keel,  is  produced 
laterally  and  posteriorly  into  two  xiphoid  processes,  and 
bears  articular  surfaces  for  the  coracoids  anteriorly,  for  the 
ribs  laterally. 

The  skeleton  of  the  wing  includes  the  stout  humerus,  the 


Isch. 

\Ac.  Ml. 

FIG.  211. — Side  view  of  pelvis  of  Cassowary. 
//.,  ilium  ;    Isck.,  ischium  ;    Pb.>  pubis  ;    Ac.,  acetabulum. 

separate  radius  and  ulna  (the  latter  the  larger),  two  free 
carpals,  a  carpo-metacarpus  of  three  metacarpals  fused  to 
one  another  and  to  some  carpal  elements,  and  three  digits 
— the  thumb  with  one  joint,  the  first  finger  with  two  joints, 
the  second  with  one. 

The  pelvic  girdle  consists  of  dorsal  ilia  fused  to  the 
complex  sacral  region,  of  ischia  sloping  backwards,  and 
of  pubes  or  post-pubic  processes  running  parallel  to  the 
ischia.  The  incomplete  ossification  of  the  socket  or 
acetabulum,  and  the  absence  of  ventral  symphyses,  are 
also  noteworthy. 

The  hind  limb  consists  of  a  short  stout  femur,  a  tibia  to 
which  the  proximal  tarsals  are  fused  (forming  a  tibio-tarsus), 


6o6 


BIRDS. 


an  incomplete  fibula  joined  to  the  tibia,  three  metatarsals 
fused  to  one  another  and  to  the  distal  tarsals  (forming  the 
tarso-metatarsus),  and,  finally,  three 
toes,  of  which  the  innermost  has  two 
phalanges,  the  next  three,  and  the 
outermost  four. 


ft 


Nervous  System. 

In  contrast  to  the  brain  of  croco- 
diles and  other  Reptiles,  the  brain 
of  the  pigeon  and  other  Birds  fills 
the  cranial  cavity.  The  cerebral 
hemispheres  are  large  and  smooth. 
Their  roof  is  thin,  their  main  mass 
consists  of  the  large  corpora  striata 
which  bulge  into  the  ventricles. 
They  meet  the  cerebellum  and 
throw  the  solid  optic  lobes  to  the 
sides.  The  olfactory  lobes  are 
very  small.  Between  the  cerebral 
hemispheres  and  the  cerebellum, 
the  pineal  body  rises  to  the  surface, 
and  a  slight  posterior  separation  of 
the  hemispheres  will  disclose  the 
region  of  the  optic  thalami.  The 
cerebellum  is  ridged  transversely 
and  divided  into  a  median  and  two 
lateral  regions.  The  curvature  of 
the  brain  is  now  well  marked  in 
the  adult,  thus  the  medulla  is 
quite  hidden  by,  and  descends  almost  vertically  from,  the 
cerebellum. 

There  are  as  usual  twelve  cranial  nerves. 

In  connection  with  the  spinal  cord,  the  brachial  plexus  of  nerves  to 
the  fore-arm,  and  the  sacral  plexus  to  the  leg,  should  be  noticed.  In 
the  lumbar  region  the  halves  of  the  cords  diverge  for  a  short  distance, 
forming  a  wide  space — the  rhomboidal  sinus — roofed  only  by  the  pia 
mater.  The  cervical  part  of  the  sympathetic  nervous  system  is  double 
on  each  side. 


FIG.  212.  —  Bones  of 
hind  leg  of  Eagle. 

/.,  Femur  :  tt.,  tibio-tarsus  ; 
/#.,  fibula  ;  «.,  ankle  joint ; 
mt.,  tarso-metatarsus  ;  mP.t 
ist  metatarsal  (free). 


SENSE   ORGANS.  607 

Sense  Organs. 

The  sense  of  smell  does  not  seem  to  be  keenly  de- 
veloped in  many  birds.  The  nostrils  are  longitudinal  slits 
overhung  by  the  swollen,  more  or  less  sensitive,  cere. 

The  sense  of  hearing  is  acute.  Externally  the  ear  is 
marked  by  an  open  tube — the  external  auditory  meatus ; 
the  aperture  of  which  is  surrounded  by  a  regular  circlet  of 
feathers.  Within  the  tube  beneath  the  surface  lies  the  drum 
or  tympanum ;  connecting  this  with  the  fenestra  ovalis  of 
the  inner  ear  is  the  well-developed  columella ;  the  tympanic 


FIG.  213. — Brain  of  Pigeon.     (After  BRONN.) 

i,  Dorsal  view;  2,  ventral  view;  3,  side  view;  olf.,  olfactory 
lobes  ;  c.,  cerebral  hemispheres  ;  o.L,  optic  lobes  ;  d>.,  cerebellum  ; 
m.o.,  medulla  oblongata;  s.c.,  spinal  cord. 

chamber  is  continued  past  the  ear  as  the  Eustachian  tube 
which  unites  with  that  of  the  opposite  side,  and  opens  into 
the  mouth  cavity  in  front  of  the  basi-sphenoid  bone.  The 
cochlea,  or  curved  protuberance  of  the  sacculus,  which  is 
incipient  in  Amphibians,  and  larger  in  Reptiles,  is  yet  more 
marked  in  Birds. 

As  to  the  eye,  its  protection  by  an  upper,  a  lower,  and  a 


6o8 


BIRDS. 


third  eyelid  or  nictitating  membrane,  is  obvious.  The  front 
of  the  sclerotic  protrudes  in  a  rounded  cone,  and  is  strength- 
ened by  a  ring  of  little  bones.  Into  the  vitreous  humour, 
the  vascular,  nutritive  pecten  projects.  Birds  have  remark- 
able powers  of  optic  accommodation. 

The  Alimentary  System. 

The  jaws  are  ensheathed  in  horn.  There  are  no  hints  of 
teeth  except  that  a  "  dental  ridge  "  (see  Mammals)  has  been 
detected  in  some  embryos.  A  narrow  tongue  lies  in  the 
floor  of  the  mouth ;  it  is  unimportant  in  the  pigeon,  but  is 
thick  in  parrots,  and  long  in  woodpeckers  and  humming- 
birds. Associated  with  the  tongue  there  are  numerous 
glands.  Into  the  mouth  there 
open  the  posterior  nares,  and 
the  united  Eustachian  tubes. 

The  gullet  expands  into  a  thin- 
walled,  slightly  bilobed,  non- 
glandular  crop,  in  which  are 
stored  the  hurriedly  swallowed 
seeds.  While  it  remains  within 
the  crop,  the  food  is  softened  a 
little.  Especially  at  the  breeding 
season  the  cells  lining  the  crop 
undergo  a  strange  degeneration, 
forming  "  pigeon's  milk,"  which 
both  males  and  females  give  to 
the  young  birds. 

From  the  crop  the  food  canal 
is  continued  into  the  glandular 
part  of  the  stomach  (the  proventriculus)  where  gastric  juice 
is  secreted. 

Beneath  the  proventriculus  is  the  gizzard,  in  which  the 
food  is  ground.  The  walls  are  very  muscular,  the  fibres 
radiating  from  two  tendinous  discs ;  the  internal  surface  is 
lined  by  a  hard  cuticle,  and  within  the  cavity  are  small 
stones  which  the  bird  has  swallowed.  The  pyloric  opening 
from  the  gizzard  into  the  duodenum,  is  very  near  the  open- 
ing from  the  proventriculus  into  the  gizzard. 

In  the  fold  of  the  long  duodenum  lies  the  pancreas  with 
three  ducts,  and  into  the  same  region  open  two  bile  ducts 


cd. 


ud.. 


•pet.. 


FIG.  214. — Diagrammatic 
section  of  cloaca  of  Male 
Bird.  (After  GADOW.) 

cd,  Upper  region  of  cloaca  into 
which  rectum  opens  ;  ud,  median 
region  into  which  ureters  (u)  and 
vas  deferens  (vd)  open  ;  pd,  pos- 
terior region  into  which  Bursa 
Fabricii  (B.F)  opens. 


VASCULAR  SYSTEM. 


609 


from  the  liver,  which  is  without  a  gall  bladder,  though  this 

is  present  in  most  birds. 

The  small  intestine  is  long;    the  large  intestine  is  very 

short ;  in  fact,  it  is  not  more  than  a  rectum  two  inches  in 

length.    At  the  junction  of  the  small  and  the  large  intestine, 

there  are  two  short  caeca.  In 
some  birds,  e.g.,  the  fowl,  these 
are  of  considerable  length. 

The  cloaca  has  three  divisions, 
an  upper  part  into  which  the 
rectum  opens,  a  median  part 
into  which  the  ureters  and  the 
genital  ducts  open,  and  a  pos- 
terior region  opening  into  which 
from  the  dorsal  surface  is  a  vas- 
cular and  glandular  sac  called 
the  bursa  Fabricii,  which  usually 
disappears  during  adolescence. 
Its  function  is  obscure. 

Vascular  System. 

The  four-chambered  heart,  the 
single  aortic  arch  bending  over 
to  the  right  side,  the  hot  blood, 
are  the  most  important  char- 
acteristics. 

The  impure  blood  having  re- 
turned by  the  venae  cavae  to  the 
right  auricle,  passes  through  the 
auriculo-ventricular  valve,  which 
has  two  muscular  flaps,  into  the 
right  ventricle,  and  is  thence 
driven  to  the  lungs.  From  the 
lungs,  the  purified  blood  returns 
to  the  left  auricle,  and  passes 
through  two  membranous  valves 
into  the  left  ventricle.  Thence 
it  is  driven  through  the  arterial  trunk  into  the  carotids, 
the  subclavians,  and  the  dorsal  aorta.  The  bases  of  the 
aortic  and  pulmonary  trunks  are  guarded  by  three  semilunar 
valves.  From  the  capillaries,  the  impure  blood  is  collected 
39 


FIG.  215. — Circulation  of 
Pigeon.  (From  PARKER, 
see  Preface.) 

ra,  Right  auricle  ;  la,  left 
auricle  ;  P,  pulmonary  artery ; 
C,  carotid  artery ;  Br.a,  brachial 
artery  ;  PC.  a,  pectoral  artery  ; 
j,  Jugular  vein  ;  Br.  v,  brachial 
vein  ;  PC.  v,  pectoral  vein  ;  ao, 
aorta  ;  h  v,  hepatic  veins  ;  i  v  c, 
inferior  vena  cava  ;  d  «,  dorsal 
aorta  ;  zY,  iliac  artery  and  vein  ; 
r p,  renal  portal  or  hypogastric  ; 
sc,  sciatic  artery  and  vein  ;  r  z>, 
renal  vein  ;  J?,  femoral  vein  ;  c  m, 
coccygeo-mesenteric  to  liver. 


6io  BIRDS. 

anteriorly  in  two  superior  venae  cavas,  and  posteriorly  in  an 
inferior  vena  cava,  composed  of  veins  from  hind  legs  and 
kidneys,  and  receiving  as  it  approaches  the  heart  the  hepatic 
veins  from  the  liver. 

The  right  auricle  of  the  heart  is  larger  than  the  left ;  the 
right  ventricle  has  thin  walls,  and  partly  surrounds  the  more 
muscular  left  ventricle. 

The  arterial  system  consists  of  the  following  vessels  : — 

(a)  The  arterial  trunk,  as  it  rises  from  the  heart,  gives  off  on  each 
side  an  innominate  artery.     Each  innominate  gives  off  a  carotid 
and  a  subclavian,  and  the  subclavian  immediately  divides  into 
a  brachial  to  the  arm  and  a  pectoral  to  the  breast  muscles. 
(b}  The  dorsal  aorta,  formed  by  a  continuation  of  the  arterial  trunk 
bending  round  on  the  right  side,  gives  off  cceliac,  mesenteric, 
renal,  femoral,  sciatic,  iliac,  and  other  arteries. 
(c]  The  pulmonary  arteries  (with  impure  blood)  from  right  ventricle 
to  lungs. 

The  venous  system  consists  of  the  following  vessels  : — 

(a)  Two   superior   venne   cavse,  each   formed   from   the   union   of 
jugulars  from  the  head,  a  brachial  from  the  arm,  and  a  pectoral 
from  the  breast. 

(b)  The  inferior  vena  cava  is  formed  from  the  junction  of  two  iliac 

veins  just  in  front  of  the  kidneys.  Each  of  these  iliacs  results 
from  the  union  of  a  femoral  from  the  leg,  an  efferent  renal 
from  the  kidney,  and  a  renal  portal,  or  hypogastric,  which 
passes  upwards  through  the  kidney.  To  understand  this  renal 
portal,  it  is  convenient  to  begin  at  the  tail.  A  short  caudal 
vein  divides  anteriorly  into  right  and  left  branches,  each  of 
these  receives  an  internal  iliac  from  the  sides  of  the  pelvic 
region ;  thus  the  hypogastric  is  formed,  which  passing  upwards 
through  the  kidney  receives  the  sciatic,  and  finally  joins  with 
the  femoral  and  with  the  renal. 

(c)  The  pulmonary  veins,  with  pure  blood,  from  lungs  to  left  auricle. 
The  hepatic  portal  system  is  as  usual : — mesenteric  veins  from  the 

intestine  combine  in  portal  veins ;  the  blood  filters  through  the  liver ; 
and  is  collected  in  hepatic  veins,  which  unite  with  the  anterior  end  of 
the  inferior  vena  cava. 

A  functional  renal  portal  system  is  represented  by  branches  which  the 
femoral  and  sciatic  give  off  to  the  kidney. 

From  the  transverse  vein  formed  between  the  two  hypogastrics  or  by 
the  division  of  the  caudal  vein,  a  coccygeo-mesenteric  arises,  which 
receives  vessels  from  the  cloaca  and  large  intestine,  and  is  continued 
along  the  mesentery  to  join  the  hepatic  portal  system. 

As  there  are  rarely  any  valves  in  the  renal  portal  veins,  the  blood 
from  the  viscera  and  hind  limbs  can  pass  freely  either  through  the  iliac 
veins  and  thence  to  the  inferior  vena  cava,  or  through  the  coccygeo- 
mesenteric  vein  to  the  hepatic  portal  system. 

The  epigastric  vein  of  the  bird  takes  blood  from  the  fat-laden  sheet 


RESPIRATORY  AND  EXCRETORY  SYSTEMS.      611 

or  great  omentum  which  covers  the  abdominal  viscera.     It  leads  not 
into  the  liver,  but  into  one  of  the  hepatic  veins. 

Associated  with  the  blood  vascular  system,  there  is  a 
lymphatic  system  with  a  few  lymphatic  glands.  The  spleen 
lies  on  the  right  side  of  the  proventriculus,  the  paired 
thyroid  lies  beside  the  origin  of  the  carotid  arteries,  and  a 
paired  thymus  is  found  in  young  birds  in  the  neck  region. 

Respiratory  System. 

The  important  facts  are,  that  there  is  as  yet  no  diaphragm, 
that  some  of  the  bronchial  branches  in  the  lungs  are  con- 
tinued into  adjacent  air  sacs,  that  expiration  is  a  more  active 
process  than  inspiration. 

The  nostrils  lie  at  the  base  of  the  beak  overlapped  by 
the  cere.  Only  in  the  kiwi  are  they  at  the  tip  of  the  beak. 

The  trachea  is  strengthened  by  bony  rings,  and  is  moved 
by  two  sterno-tracheal  muscles  from  the  sternum.  It  has  a 
larynx  at  its  anterior  end,  and  a  syrinx,  with  vocal  cords,  at 
its  lower  end,  where  the  bronchi  diverge.  The  bronchial 
tubes  branch  in  a  kind  of  tree-like  fashion  in  the  lungs. 
These  lie  attached  to  the  dorsal  wall  of  the  thorax,  indented 
by  six  of  the  ribs,  and  covered  with  pleural  membrane  on 
their  ventral  surface  only. 

Around  the  lungs,  and  connected  with  the  ends  of  some 
of  the  bronchial  tubes,  are  nine  air  sacs,  four  lateral  and 
one  median.  In  order  from  behind  forwards,  lie  posterior 
or  abdominal  sacs,  the  posterior  thoracics,  the  anterior 
thoracics,  the  cervicals,  and  the  interclavicular  in  the  middle 
line  in  front.  The  anterior  and  posterior  air  sacs  are  con- 
tinuous with  air  spaces  in  the  bones. 

Excretory  System. 

The  kidneys  are  three  lobed,  and  lie  embedded  in  the 
ilia.  They  receive  blood  from  the  dorsal  aorta  by  renal 
arteries,  and  the  filtered  blood  leaves  them  by  renal  veins 
which  unite  with  femorals  and  renal  portals  to  form  the 
iliacs,  or,  we  may  almost  say,  the  inferior  vena  cava.'  But 
the  kidney  also  receives  venous  blood  from  the  sciatic  and 
other  posterior  veins.  Thus  to  some  extent  there  is  a  renal 
portal  system,  which  does  not  occur  in  Mammals. 

The  waste  products,  consisting  for  the  most  part  of  urates,  pass  in 
semi-solid  form  down  the  ureters  into  the  median  compartment  of  the 


612 


BIRDS. 


cloaca.  The  predominance  of  urates  in  the  active  birds,  with  high 
oxidation  and  blood  very  rich  in  red  corpuscles  should  be  borne  in 
mind  when  the  physiological  relations  of  uric  acid  to  metabolism  are 
studied. 

In  front  of  each  kidney,  at  the  base  of  the  iliac  vein,  there  lies  a  supra- 
renal body. 

Reproductive  System. 

The  testes  lie  in  front  of  the  kidneys.  They  increase  in 
size  before  the  breeding  season.  The  sexual  period  in  birds 
is  often  narrowly  limited. 

The  spermatozoa  pass  from  the  testis  into  a  vas  deferens, 
which  lies  to  the  outside  of  the  corresponding  ureter.  The 


FIG.  216. — Urinogenital  organs  of  Pigeon. 

K.,  kidney;  £/.,  ureter;  cl.,  cloaca;  7\,  Testis;  v.d.,  vas  de- 
ferens ;  v.s.,  seminal  vesicle  ;  ov.,  ovary  ;  od.,  oviduct ;  r.r.od., 
rudimentary  right  oviduct ;  f.t.,  funnel  at  end  of  oviduct. 

vasa  deferentia,  slightly  convoluted  when  full  of  sperms, 
open  separately  into  the  median  compartment  of  the  cloaca. 

In  the  adult  pigeon,  and  in  most  birds,  there  is  only  one 
ovary  ;•  that  of  the  right  side  atrophies  very  early  in  life. 
The  right  oviduct  is  represented  by  a  small  rudiment  close 
to  the  cloaca. 

The  ovary  is  covered  with  follicles  containing  ova  at 
various  stages  of  ripeness.  As  these  ova  become  dilated 
with  yolk  and  otherwise  mature  they  burst  from  the  ovary, 


DEVELOPMENT  OF  THE   CHICK. 


613 


and  are  caught  by  the  dilated  end  of  the  oviduct.  The 
first  part  of  the  duct  is  narrow,  and  there  the  ova  may  be 
fertilised ;  the  second  part  is  wide  and  glandular,  secreting 
the  white  of  egg ;  in  the  third  region,  which  is  muscular  and 
glandular,  the  shell  of  the  egg  is  made. 

How  the  shell  is  made  we  do  not  precisely  know, 
but  it  seems  certain  that  it  is  not  by  the  mechanical  apposi- 
tion of  the  secretions  of  the  oviduct.  A  rudiment  is  present 
from  the  first,  and  this  rudiment  uses  materials  provided  by 
the  oviduct,  and,  although  not  cellular,  grows  by  "  intus- 
susception." 

In  sexual  union  the  cloaca  of  the  male  is  closely  apposed 
to  that  of  the  female ;  only  in  a  few  cases  (in  ducks  and 
geese,  and  in  the  Ratitae),  is  there  a  copulatory  organ. 

Development  of  the  Chick. 

The  ovarian  ovum  of  the  hen  is  a  large  spherical  body,  consisting 
largely  of  yolk,  but  exhibiting  at  one  region  a  disc  of  formative  proto- 
plasm with  a  large  nucleus  or  ger- 
minal vesicle.  The  ripening  of  the 
egg  is  accompanied  by  the  dis- 
appearance of  the  nuclear  mem- 
brane, and  also,  judging  from 
analogy,  by  the  formation  of  polar 
bodies ;  but  the  details  of  the 
process  are  still  obscure  in  the  hen. 
Either  before  it  leaves  the  ovary, 
or  in  the  upper  part  of  the  oviduct, 
the  egg  is  fertilized  by  a  spermato- 
zoon. During  the  rest  of  its  passage 
down  the  female  ducts,  it  undergoes 
two  sets  of  changes.  On  the  one 
hand  it  becomes  surrounded  by 
various  envelopes  added  to  the  deli- 
cate vitelline  membrane  with  which 
it  is  already  invested,  on  the  other  the 
process  of  segmentation  goes  on 
rapidly  in  the  formative  area.  As  a 
result  of  these  processes,  we  find  that  when  laid  the  egg  is  surrounded 
first  by  a  firm  porous  shell  of  carbonate  of  lime,  formed  in  the  lowest 
part  of  the  oviduct ;  beneath  this  there  is  a  double  shell  membrane,  the 
two  layers  of  which  are  separated  at  the  broad  end  of  the  shell  to  form 
an  air  chamber.  This  chamber  grows  larger  as  development  proceeds, 
and  is  of  some  importance,  in  connection  with  respiration,  as  an  inter- 
mediate region  between  the  embryo  and  the  external  medium.  Beneath 
the  shell  membranes  lies  the  albumen,  or  "white  of  egg,"  which  is 
secreted  by  the  thin-walled  region  of  the  oviduct ;  in  it  lie  two  spirally 
twisted  cords  or  chalazse,  produced  by  the  rotation  of  the  egg  in  the 


FIG.  217. — Diagrammatic  sec- 
tion of  Egg.  (After  ALLEN 
THOMSON.) 

g.v.,  Position  of  germinal  vesicle  ; 
a.c.,  air  chamber  ;  K.,  yolk  (alternate 
layers  of  "yellow"  and  "white"); 
ch.  chalaza. 


614  BIRDS. 

oviduct.  Within  the  enveloping  albumen  lies  the  ovum  proper,  with  its 
enormous  mass  of  yolk.  The  yolk  is  not  homogeneous,  but  consists  of 
two  substances,  known  respectively  as  white  and  yellow  yolk.  The 
white  yolk  forms  a  central  flask-shaped  mass,  and  occurs  also  as  thin 
concentric  layers  in  the  yellow  yolk. 

On  the  upper  surface  of  the  yolk,  in  whatever  position  the  egg  be 
held,  lies  the  segmented  blastoderm,  whose  exact  origin  we  must  con- 
sider more  precisely. 

In  accordance  with  the  principles  of  development,  with  which  the 
student  is  already  familiar,  yolk  is  to  be  regarded  as  an  inert  and  passive 
substance.  In  the  hen's  egg  we  have  an  increased  specialisation  along 
the  line  indicated  by  the  egg  of  the  frog.  Here  we  have  a  small  patch 
of  formative  protoplasm  at  one  pole,  and  a  large  aggregate  of  yolk  com- 
posing the  remainder  of  the  egg.  In  consequence,  the  activity  of  the 
protoplasm  is  unable  to  overcome  the  inertia  of  the  yolk,  and  segmenta- 
tion is  meroblastic  and  discoidal. 

In  the  protoplasm  of  the  egg  horizontal  and  vertical  furrows  appear  in 
rapid  succession.  The  result,  as  exhibited  by  vertical  sections,  is  to 
produce  an  upper  epithelial  layer  of  cells,  separated  by  a  small  space 
from  larger,  more  irregular  cells,  which  are  still  in  connection  with  the 
yolk  on  which  they  lie.  At  the  circular  border  of  the  germinal  disc  the 
two  sets  of  cells  are  continuous.  According  to  some  authorities,  this 
stage  represents  the  blastula,  the  upper  layer  of  cells  corresponding  to 
the  cells  of  the  animal  pole  in  the  frog,  the  lower  with  the  enormous 
mass  of  yolk  on  which  they  lie  to  the  cells  of  the  vegetative  pole,  the 
space  to  the  segmentation  cavity. 

At  the  next  stage  there  appears  at  the  future  posterior  end  a  crescent- 
shaped  groove.  In  this  region  there  is  an  ingrowth  of  cells,  which  pro- 
bably represents  a  modified  process  of  gastrulation,  and  results  in  the 
obliteration  of  the  segmentation  cavity,  and  the  formation  of  a  "  sub- 
germinal  "  cavity  or  archenteron.  The  floor  of  the  sub-germinal  cavity 
is  formed  by  the  yolk,  in  which,  by  a  process  of  supplementary  cleavage, 
yolk  nuclei  appear. 

This  condition  is  that  attained  when  the  egg  is  laid.  On  surface  view 
we  see  a  central  ill-defined  "pellucid  area."  This,  which  becomes 
much  more  distinct  during  the  early  hours  of  incubation,  is  the  area  of 
the  blastoderm  which  overlies  the  sub-germinal  cavity,  and  is  contrasted 
with  the  surrounding  "opaque  area,"  which  lies  directly  on  the  yolk. 
At  the  posterior  region  of  the  opaque  area,  as  already  noted,  there  is 
the  crescentic  groove,  where  the  outer  and  inner  layers  are  continuous. 

After  the  commencement  of  incubation,  the  blastoderm  spreads 
rapidly  over  the  yolk,  chiefly  by  the  extension  of  the  area  opaca  ;  the 
area  pellucida  meanwhile  elongates  and  becomes  oval. 

Another  important  change  which  also  occurs  in  the  early  hours  of 
incubation  is  the  conversion  of  the  transverse  crescentic  groove  into  the 
longitudinal  primitive  streak.  The  precise  meaning  of  this  change  is 
difficult  and  uncertain,  but  there  seems  no  doubt  that  the  primitive 
streak  represents  the  anterior  lip  of  the  blastopore  of  the  frog.  It  runs 
down  the  centre  of  the  area  pellucida,  and  is  marked  by  a  central  furrow, 
the  primitive  groove.  At  its  sides  two  wings  of  cells  are  obvious  ;  these 
soon  spread  out  laterally  and  anteriorly,  and  constitute  the  mesoblast. 


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FIG.  218. — Stages  in  development  of  Chick.     (After  MARSHALL.) 

1.  Segmentation,  superficial  view  of  blastoderm. 

2.  Vertical  section  of  blastoderm  ;  Ep.,  epiblast  ;  I.e.,  lower  layer  of  cells  ;  s.g.c.,  seg- 
mentation cavity  ;  y.,  yolk. 

3.  Diagrammatic  surface  view;   «./.,  area  pellucida;  a.o.,  area  opaca ;  n.p.,  neural 
groove  ;  p.s.,  primitive  streak. 

4.  Diagrammatic  surface  view  at  later  stage  ;  ap.,  area  pellucida  ;  a.o.,  area  opaca  ; 
m.s.,  mesoblast  segments  ;  p.s.,  primitive  streak. 

5.  Cross  section  ;  s.c.,  spinal  cord  ;  s.g.)  rudiment  of  spinal  ganglia  ',  JV.,  notochord  ; 
m.p.,  mesoblastic  plates  ;  A.,  aorta  ;  am.,  amnion  fold. 

6.  Embryo  ;  cb.,  cerebellum  ;  F.,  ear  ;  ff.,  heart  ;  f.L,  fore  limb  ;  k.I.,  hind  limb  ;  y.s., 
stalk  of  cut-off  yolk  sac  ;  AL,  allantois  ;  E.,  eye  ;  c. ,  cerebrum. 


6i6 


BIRDS. 


The  precise  origin  of  the  constituents  of  this  middle  layer  is  uncertain, 
but  it  is  important  to  notice  that  all  three  layers  of  the  embryo  are  con- 
nected at  the  sides  of  the  primitive  streak,  as  at  the  margin  of  the  blas- 
topore  in  the  frog. 

In  the  region  in  front  of  the  primitive  streak,  a  row  of  hypoblast 
cells  becomes  differentiated  to  form  the  notochord.  At  its  sides  the 
sheets  of  mesoblastic  cells  split  into  an  inner  or  splanchnic  layer,  and  an 
outer  or  somatic  layer.  A  little  later  the  mesoblast  divides  into  the 
segmentally  arranged  mesoblastic  somites,  lying  at  the  sides  of  the  noto- 
chord, and  the  unsegmented  lateral  plate,  whose  outer  and  inner  walls 
form  the  corresponding  boundaries  of  the  ccelome. 

At  the  time  when  the  notochord  has  appeared  internally,  the  external 
epiblast  becomes  differentiated  to  form  the  medullary  groove,  which  gives 
rise  in  the  usual  way  to  the  medullary  canal.  The  folds  at  first  diverge 
posteriorly  on  either  side  of  the  primitive  streak,  but  as  the  union  travels 
backwards,  this  is  included  in  the  medullary  canal,  and  so  disappears. 

During  the  course  of  the  second  day,  the  embryo  seems  to  sink 
further  into  the  yolk,  while  both  anteriorly  and  posteriorly  double  folds 
known  respectively  as  the  head 
and  the  tail  folds,  rise  up.  In 
the  course  of  their  development 
the  embryo  becomes  completely 
"  folded  off"  from  the  yolk.  At 
a  slightly  later  stage,  side  folds 
also  appear  ;  all  the  folds  now 
consist  of  a  double  layer  of  soma- 
topleure  covered  externally  by 
epiblast.  The  folds  meet  above 
the  back  of  the  embryo  and 
coalesce.  The  inner  layer  forms 
the  true  amnion,  the  outer  the 
false  amnion  or  subzonal  mem- 
brane. Into  the  space  between 
the  amniotic  folds,  an  outgrowth 
from  the  posterior  region  of  the 
gut,  the  allantois,  grows  out. 

Before  the  end  of  the  first 
day,  blood  vessels  begin  to  be 
developed  in  the  extra-embryo- 
nic region  of  the  blastoderm. 
These  form  the  beginning  of  the 
vitelline  vessels,  which  are  of 
great  importance  in  the  early 
stages  of  development,  and  have  probably  at  first  some  respiratory  im- 
portance. As  development  proceeds,  the  allantois  increases  greatly,  and, 
fusing  with  the  subzonal  membrane,  approaches  close  to  the  egg  shell. 
It  has  a  large  blood  supply,  and  functions  as  an  organ  of  respiration,  in 
addition  it  absorbs  the  white  of  egg,  thus  serving  as  an  organ  of  nutri- 
tion, and  also  receives  deposits  of  urates,  thus  functioning  as  an  organ 
of  excretion  in  the  narrow  sense  of  the  term. 

We  have  spoken  of  the  "folding  off"  of  the  embryo  ;  it  is  important 


FIG.  219. — Diagrammatic  section 
of  embryo  within  Egg.  (After 
KENNEL.  ) 

Z>.,  Yolk  sac  ;  da.,  gut  of  embryo  ;  «/., 
«/.,  inner  and  outer  wall  of  the  allantois  ; 
am.,  amnion  proper  ;  a.,  within  amniotic 
cavity  ;  s.,  sub-zonal  membrane. 


CLASSIFICATION  OF  BIRDS.  617 

.  to  realise  that,  as  a  result  of  this,  the  still  small  embryo  is  attached  by  a 
relatively  narrow  stalk  to  the  large  yolk  sac,  over  which  the  blastoderm 
is  now  slowly  spreading.  In  the  young  tadpole,  the  yolk  lies  heaped  up 
on  the  floor  of  the  gut,  and  causes  a  certain  amount  of  distortion.  In 
the  chick  the  amount  of  yolk  is  so  great  that  it  forms  a  hernia-like  pro- 
trusion of  the  gut,  and  only  at  a  very  late  stage  is  the  greatly  reduced 
sac  withdrawn  into  the  gut,  after  which  the  dermal  and  intestinal 
umbilical  openings  are  closed. 

With  regard  to  the  development  of  the  various  organs  of  the  body,  the 
conditions  are  much  the  same  as  for  the  frog.  The  chick  embryo  never 
exhibits  any  trace  of  gills,  but  yet  gill  clefts  perforate  the  pharynx.  The 
embryonic  organ  of  respiration  is  the  allantois,  but  yet  that  arrangement 
of  aortic  arches  by  means  of  which  in  the  tadpole  blood  is  carried  to  the 
gills,  is  repeated  here. 

About  the  twentieth  day,  the  beak  perforates  the  membranes  of  the 
air  chamber,  and,  the  air  rushing  in,  expands  the  hitherto  functionless 
lungs.  At  the  same  time  important  changes  occur  in  the  circulatory 
system,  "  the  umbilicus  becomes  completely  closed,  the  allantois  shrivels 
up,  and  the  chick,  piercing  the  broad  end  of  the  shell  with  repeated 
blows  of  its  beak,  steps  out  into  the  world." 


CLASSIFICATION    OF    BIRDS. 

I. — Sub-Class.    ARCH^EORNITHES  or  SAURUR^:.    Ancient  extinct  birds, 
with  reptilian  affinities  more  marked  than  in  any  living  forms. 

The  oldest  known  bird  is  Archczopteryx,  remains  of  which  have  been 
found  in  the  Solenhofen  slates  in  the  Upper  Oolite  (Jurassic)  of  Bavaria. 
"The  stone  is  so  fine  grained  that,  besides  the  bones  of  the  wings,  the 
furculum  or  merrythought,  the  pelvis,  the  legs,  and  the  tail,  we  have 
actually  casts  or  impressions  on  the  stone  (made  when  it  was  as  yet  only 
soft  mud)  of  all  the  feathers  of  the  wings,  and  of  the  tail." — (Nicholson 
and  Lydekker). 

This  link  between  Birds  and  Reptiles  seems  to  have  been  a  land  bird 
about  the  size  of  a  crow.  It  had  feathers,  and  the  upper  jaw  bore 
conical  teeth.  In  the  fossil  specimen  the  feathers  are  confined  to  the 
wings,  legs,  and  tail,  those  on  the  head,  neck,  and  trunk,  having 
perhaps  fallen  off.  Each  joint  of  the  long  tail  bears  a  pair  of  lateral 
feathers — a  unique  arrangement.  The  vertebrae  have  flat  ends  ;  the 
ribs  are  very  slender,  without  uncinate  processes ;  the  sternum  is  not 
known  ;  there  seem  to  have  been  abdominal  ribs.  There  are  separate 
metacarpals ;  the  first  finger  has  two  phalanges,  the  second  three,  the 
third  three  or  four,  and  all  are  clawed.  Dr.  Hurst  has  recently  main- 
tained that  the  metacarpals  of  digits  4  and  5  are  also  present.  The  tail 
was  long  like  that  of  a  lizard,  with  about  a  score  of  vertebrae,  and  no 
ploughshare  bone. 

II. — Sub-Class.    NEORNITHES. 

The  metacarpals  are  fused.  The  second  finger  is  the  longest,  and  the 
third  is  reduced.  Caudal  vertebrae  are  apparently  not  more  than  thirteen 
in  number. 


6i8 


BIRDS. 


The  extant  Neornithes  are  conveniently  distinguished  as  Ratitse  and 
Carinatse,  but  the  distinctions  do  not  seem  very  well  grounded, 
and  to  most  of  them  there  are  exceptions. 

Some  Contrasts  between  modern  Ratita  and  modern  Carinatcz. 


CARINAT.E. 


Running  Birds,  with  wings  more  or 
less  degenerate  and  unused  in 
flight,  with  a  keelless  raft-like 
breast  bone. 


The  skull  is  droma?ognathous,  i.e. , 
the  vomer  is  interposed  between 
the  palatines,  the  pterygoids, 
and  the  basi-sphenoidal  rostrum. 


The  sutures  in  the  skull  remain  for  a 
long  time  distinct. 

The  long  axes  of  the  adjacent  por- 
tions of  the  scapula  and  coracoid 
lie  almost  in  the  same  line,  or 
form  a  very  obtuse  angle,  and 
the  two  bones  are  fused. 

The  clavicles  are  small  or  absent. 

The  ilium  and  ischium  are  not  united 
behind,  except  in  adult  Rhea 
and  Dromceus.  There  is  no 
ploughshare  bone. 

The  feathers  of  the  adult  have  free 
barbs.  There  is  no  oil  gland. 


Flying  Birds,  with  wings  almost 
always  well  exercised  in  flight, 
with  a  keeled  breast  bone. 

(The  keel  is  rudimentary  in  the  New 
Zealand  parrot  Stringops,  in  the 
exterminated  Dodo  (JDidus],  and 
in  the  extinct  Aptornis — one  of 
the  rails.  The  penguins  do  not 
fly  at  all,  the  Tinamou,  the 
Hoatzin,  and  some  other  birds, 
fly  very  little. 

Except  in  the  Tinamous,  the  skull  is 
never  dromagognathous,  i.e.,  the 
vomer  is  not  fused  with  the 
neighbouring  bones  of  the  pal- 
ate, and  the  palatines  articulate 
with  thebasi-sphenoidal  rostrum. 

The  sutures  in  the  skull  almost  always 
disappear  very  early. 

The  scapula  and  coracoid  meet  at  a 
sharp  angle,  and  are  separate 
from  one  another. 


The  clavicles  are  in  most  cases  very 
well  developed. 

The  ilium  and  ischium  unite,  enclos- 
ing a  sciatic  foramen.  The  ter- 
minal caudal  vertebrae  fuse  to 
form  a  ploughshare  bone  or 
pygostyle. 

The  barbs  of  the  feathers  are  gener- 
ally united. 


i.  Division 


Running  Birds  with  raft-like  unkeeled 
breastbone. 


The  African  Ostrich  (Struthid]  is  represented  by  two  or  three  species, 
at  home  in  the  plains  and  deserts  of  Africa,  and  notable  for  their  size, 
swiftness  of  foot,  and  beauty.  There  are  but  two  toes,  the  third  and  the 
fourth,  with  stunted  nails.  There  are  no  clavicles.  The  pubes  form 
a  ventral  symphysis.  The  enormous  size  of  rectum  and  cseca  is  a 
unique  character.  The  ostrich  is  monogamous,  and  at  the  breeding 
season  the  hen  lays  its  eggs,  at  intervals  of  two  days,  in  a  hollow 
dug  out  in  the  sand  by  the  male.  The  eggs  are  incubated  by  the  parents 
alternately,  the  male  sitting  during  the  night,  but  in  the  hottest  regions 


CLASSIFICATION  OF  BIRDS.  619 

they  are  sometimes  left  during  part  of  the  day  simply  covered  by  the 
sand. 

The  American  Ostrich  (Rhed]  is  represented  by  three  species  in  the 
S.  American  Pampas.  In  the  Rhea  there  are  three  toes,  all  clawed, 
and  the  ischia  form  a  ventral  symphysis.  There  are  no  clavicles.  Only 
here  among  Ratitae  is  there  a  well-developed  syrinx.  -The  caeca  are 
large.  The  male  excavates  a  shallow  nest  in  the  ground,  and  there, 
surrounded  by  a  few  leaves  and  grasses,  the  numerous  eggs  are  usually 
laid.  It  seems  that  the  male  bird  alone  hatches  the  eggs.  Single  eggs 
are  often  laid  here  and  there  on  the  plains,  but  these  are  not  incubated. 

The  Emu  (Dromtzus]  is  represented  by  two  species  in  Australian 
deserts  and  plains.  The  fore  limb  is  greatly  reduced,  the  feathers  have 
long  aftershafts.  Nearly  related  are  the  Cassowaries  ( Casuarius]  living 
in  the  Austral- Malayan  region,  eight  species  in  the  Papuan  Islands,  one 
in  N.  E.  Australia,  and  one  in  Ceram.  They  live  in  the  forests  and 
scrub.  The  fore  limb  is  very  small,  with  the  shafts  of  the  wing  feathers 
reduced  to  spines  ;  the  ordinary  feathers  having  long  aftershafts.  On 
the  top  of  the  skull  there  is  a  horny  helmet,  covering  a  core  of  light 
spongy  bone  ;  this  protects  the  bent  head  as  the  bird  rushes  through  the 
scrub.  There  are  three  toes,  the  inner  one  with  a  long  sharp  claw — a 
formidable  weapon.  In  both  these  genera  the  clavicles  are  rudimentary 
and  the  caeca  small. 

The  Kiwi  (Apteryx]  forms  a  very  distinct  genus  of  Ratitae,  represented 
by  four  species,  restricted  to  New  Zealand.  It  is  not  larger  than  a 
hen,  and  has  simple  hair-like  or  bristle-like  feathers,  a  long  bill  and 
terminal  nostrils,  a  very  rudimentary  wing  and  no  clavicles,  and  no 
distinct  tail  feathers.  There  are  four  clawed  toes.  The  caeca  are  large, 
It  is  a  nocturnal  bird,  swift  and  noiseless  in  its  movements,  feeding  in 
great  part  on  earthworms.  The  egg  is  very  large  for  the  size  of  the 
bird. 

Among  the  extinct  forms  are  the  gigantic  Moas  (Dinornis],  which 
seem  to  have  been  exterminated  in  New  Zealand  in  comparatively 
recent  times.  The  fore  limbs  were  almost  completely  reduced,  the 
hind  legs  were  very  large,  and  some  forms  attained  a  height  of  ten  feet 
or  even  more. 

Another  recently  lost  order  of  giant  birds  is  represented  by  remains  of 
^Epyornis  found  in  Madagascar.  Some  of  these  indicate  birds  as  large 
as  ostriches,  but  eggs  have  been  found  holding  six  times  as  much  as  that 
of  an  ostrich. 

Besides  these  there  are  other  extinct  Ratitae  (Stereornithes)  such 
as  BrontomiS)  Dasornis,  and  Gastornis. 

We  may  think  of  the  Ratitae,  according  to  W.  K.  Parker,  as  "  over- 

frown,  degenerate  birds  that  were  once  on  the  right  road  for  becoming 
ying   fowl,   but    through  greediness  and  idleness  never  reached    the 
'  goal,'  went  back,  indeed,  and  lost  their  sternal  keel,  and  almost  lost 
their  unexercised  wings," 

2.  Division  ODONTOLC/E.  Extinct  toothed  birds  from  N.  American 
Cretaceous  strata,  e.g.,  Hesperornis,  somewhat  like  a  swimming 
ostrich,  with  sharp  teeth  sunk  in  a  groove,  with  vertebrae  like  those 
of  modern  birds. 


620  BIRDS. 

3.  Division  CARINATVE.     Flying  Birds  with  a  keeled  breast  bone. 

The  detailed  classification  of  Birds  is  difficult.  There  are  so  many 
that  ornithologists  have  not  yet  been  able  to  decipher  all  their  relation- 
ships. It  is  only  of  recent  years  that  anatomists  like  Huxley,  and 
embryologists  like  Parker,  have  placed  the  classification  on  a  secure 
foundation. 

For  though  the  old  classification  of  birds  into  snatchers  (Raptores), 
perchers  (Insessores),  climbers  (Scansores),  scratchers  (Rasores),  stilt- 
walkers  (Grallatores),  and  swimmers  (Natatores)  was  interesting  and 
suggestive,  yet  it  is  easy  to  understand  that  an  arrangement  of  this  sort 
may  be  misleading,  since  birds  of  entirely  different  structure  may  have 
similar  habits. 

Huxley  classified  birds  according  to  the  structure  of  their  skulls,  and 
though  this  might  seem  a  one-sided  method  of  classification,  its  natural- 
ness depends,  as  Parker  notes,  on  the  striking  fact  that  "the  structure 
of  the  skull  and  face  govern  the  whole  body,  as  it  were  ;  every  other 
part  of  the  organism  corresponds  to  what  is  observable  there." 

Huxley's  classification,  slightly  altered  by  Parker,  is  as  follows  :  — 

A.  The  vomer  broad  behind,  and  interposing  between  the  pterygoids, 
the  palatines,  and  the  basi-sphenoidal  rostrum  :  —  DROM^EO- 


The  Tinamous. 

B.  The  vomer  narrow  behind  ;  the  pterygoids  and  palatines  articulating 
largely  with  the  basisphenoidal  rostrum. 

a.  The  maxillo-palatines  free. 

1.  The  vomer  pointed  in  front  :  —  SCHIZOGNATH^:. 
The    plovers,    gulls,    penguins,   cranes,   fowls,    sand 

grouse,    pigeons,    the    hoatzin,    the    goat  suckers,   the 
humming  birds. 

2.  The  vomer  truncated  in  front  :  —  yEciTHOGNATH^:. 
The  passerines,  swifts,  and  the  hemipods. 

3.  The  vomerine  halves  permanently  distinct,  and  the 
maxillo-palatines  arrested  :  —  SAUROGNATH^E. 

The  woodpeckers. 

b.  The  maxillo-palatines  united  :  —  DESMOGNATH^E. 

The  birds  of  prey,  parrots,  cuckoos  and  kingfishers, 
ducks  and  geese,  flamingoes,  storks,  and  cormorants. 
We  give  an  outline  of  the   arrangement   of  Carinatoe  proposed   by 
Professor  Gadow  :  — 

BRIGADE  I. 

1.  Legion  Colymbomomphse. 

1.  Ichthyornithes  ;    the    extinct    Icthyornis,    with    teeth    and 

biconcave  vertebrae. 

2.  Colymbiformes,  e.g.  ,  grebe  and  dabchick. 

3.  Sphenisciformes,  e.g.,  penguins. 

4.  Procellariiformes,  e.g.,  petrels. 

2.  Legion  PelargomorphDe. 

5.  Ciconiiformes,  e.g.  ,  storks,  herons,  solan  goose,  cormorant. 
'  6.   Anseriformes,  e.g.,  ducks,  geese,  and  horned  screamers. 

7.   Falconiformes,  e.g.,  falcons,  vultures,  hawks,  eagles. 


GENERAL  LIFE.  621 

BRIGADE  II. 

3.  Legion  Alectomorphoe. 

8.  Tinamiformes,  tinamou. 

9.  Galliformes,  e.g.,  fowl,  pheasants,  Opisthocomus. 

10.  Gruiformes,  e.g.,  cranes,  rails,  bustards, 

11.  Charadriiformes,  e.g.,  plovers,  gulls,  sand  grouse,  pigeons. 

4.  Legion  Coracomorphse. 

12.  Cuculiformes,  e.g.,  cuckoos,  parrots. 

13.  Coracioformes,  e.g.,  rollers,  owls,  woodpeckers. 

14.  Passeriformes,    e.g.,     crows,     birds     of    paradise,    finches, 

thrushes,  swallows,  &c.  &c.,  in   fact  more  than  half  the 
known  birds. 

GENERAL  LIFE. 

FLIGHT. — As  birds  are  characteristically  flying  animals, 
many  of  their  peculiarities  may  be  interpreted  in  adaptation 
to  this  mode  of  motion. 

(a)  Shape  and  General  Structure  of  the  Body.  —  The 
resistance  offered  by  the  air  to  the  passage  of  a  body  through 
it  depends  in  part  on  the  shape  of  the  body,  and  the  boat- 
like  shape  of  the  bird  is  such  that  it  offers  relatively  little 
resistance.  The  attachment  of  the  wings  high  up  on  the 
thorax,  the  high  position  of  such  light  organs  as  lungs  and 
air  sacs,  the  low  position  of  the  heavy  muscles  and  digestive 
organs,  the  consequently  low  centre  of  gravity,  are  also 
structural  facts  of  some  importance.  It  must  be  remem- 
bered, however,  that  the  frictional  resistance  of  the  air  is 
slight. 

(fr)  The  Muscles  of  Flight. — The  most  important  is  that 
which  covers  the  whole  of  the  breast  bone,  the  pectoralis 
major.  It  brings  the  wing  downward,  forward,  and  back- 
ward, keeping  the  bird  up  and  carrying  it  forward.  As  it 
has  most  work  to  do,  it  is  by  far  the  largest.  Internal  to  it 
lies  a  second  muscle,  the  pectoralis  minor,  which  raises  the 
wing  for  the  next  stroke.  Besides  these  two  great  muscles, 
there  are  others  of  minor  importance,  the  deltoides  externus 
and  three  coraco-brachials,  all  of  which  help  to  raise  the 
wing.  On  an  average  these  muscles  of  flight  weigh  about 
one-sixth  of  the  whole  bird,  but  the  proportion  is  often 
much  greater,  amounting  to  nearly  one-half  in  some  pigeons. 
Buffon  noted  that  eagles  disappeared  from  sight  in  about 
three  minutes,  and  a  common  rate  of  flight  is  about  fifty 
feet  per  second. 


622 


BIRDS. 


cl. 


-sc. 


cor. 


(c)  The  Skeleton. — The  rigidity  of  the  greater  part  of  the 
backbone,  due    to    fusion  of  vertebrae,  is  of  advantage  in 
affording  a  firm  fulcrum  for  the  wing  strokes,  while  the  arched 
clavicles  (meeting  in  an  interclavicle  and  often  fused  in  front 
to  the  sternum)  and  the  strong  coracoids  (which  articulate 
with  the  sternum)  are  adapted  to  resist  the  inward  pressure 
of  the  down  stroke.     As  the  keel  of  the  breast  bone  serves 
in  part  for  the  insertion  of  the  two  chief  muscles,  its  size 
bears    some    propor- 
tion to  the    strength 

of  flight.  It  is  absent 
in  the  running  birds, 
such  as  the  ostriches, 
and  has  degenerated 
in  the  New  Zealand 
parrot  (Stringops\ 
which  has  ceased  to 
fly  and  taken  to  bur- 
rowing. 

(d)  Air  Sacs  and 
Air    Spaces.  -  -  The 
lungs  of  birds  open 
into  a  number  of  air 
sacs,   which    have    a 
larger  cubic  content 
than  the  lungs,    and 
in  many  cases  these 
air  sacs  are  continued 
in  development  into 
air  spaces  within  the 
bones  and  even  under 
the    skin.       From    a 
broken    bone    it    is 

e  to 


7/! 


FIG.  220. — Pectoral  girdle  and  sternum 
of  Swan. 


A  Part  of  carina  removed  shows  peculiar  loop  of 
trachea    (^);     ^    clavicle;    cor.,    coracoid;     sc., 
air  SaCS,  and  through    scapula;  £/.,  glenoid  cavity  for  head  of  humerus  ; 
i  r..  sternal  .parts  of  ribs. 

a     broken     bone     a 

bird  with  choked  windpipe  may  for  a  time  breathe.  The  whole 
system  of  air-containing  cavities  is  continuous,  except  in  the 
case  of  the  skull  bones,  whose  spaces  receive  air  from  the  nasal 
and  Eustachian  tubes.  In  view  of  these  facts,  it  used  to  be 
supposed  that  a  bird  with  heated  air  in  the  sacs  and  spaces 


FLIGHT  OF  BIRDS.  623 

was  comparable  to  a  balloon.  But  this  is  fallacious.  The 
air  must  indeed  lessen  the  specific  gravity  of  the  bird,  but  a 
few  mouthfuls  of  food  are  sufficient  to  counteract  the 
lightening.  Moreover,  in  many  small  birds  which  are  good 
flyers  all  the  large  bones,  or  all  except  the  humerus,  contain 
marrow,  and  are  therefore  not  "  pneumatic,"  and  the  horn- 
bill,  which  is  but  a  poor  flyer,  is  one  of  the  most  pneumatic 
of  birds.  It  is  likely  that  the  chief  importance  of  the  air 
sacs  and  air  spaces  is  in  connection  with  respiration  and 
with  the  regulation  of  the  body  temperature.  It  is  certain 
that  in  ordinary  flight  the  lightest  of  birds  has  to  keep 


FIG.  221. — Position  of  wings  in  Pigeon  at  maximum 
elevation.     (From  MAREY.) 

itself  from  falling  by  constant  effort.  The  bird  is  not  com- 
parable to  a  balloon,  but  to  a  flying  machine  ;  "  it  has  to  be 
not  a  buoyant  cork,  but  a  buoyant  bullet." 

There  is  no  motion  more  marvellous  or  more  beautiful  than  the  flight 
of  a  bird.  It  is  harmonious  with  the  bird's  true  nature.  For  there  is 
more  than  poetic  insight  in  Ruskin's  description: — "The  bird  is  little 
more  than  a  drift  of  the  air  brought  into  form  by  plumes  ;  the  air  is  in 
all  its  quills,  it  breathes  through  its  whole  frame  and  flesh,  and  glows 
with  air  in  its  flying  ;  like  a  blown  flame  it  rests  upon  the  air,  subdues 


624 


BIRDS. 


it,  surpasses  it,  outraces  it ;  is  the  air,  conscious  of  itself,  conquering 
itself,  ruling  itself. " 

Ruskin  has  compared  the  flight  of  a  bird  to  the  sailing  of  a  boat.     "  In 
a  boat,  the  air  strikes  the  sail ;  in  a  bird,  the  sail  strikes  the  air ;  in  a 


FIG.  222. — Wings  coming  down.     (From  MAREY.) 

boat,  the  force  is  lateral,  and  in  a  bird  downwards ;  and  it  has  its  sail 
on  both  sides. "  But,  as  he  says,  the  sail  of  a  boat  serves  only  to  carry 
it  onwards,  while  wings  have  not  only  to  waft  the  bird  onwards,  but  to 


FIG.  223. — Wings  completely  depressed.     (From  MAREY.) 

keep  it  up.     To  carry  the  weight  of  the  bird  the  wings  strike  vertically, 

sometimes  the  direction 
bird  mounts   upwards ; 


K-CCp    11    U]J.  JL  U    L,fcUI_y     LUC     WClgUL    Ul     LUC     UUU     LUC     W         _ 

to  carry  the  bird  onwards  they  strike  obliquely  ;  sometimes  the  direction 
of  the  stroke  is  more  vertical,  and  then  the  bir 


THE  SONG   OF  BIRDS.  625 

sometimes  it  is  more  oblique,  and  then  the  bird  speeds  onwards  ;  usually 
both  directions  are  combined.  The  raising  of  the  wing  after  each  stroke 
requires  relatively  little  effort,  the  resistance  to  be  overcome  being  very 
slight.  In  steering,  the  feathers  of  the  tail  often  bear  to  the  wings  a 
relation  comparable  to  that  between  rudder  and  sail. 

Modes  of  flight. — There  are  three  chief  modes  of  flight : — 

(1)  By  gliding  or  skimming,  during  which  the  bird  has  its  wings 
spread,  but  does  not  flap  them,  depending  for  its  movement  on  the 
velocity  acquired  by  previous  strokes,  by  descending  from  a  higher  to  a 
lower  level,  or  by  the  wind.     This  may  be  readily  observed  in  gull  and 
heron,  in  a  pigeon  gliding  from  its  loft  to  the  ground,  or  in  a  falcon 
swooping  upon  its  quarry. 

(2)  By  active  strokes  of  the  wings,  in  which  the  wings  move  down- 
ward and  forward,  backward  and  upward,  in  a  complex  curve.     This  is 
of  course  the  commonest  mode  of  flight.     It  has  been  carefully  studied 
and  photographed  by  Marey  in  the  gull  and  pigeon. 

(3)  By  sailing  or  soaring  with  motionless  spread  wings,  in  which  the 
bird  does  not  necessarily  lose  in  velocity  or  in  vertical  position  as  is  the 
case  in  gliding.      It  is  illustrated  by  such  birds  as  crow,  falcon,  stork, 
and  albatros,  and  has  been  observed  only  when  there  was  wind.     It  is 
still  imperfectly  understood,  but  there  seems  most  to  be  said  in  favour  of 
the  theory  that  it  depends  on  the  varying  velocity  of  the  wind  at  different 
heights. 

The  Song  of  Birds. — Singing  is  a  natural  expression  of 
emotional  intensity.  The  song  rises  in  the  bird,  Richard 
Jefferies  said,  as  naturally  as  the  sap  in  the  bough.  It  is 
richest  at  the  climax  of  emotion  in  the  breeding  season,  and 
is  always  best  and  often  solely  developed  in  the  males.  But 
song  in  any  excellence  is  the  gift  of  comparatively  few  birds, 
though  nearly  all  have  a  voice  of  some  sort,  often  so 
characteristic  that  the  species  may  be  recognised  by  its  call 
alone.  The  twittering  of  swallows,  the  cawing  of  rooks,  the 
melancholy  voice  of  the  sea-mew,  the  lapwing's  prayerful 
cry,  the  weird  call  of  the  curlew,  are  familiar  to  most  of  us. 
A  few  birds,  notably  the  parrot  and  the  jackdaw,  can  be 
taught  to  pronounce  articulate  words  :  but  the  power  of 
imitation  is  widespread  among  birds,  the  case  of  the  canary 
learning  the  song  of  the  nightingale  being  a  well  known 
instance.  This  power  of  imitation  has  some  importance  in 
relation  to  the  general  theory  of  instinct,  for  the  song  of  all 
birds  is  probably  in  great  part  imitative,  though  to  a  certain 
extent  the  musical  talent  is  really  inherited.  Young  birds 
taken  away  from  their  nests  when  very  young,  so  that  they 
have  hardly  heard  the  voices  of  their  kind,  will  sing  the 
characteristic  song  of  the  species,  but  do  so  imperfectly. 

40 


626  BIRDS. 

Many  birds,  apart  from  those  who  have  been  educated, 
have  "  words."  expressing  pleasure,  pain,  sense  of  danger, 
presence  of  food,  and  the  like.  But  there  is  a  difference 
between  this  power  of  utterance  and  the  possession  of 
language,  which  implies  the  expression  of  a  judgment,  e.g., 
food  is  good. 

The  vocal  organ  of  Birds  is  not  situated  in  the  larynx  as  it 
is  in  Mammals,  but  in  the  syrinx — a  song  box  at  the  base  of 
the  windpipe.  In  this  syrinx  there  are  vocal  membranes  or 
folds  of  skin ;  their  vibration  as  the  air  passes  over  them 
causes  sound ;  the  note  varies  with  the  muscular  tension  of 
the  folds,  with  the  muscular  state  of  the  complex  associated 
parts,  and  with  the  column  of  air  in  the  windpipe. 

Courtship.  — Birds  usually  pair  in  the  springtime,  but  there  are  many 
exceptions.  Some,  such  as  eagles,  live  alone  except  at  the  pairing 
time  ;  others,  notably  the  doves,  always  live  together  in  pairs  ;  many, 
such  as  rooks,  parrots,  and  cranes,  are  sociable  gregarious  birds.  A 
few,  like  the  fowls,  are  polygamous ;  the  cuckoo  is  polyandrous. 

In  most  cases,  however,  birds  pair,  and  the  mates  are  true  to  one 
another  for  a  season.  The  pairing  is  often  preceded  by  a  courtship  in 
which  the  more  decorative,  more  vocal  males  win  their  desired  mates, 
being,  according  to  Darwin,  chosen  by  them.  Darwin  attributed  the 
captivating  characteristics  of  the  males,  well  seen  in  peacocks  and  birds 
of  paradise,  or  as  regards  musical  powers  in  most  of  our  own  British 
songsters,  to  the  sexual  selection  exercised  by  the  females  ;  for  if  the 
more  decorative  or  the  more  melodious  males  always  got  the  preference 
in  courtship,  the  qualities  which  contributed  to  their  success  would  tend 
to  predominate  in  the  race.  He  believed,  moreover,  that  characteristics 
of  male  parents  were  entailed  on  male  offspring.  Wallace  regarded  the 
differences  between  males  and  females  in  another  way,  arguing  that 
in  the  course  of  natural  selection  the  more  conspicuous  females  had 
been  eliminated,  brightness  being  disadvantageous  during  incubation. 
It  seems  likely  enough  that  both  conclusions  are  to  some  extent  true, 
while  there  is  much  to  be  said  in  favour  of  a  deeper  explanation,  to 
which  Wallace  inclines,  that  the  secondary  differences  between  the  sexes 
are  natural  and  necessary  expressions  of  the  fundamental  constitutional 
differences  involved  in  maleness  and  femaleness. 

Nests. — After  pairing,  the  work  of  nest-building  is  begun.  Almost 
all  birds  build  nests  ;  the  well-known  habit  is  a  characteristic  expression 
of  their  parental  care.  Other  creatures,  indeed,  such  as  sticklebacks 
among  Fishes,  and  squirrels  among  Mammals,  besides  numerous 
Insects,  build  nests,  but  the  habit  is  most  perfectly  developed  among 
Birds.  As  is  well  known,  each  species  has  its  own  peculiar  style  of 
nest,  and  builds  it  of  special  materials.  Generally  the  nest  is  solitary, 
hidden  in  some  private  nook.  The  perfection  of  art  which  is  reached 
by  some  birds  in  the  making  of  their  nests  is  marvellous  ;  they  use  their 
bills  and  their  feet,  and  smooth  the  inside  by  twisting  round  and  round. 


THE  EGGS   OF  BIRDS.  627 

Usually  the  hen  does  most  of  the  work,  but  her  mate  sometimes  helps, 
both  in  building  the  nest  and  in  hatching  the  young. 

The  nest  is  a  cradle  rather  than  a  house,  for  its  chief  use  is  to  secure 
an  approximately  constant  warmth  for  the  young  which  are  being 
formed  within  the  eggs,  and  to  afford  protection  for  the  helpless 
fledglings.  At  the  same  time,  the  nest  secures  the  comfort  of  the 
parent  bird  during  the  days  and  nights  of  brooding. 

The  variety  of  nests  may  be  illustrated  by  mentioning  the  burrowed 
nests  of  sand-martins  and  kingfishers,  the  ground-nests  of  game-birds 
and  gulls,  the  mud-nests  of  house-swallow  and  flamingo,  the  holes 
which  the  woodpecker  fashions  in  the  tree  stem,  the  platforms  built  by 
doves  and  eagles,  storks  and  cranes,  the  basket-nests  of  most  singing- 
birds,  the  structures  delicately  woven  by  the  goldfinch,  bullfinch,  and 
humming-birds,  the  sewed  nest  of  the  tailor-bird,  the  mossy  nests  of  the 
wrens,  the  edible  nest  of  the  Collocalia^  which  is  chiefly  composed  of 
mucin  secreted  by  the  salivary  glands. 

The  Eggs  of  Birds. — When  the  nest  is  finished,  the  eggs  are  ready  to 
be  laid.  After  they  are  laid,  the  patience  of  brooding  begins.  With 
the  great  care  that  Birds  take  of  their  young  we  may  associate  the 
comparatively  small  number  of  the  eggs ;  but  it  is  more  accurate  to 
recognise  that,  as  animals  become  more  highly  evolved,  the  number  of 
offspring  decreases.  Yet  it  must  be  remembered  that  inductions  of  this 
kind  are  only  generally  true,  for  subsidiary  conditions  often  bring  about 
the  apparent  contradiction  of  a  general  truth.  Thus  we  are  not  justified 
in  saying  that  the  Apteryx,  which  lays  one  egg,  is  a  more  highly  differen- 
tiated bird  than  the  ostrich,  which  lays  many. 

The  size  of  the  egg  usually  bears  some  relation  to  the  size  of  the  bird. 
Of  European  birds,  the  swans  have  the  largest  eggs,  the  golden-crested 
wren  the  smallest.  It  is  said  that  the  egg  of  the  extinct  Moa  sometimes 
measured  nine  inches  in  breadth  and  twelve  inches  in  length ;  while 
that  of  the  extinct  jEpyornis  held  over  two  gallons,  some  six  times  as 
much  as  an  an  ostrich's  egg,  or  a  hundred  and  fifty  times  as  much  as 
a  fowl's.  Yet  the  size  of  the  egg  is  only  generally  proportional  to  that 
of  the  bird  ;  for  while  the  cuckoo  is  much  larger  than  the  lark,  the  eggs 
of  the  two  are  about  the  same  size  ;  and  while  the  guillemot  and  the 
raven  are  almost  of  equal  size,  the  eggs  of  the  former  are  in  volume 
about  ten  times  larger  than  those  of  the  latter.  The  eggs  of  birds, 
whose  young  are  rapidly  hatched  and  soon  leave  the  nest  are  large. 
Professor  Newton  remarks  that  * '  the  number  of  eggs  to  be  covered  at 
one  time  seems  also  to  have  some  relation  to  their  size,"  while  from  what 
one  notices  in  the  poultry  yard,  and  from  a  comparison  of  the  habits  of 
different  birds,  it  seems  probable  that  a  highly  nutritive,  sluggish  bird, 
will  have  larger  eggs  than  a  bird  of  more  active  habit  and  sparser  diet. 

The  shell  of  the  egg  is  often  very  beautifully  coloured  ;  there  is  a 
predominant  tint  upon  which  are  spots,  streaks,  and  blotches  of  varied 
colour  and  disposition,  so  that  the  egg  is  almost  always  characteristic  of 
the  species.  The  colouring  matter  consists  of  pigments  related  to  those 
of  the  blood  and  the  bile,  and  is  deposited  while  the  shell  is  being 
formed  in  the  lower  part  of  the  oviduct.  As  the  eggs  may  move  before 
the  pigments  are  fixed,  blotchings  and  markings  naturally  result.  But 
the  most  interesting  fact  in  regard  to  the  colouring  of  the  egg  shells,  is 


628  BIRDS. 

that  the  tints  are  often  protectively  harmonious  with  those  of  the 
surroundings.  Thus,  eggs  laid  almost  on  the  ground  are  often  brownish 
like  the  soil,  those  laid  in  rocky  places  by  the  sea  often  look  very  like 
stones,  while  conspicuous  eggs  are  usually  found  in  covered  nests. 

The  state  of  the  newly  hatched  young  is  very  various.  Some  are 
born  naked,  blind,  and  helpless,  and  have  to  be  carefully  fed  by  their 
parents  until  they  are  fully  fledged.  This  is  true  of  the  thrush  and  of 
many  other  song-birds.  Others  are  born  covered  with  down  but  still 
helpless  ;  while  a  few,  like  the  chicks,  are  able  to  run  about  and  feed 
themselves  a  few  minutes  after  they  leave  the  egg.  Those  which 
require  to  be  fed  and  brooded  over  are  sometimes  called  Altrices  or 
Insessores,  while  those  which  are  at  once  active  and  able  to  feed  them- 
selves are  called  Prsecoces  or  Autophagse. 

Moulting. — Every  year  birds  lose  their  old  feathers.  This  moulting 
generally  takes  place  after  the  fatigue  of  the  breeding  season,  but  in  the 
case  of  the  swallows  and  the  diurnal  birds  of  prey  and  some  others  the 
moult  is  in  mid-winter.  The  process  is  comparable  to  the  casting  of 
scales  in  Reptiles,  and  to  the  shedding  of  hair  in  Mammals.  Feathers 
are  so  easily  injured  that  the  advantage  of  the  annual  renewal  is 
evident,  especially  when  it  takes  place  just  before  the  time  at  which  it 
may  be  necessary  to  set  forth  on  a  long  migratory  flight. 

In  moulting,  the  feathers  fall  out  and  are  replaced  gradually,  but 
sometimes  they  are  shed  so  rapidly  that  the  bird  is  left  very  bare, 
thus  moulting  ducks  are  unable  to  fly.  There  are  many  birds  that 
moult,  more  or  less  completely,  more  than  once  a  year ;  thus  the 
garden  warbler  sheds  its  feathers  twice.  The  males  of  many  bright 
birds  assume  special  decorations  after  a  partial  moult  which  occurs 
before  the  time  of  pairing.  Most  remarkable  is  the  case  of  the 
ptarmigan,  which  changes  its  dress  three  times  in  the  year ;  after  the 
breeding  season  is  over  the  plumage  becomes  grey ;  as  the  winter  sets 
in  it  grows  white,  and  suited  to  the  surrounding  snow ;  in  the  spring, 
the  season  of  courtship,  the  wedding  robes  are  put  on. 

Diet. — The  food  of  birds  varies  greatly,  not  only  in  different  kinds, 
but  also  at  different  seasons.  Many  are  herbivorous,  feeding  on  the  soft 
green  parts  of  plants,  and  in  these  birds  the  intestine  is  long.  Some 
confine  themselves  to  grain,  and  these  have  large  crops  and  strong 
grinding  gizzards,  while  those  which  combine  cereals  and  insects  have 
in  most  cases  no  crop.  A  few  sip  honey,  and  may  even  help  in  the 
cross-fertilisation  of  flowers  ;  those  that  feed  on  fruits  play  an  important 
part  in  the  dissemination  of  seeds  ;  those  that  devour  insects  are  of 
great  service  to  man.  In  fruit-eating  and  insectivorous  birds  the  crop 
is  usually  small,  and  the  gizzard  only  slightly  muscular.  But  many 
birds  feed  on  worms,  molluscs,  fishes,  and  small  mammals  ;  in  these 
the  glandular  part  of  the  stomach  is  more  developed  than  the  muscular 
part.  It  has  been  shown  that  the  nature  of  the  stomach  in  the  Shetland 
gull  changes  twice  a  year,  as  the  bird  changes  a  summer  diet  of  grain 
and  seeds  for  a  winter  diet  offish,  and  vice  versa.  In  the  case  of  canaries, 
bullfinches,  parrots,  etc.,  it  has  been  noted  that  the  food  influences 
the  colouring  of  the  plumage. 

Migration  of  Birds.  —  Migration  remains  in  no  small  degree  a 
zoological  mystery.  On  certain  points  we  need  more  facts,  and  even 


MIGRATION  OF  BIRDS.  629 

where  facts  are  abundant  we  but  imperfectly  understand  them.      Let  us 
first  state  some  of  the  outstanding  facts. 

(1)  Most  birds  seem  to  be  more  or  less  migratory,  but  the  range 
differs  greatly.     It  is  said  that   the  dotterel  may   sup  on  the  North 
African  steppe  and  breakfast  next  morning  on  the  Arctic  tundra,  and 
although  the  alleged  rate  may  not  be  demonstrable,  there  is  no  doubt 
that  a  distance  of  about  2000  miles  is  traversed  by  this  bird  and  by 
many  others.      Indeed,   flights  of  7-10,000  miles   arev  said   to   occur. 
In  the  Tropics,  on  the  other  hand,  the  migration  may  simply  be  from 
valley  to  hillside. 

(2)  Observers  in  temperate  countries  long  ago  noticed  that  the  birds 
they  saw  might  be  grouped  in  reference  to  their  migrations.     Thus  (a) 
some  arrive  in  spring  from  the  South,  remain  to  breed,  and  leave  for  the 
South  in  autumn,  e.g.,  swallow  and  cuckoo  in  Britain  ;  (b]  some  arrive 
in  autumn  chiefly  from  the  North,  stay  throughout  the  winter,  and  fly 
northwards  again    in    spring,   e.g.,   the  fieldfare  and  the   redwing   in 
Britain;  (c)  some — the  "  Birds  of  Passage  " — are  seen  only  for  a  short 
time  twice  a  year  on  their  way  to  colder  or  warmer  countries  in  spring 
or  autumn,  e.g.,  sandpipers ;  and  (d]  some  seem  to  deserve  the  name  of 
"residents,"  but  really  exhibit  a  partial  migration,  such  as  the  song- 
thrush  and  redbreast  in  Britain.     In  spring  the  tide  is  on  the  whole 
northwards,  in  autumn  southwards,  but  the  paths  are  great  curves,  and 
easterly  and  westerly  waves  pass  from  one  country  to  another.     The 
migrants  always  breed  in  the  colder  countries  included  in  their  range. 

(3)  There  is  striking  regularity  in  the  advent  and  departure  of  many 
of  the  migrants.     In  spite  of  the  immense  distances  which  many  of  our 
immigrants  travel,  and  in  spite  of  unpropitious  weather,  they  are  often 
punctual  within  a  day  or  two  to  their  average  time  of  arrival  for  many 
years.     Similarly  some  birds,  such  as  the  swifts,  are  hardly  less  precise 
in  leaving  our  shores. 

(4)  It  is  beyond  all  doubt  that  many  individual  birds  find  their  way 
back  to  the  same  district,  even  to  the  same  spot,  where  they  had  made 
their  nest  in  previous  years.     Not  less  marvellous  is  the  security  with 
which  the  flight  from  country  to  country  is  continued  in  darkness,  at 
great  heights,  and  over  the  trackless  sea.     At  the  same  time  it  must  be 
noticed  that  the  mortality  during  migration  is  very  great. 

Having  stated  a  few  of  the  outstanding  facts,  let  us  note  some  of  the 
interpretations  and  suggestions  which  help  us  to  understand  them. 

The  impulse  to  migrate  is  instinctive  ;  it  is  exhibited  by  well-fed 
caged  birds  ;  migrating  is  an  inherited  habit.  But  it  is  likely  that  there 
are  always  immediate  causes  which  prompt  the  habit,  such  as  scarcity  of 
food,  and  to  a  less  degree,  increasing  cold  in  the  case  of  many  birds 
which  leave  us  in  autumn.  It  is  more  difficult  to  recognise  the  immed- 
iate causes  prompting  their  return. 

It  seems  likely  that  the  origin  of  the  migrating  habit  is  wrapped  up 
with  the  history  of  climates,  and  we  can  understand  how  the  setting  in 
of  glacial  conditions  from  the  north  would  gradually  force  birds,  century 
by  century,  to  a  longer  flight  southwards.  And  if  the  climatic  condi- 
tions limit  the  area  of  safe  and  comfortable  breeding  to  one  country  (the 
more  northerly),  and  the  possibility  of  food  during  winter  to  another 
country  (the  more  southerly),  we  can  understand,  with  Wallace,  "  that 


630  BIRDS. 

those  birds  which  do  not  leave  the  breeding  area  at  the  proper  season 
will  suffer,  and  ultimately  become  extinct ;  which  will  also  be  the  fate 
of  those  which  do  not  leave  the  feeding  area  at  the  proper  time."  In 
short,  given  environmental  changes  of  climate  on  the  one  hand,  and  a 
measure  of  plasticity  and  initiative  on  the  part  of  the  organism,  the  habit 
of  migrating  would  be  perfected  in  the  course  of  natural  elimination. 

But  while  this  view  is  so  far  satisfactory,  it  leaves  us  face  to  face  with 
the  problem  how  birds  migrate  as  safely  and  surely  as  they  do  on  their 
pathless  way.  To  say  that  they  do  so  by  instinct  only  shelves  the 
difficulty,  even  if  it  were  true ;  and  to  point  out  that  the  merciless 
elimination  which  continually  goes  on  keeps  up  the  standard  of  racial 
fitness,  leaves  us  still  wondering  how  any  became  fit  at  all. 

One  welcomes  therefore  any  suggestion  as  to  the  manner  in  which 
birds  learn  or  have  learned  to  find  their  way.  The  power  has  been 
compared  to  the  "homing"  faculty  of  some  pigeons,  but  most  believe 
that  pigeons  are  guided  solely  by  noticing  landmarks,  which  could 
hardly  be  done  over  10,000  miles  of  land,  and  obviously  not  over  1000 
miles  of  sea,  or  during  the  night.  Some  have  urged  that  birds  follow 
river  valleys,  the  lines  of  old  "land  bridges"  connecting  continents, 
the  roll  of  the  waves,  and  so  forth,  but  the  difficulty  remains  of  flight  by 
night  and  at  very  great  heights.  Attractive  is  the  suggestion  that  birds 
are  guided  by  what  may  be  called  a  "  tradition  "  based  on  experience  ; 
those  guide  well  one  year  who  have  followed  well  in  previous  years. 
But  some  young  birds  fly  apart  from  their  parents,  and  some  birds  do 
not  fly  in  flocks  at  all.  Moreover,  it  is  difficult  to  understand  how  the 
experience  could  be  gained  except  by  sight,  which  in  many  cases  is 
excluded  by  the  darkness.  In  face  of  these  difficulties,  some  authorities, 
such  as  Professor  Newton,  have  been  led  to  believe  that  birds  have,  in 
an  unusual  degree,  "a  sense  of  direction." 

Pedigree  of  Birds. 

Birds  have  many  structural  affinities  with  Reptiles,  some  of 
the  ancient  Dinosaurs  present  approximations  to  Birds,  the 
extinct  flying  Pterodactyls  show  that  it  was  possible  for  flight 
to  be  developed  among  Reptiles,  the  oldest  bird — Archceo- 
pteryx — is  in  many  ways  a  connecting  link  between  the  two 
classes,  and  the  development  of  some  Birds  reveals  many 
remarkable  resemblances  with  that  of  Reptiles, — therefore, 
with  the  strength  of  the  general  argument  for  evolution  to 
corroborate  us,  we  conclude  that  Birds  evolved  from  a 
Reptile  stock. 

Speaking  of  his  work  on  the  development  of  the  fowl, 
W.  K.  Parker  wrote  in  1868  :— "  Whilst  at  work  I  seemed 
to  myself  to  have  been  endeavouring  to  decipher  a 
palimpsest^  and  one  not  erased  and  written  upon  again  just 
once,  but  five  or  six  times  over.  Having  erased,  as  it  were, 


PEDIGREE   OF   BIRDS.  631 

the  characters  of  the  culminating  type — those  of  the  gaudy 
Indian  bird — I  seemed  to  be  amongst  the  sombre  Grouse ; 
and  then,  towards  incubation,  the  characters  of  the  Sand- 
grouse  and  Hemipod  stood  out  before  me.  Rubbing  these 
away,  in  my  downward  work  the  form  of  the  Tinamou 
looked  me  in  the  face ;  then  the  aberrant  Ostrich  seemed 
to  be  described  in  large  archaic  characters  ;  a  little  while, 
and  these  faded  into  what  could  just  be  read  off  as  pertain- 
ing to  the  Sea  Turtle ;  whilst  underlying  the  whole,  the  Fish, 
in  its  simplest  Myxinoid  form,  could  be  traced  in  morpho- 
logical hieroglyphics." 

More  than  twenty  years  later,  the  same  accomplished 
embryologist  described  the  development  of  the  "  Reptilian 
Bird" — Opisthocomus  cristatus.  In  this  form  the  unhatched 
chick  has  a  paw-like  hand,  three  clawed  ringers  and  a  rudi- 
ment of  a  fourth,  a  wrist  of  numerous  carpal  elements,  and 
many  other  features  suggestive  of  reptilian  descent.  It  is 
not  surprising,  then,  that  to  Parker,  a  bird  seemed  as  "a 
transformed  and,  one  might  even  say,  a  glorified  Reptile." 

It  is  likely  that  Birds  arose  from  the  Dinosaurian  stock, 
but  by  what  steps  and  under  what  impulses  we  do  not  know. 
To  one  it  is  enough  to  say  that  the  evolution  was  accom- 
plished gradually  in  the  course  of  natural  selection  by  the 
fostering  of  fit  variations  and  the  elimination  of  the  disad- 
vantageous ;  to  another  it  seems  that  the  incipient  birds 
were  "fevered  representatives  of  reptiles,  progressing  in  the 
direction  of  greater  and  greater  constitutional  activity ; " 
but  both  these  suggestions  leave  much  in  the  dark,  leave  us 
still  to  "  wonder  how  the  slow,  cold-blooded,  scaly  beast 
ever  became  transformed  into  the  quick,  hot-blooded, 
feathered  bird,  the  joy  of  creation." 


CHAPTER    XXVI 


CLASS    MAMMALIA.1 


As  Birds  and  Mammals  have  evolved  along  very  different 
lines,  Birds  possessing  the  air  and  Mammals  the  earth,  it  is 
difficult  to  say  that  either  class  is  the  higher.  But  apart 
from  the  fact,  which  prejudices  us,  that  man  himself  is 
zoologically  included  among  Mammals,  this  class  is  superior 
to  Birds  in  two  ways — in  brain  development  and  in  the 
relation  between  mother  and  offspring.  In  most  Mammals 
there  is  a  prolonged  organic  connection  between  the  mother 
and  the  unborn  young,  and  perhaps  Robert  Chambers  was 
right  in  suggesting  that  this  prolonged  gestation  was  one  of 
the  conditions  of  progress,  connected,  it  may  be,  with  the 
development  of  large  brains.  Moreover,  it  is  characteristic 
of  Mammals  that  the  young  are  nourished  after  birth  by 
their  mother's  milk,  and  it  has  been  suggested  that  the 
prolonged  infancy  of  young  Mammals  was  one  of  the  factors 
in  the  evolution  of  gentleness.  It  is  certain  at  least  that 
the  carefulness  and  sacrifice  of  the  mothers  has  been  a 
condition  of  the  survival  and  success  of  Mammals,  and  of 
Birds  also.  We  may  find  in  the  term  Mammalia,  which 
Linnaeus  first  applied  to  the  class,  a  hint  of  the  idea  that  in 
the  evolution  of  these  forms  of  life,  the  mothers  led  the 
way. 

General  Survey  of  Mammals. 

There  are  three  grades  of  Mammalian  development : — 
(A.)  The  duckmole  (Ornithorhynchus)  and  the  spiny  ant- 

1  In  the  systematic  part  of  this  chapter  I  have  been  especially  in- 
debted to  the  "  Introduction  to  the  Study  of  Mammals"  by  Sir  W.  H. 
Flower  and  Mr.  Lydekker. — (Lond.,  1891.) 


GENERAL  SURVEY  OF  MAMMALS.  633 

eaters  (Echidna  and  Proechidnd)  differ  markedly  from  other 
Mammals.  The  young  are  hatched  outside  of  the  body; 
in  other  words,  the  mothers  are  oviparous.  The  brain  is 
poorly  developed  when  compared  with  that  of  other  Mam- 
mals. Some  of  the  characteristics  of  the  skeleton,  &c.,  sug- 
gest Reptilian  affinities.  To  this  small  sub-class,  the  titles 
Monotremata,  Ornithodelphia,  and  Prototheria  are  applied. 

(£.)  The  kangaroos  and  bandicoots,  phalangers  and 
opossums,  and  the  like,  form  the  second  sub-class.  In 
these  the  young  are  born  prematurely  after  a  short  gesta- 
tion, during  which  the  organic  connection  between  the 
mother  and  the  young  is  comparatively  slight.  Most 
female  Marsupials  have  an  external  pouch  or  marsupium, 
to  which  the  tender  young  are  transferred,  and  within 
which  they  are  nourished  and  protected  for  some  time. 
Moreover,  the  brains  even  of  the  most  intelligent  Marsupials 
are  not  so  well  developed  as  those  of  higher  Mammals. 
To  this  heterogeneous  sub-class,  the  titles  Marsupialia, 
Didelphia,  and  Metatheria  are  applied. 

(C.)  In  all  the  other  Mammals  there  is  a  placenta  uniting 
the  unborn  young  to  the  mother.  It  is  among  these 
placental  Mammals  that  the  brains  begin  to  be  much  con- 
voluted,— as  it  were,  wrinkled  with  thought.  To  this  sub- 
class, including  sloths  and  ant-eaters  (Edentata),  sea-cows 
(Sirenia),  hoofed-animals  (Ungulata),  Cetaceans,  Rodents, 
Carnivores,  Insectivores,  Bats,  Lemurs,  and  Monkeys,  the 
titles  Placentalia,  Monodelphia,  and  Eutheria  are  applied. 

Among  these  orders  of  placental  Mammals,  it  seems 
likely  that  the  Edentata  and  Sirenia  should  be  placed  lowest, 
for  many  of  their  characteristics  are  old-fashioned.  The 
rest  may  be  provisionally  grouped  in  three  sets,  perhaps 
representing  three  main  lines  of  evolution. 

On  one  side  we  place  the  great  series  of  hoofed  animals 
or  Ungulata,  including  (a)  those  with  an  even  number  of 
toes  (Artiodactyla),  such  as  pigs,  hippopotamus,  camels, 
cattle,  and  deer;  (fr)  those  with  an  odd  number  of  toes 
(Perissodactyla),  such  as  tapir,  rhinoceros,  and  horse ;  (c) 
the  elephants  (Proboscidea) ;  (d]  the  Hyraxes  (Hyracoidea). 
And  near  the  Ungulata  it  seems  legitimate  to  rank  (a)  the 
whales  and  dolphins  (Cetacea),  and  (fr)  the  rabbits  and  hares, 
rats  and  mice,  &c.  (Rodentia). 


634 


MAMMALIA. 


On  the  other  side  we  place  the  great  series  of  Carnivora, 
such  as  cats,  dogs,  bears,  and  seals.  Beside  these  may  be 
ranked  the  Insectivora,  such  as  hedgehog,  mole,  and  shrew, 
and  the  bats  or  Chiroptera,  which  seem  to  be  specialised 
Insectivores. 

In  the  middle  we  place  the  series  which,  beginning  with 
the  Lemurs,  leads  through  various  grades  of  monkeys  to  a 
climax  in  man.  Among  the  monkeys  are  the  small  and 
simple  marmosets,  the  flat-nosed  American  monkeys,  the 
dog-like  apes  of  the  Old  World,  and  the  anthropoid  apes, 
which  most  nearly  approach  ourselves. 

But  it  must  be  carefully  noted  that  these  orders  are  often 
linked  by  extinct  types.  Thus,  to  take  one  instance  only,  it 
is  believed  by  some  that  the  extinct  Phenacodus  has  affinities 
with  Ungulates,  Carnivores,  and  Lemurs. 

We  may  summarise  our  general  classification  thus  : — 

MAN 

and 

MON  KEYS 


UNGU 


\ 


LATES 


LEM 


URS 


RODENTS 


CAR  NIVORES 
BATS 


INSECTIVORES 


CETACEANS 


EXTINCT   SYNTHETIC   TYPES 


SIRENIA 


EDENTATA 


MARSUPIALS 


MONOTREMES 


GENERAL  CHARACTERS. — All  Mammals  are  quadrupeds, 
except  the  Cetaceans  and  Sirenians,  in  which  the  hind  limbs 
have  disappeared,  leaving  at  most  internal  vestiges.  There  is 
generally  a  distinct  neck  between  the  head  and  the  trunk,  and 
the  vertebral  column  is,  in  most  cases,  prolonged  into  a  tail. 

Hairs  are  never  entirely  absent.  In  most  they  form  a  thick 
covering,  but  they  are  scanty  in  Sirenians  and  in  the  hippo- 


GENERAL    CHARACTERS.  635 

potamus,  and  almost  absent  in  Cetaceans,  in  which  they  are 
sometimes  restricted  to  early  stages  in  life.  The  skin  has 
abundant  sebaceous  and  sudorific  glands.  In  the  female, 
milk-giving  or  mammary  glands  develop,  as  specialisations 
of  sebaceous  glands,  except  in  Monotremes,  where  they  are 
nearer  the  sudorific  type. 

A  complete  muscular  partition  or  diaphragm  separates  the 
chest  cavity  containing  the  heart  and  lungs  from  the  abdominal 
cavity,  and  is  of  great  importance  in  respiration. 

All  the  important  bones  have  distinct  terminal  ossifications 
or  epiphyses,  absent,  however,  in  the  vertebra  of  Monotremes 
and  Sirenia.  The  centra  of  the  vertebrce  have  generally  flat 
faces,  and  there  are  seven  cervical  vertebra,  except  in  the 
manatee  and  the  two-toed  sloth  (Cholcepus  hoffmanni),  which 
have  six  ;  the  three-toed  sloth  (Bradypus  tridactylus),  which 
has  nine  ;  and  the  pangolin  (Manis),  which  has  sometimes 
eight, — variations  ivhich,  it  will  be  observed,  are  limited  to  the 
two  most  old-fashioned  orders  of  placental  Mammals. 

The  bones  of  the  skull  are  firmly  united  by  sutures,  which 
generally  persist.  Only  the  lower  jaw,  the  ear  ossicles,  and 
the  hyoid  are  movable.  There  are  tivo  occipital  condyles, 
as  in  Amphibians.  It  may  be  noted,  however,  that  for 
various  reasons,  e.g.,  that  some  Birds  and  Reptiles  are  not 
very  clearly  single-condyled,  morphologists  no  longer  attach  so 
much  importance  to  this  character  as  they  once  did.  The 
lower  jaw  on  each  side  consists,  in  adult  life,  of  a  single  bone 
which  works  on  the  squamosal ;  the  quadrate  which  intervenes 
in  Sauropsida  has  disappeared,  or  has  been  shunted  to  become 
one  of  the  ear  ossicles.  For  it  is  one  theory  of  the  three 
ossicles — malleus,  incus,  and  stapes — which  connect  the  drum 
with  the  inner  ear,  that  they  correspond  respectively  to  the 
articular,  quadrate,  and  columella  or  hyo-mandibular  of 
other  Vertebrates.  The  otic  bones  fuse  to  form  a  compact 
periotic.  A  bony  palate,  formed  from  premaxillce,  maxilla, 
and  palatines,  separates  the  buccal  cavity  from  the  nasal 
passages.  In  most  cases  there  are  teeth,  borne  in  sockets  by 
the  premaxillce,  maxilla,  and  mandible. 

Except  in  Monotremes,  the  coracoid  is  represented  by 
a  small  process  from  the  scapula,  forming  part  of  the 
glenoid  cavity  in  which  the  head  of  the  humerus  works, 
but  not  reaching  the  sternum.  The  latter  includes  (a)  a 


636  MAMMALIA. 

presternum  with  which  in  Monotremes  an  interclavicle  is 
fused,  and  with  ivhich  the  clavicles  (if  well-developed) 
articulate ;  (b)  a  mesosternum  divided  into  segments,  with 
ivhich  the  sternal  parts  of  the  ribs  articulate ;  and  (c)  a 
xiphisternum,  often  cartilaginous.  There  are  generally  two 
sacral  vertebra,  but  to  these  several  caudals,  and  more  rarely 
a  lumbar,  may  be  fused.  The  ilia  slope  downwards  and 
backwards,  the  ischia  have  no  symphysis,  but  the  pubes  are 
almost  always  united  ventrally. 

The  cerebral  hemispheres  have  usually  a  convoluted  surface, 
and  always  cover  the  optic  thalami  and  the  optic  lobes  (now 
four-fold  corpora  quadrigemina),  and  in  higher  forms  the 
cerebellum  as  well.  The  commissural  system  is  well  developed, 
being  especially  represented  by  a  large  corpus  callosum,  except 
in  Monotremes  and  Marsupials,  in  which  the  anterior  com- 
missure is  large  and  the  corpus  callosum  small  (according  to 
some),  or  absent  (according  to  others).  There  is  also  an 
important  set  of  longitudinal  fibres  called  the  for  nix. 

Except  in  Monotremes,  in  which  there  is  a  cloaca,  the  food 
canal  ends  separately  from  the  urinogenital  aperture. 

The  heart  is  four-chambered,  and  the  temperature  of  the 
blood  is  high,  though  less  than  that  of  Birds.  There  is  but 
one  aortic  trunk,  which  curves  over  the  left  bronchus.  The 
red  blood  corpuscles  are,  when  fully  formed,  non-nucleated,  and 
are  circular  in  outline,  except  in  the  Camelida  where  they  are 
oval. 

The  lungs  are  invested  by  pleural  sacs,  and  lie  freely  in 
the  chest  cavity.  Within  the  lungs  the  bronchial  tubes  fork 
repeatedly  into  finer  and  finer  branches.  At  the  top  of  the 
trachea  there  is  a  complex  larynx  with  the  vocal  cords. 

The  kidneys  are  generally  compact  and  rounded  bodies  ; 
the  ureters  open  into  the  bladder,  except  in  Monotremes  in 
which  they  enter  a  urinogenital  sinus.  Except  in  Monotremes, 
the  outlet  or  urethra  of  the  bladder  unites  in  the  male  with 
the  genital  duct,  to  form  a  urinogenital  canal ;  in  the  female, 
except  in  Monotremes  and  a  few  other  cases,  the  urethra  and 
the  genital  duct  open  into  a  common  vestibule. 

In  the  more  primitive  mammals  the  testes  lie  in  the  abdomen  ; 
in  the  majority  they  descend  permanently  (in  a  few  cases 
temporarily)  into  a  single  or  paired  scrotal  sac,  lying,  except 
in  Marsupials,  behind  the  penis. 


GENERAL  LIFE   OF  MAMMALS.  637 

The  ovaries  are  small.  Except  in  Monotremes,  the  genital 
ducts  of  the  female  are  differentiated  into  (a)  Fallopian  tubes, 
which  catch  the  ova  as  they  burst  from  the  ovaries ;  (b)  a 
uterine  portion  in  which  the  young  develop ;  and  (c)  a  vaginal 
portion  ending  in  the  urinogenital  aperture.  In  Monotremes 
the  tivo  ducts  are  simple,  and  open  separately  into  the  cloaca  ; 
in  Marsupials  there  are  two  uteri  and  two  vagince. ;  in 
Placental  Mammals  the  uterine  regions  are  more  or  less 
united,  and  the  vaginal  regions  are  always  fused. 

In  Monotremes  the  eggs  are  large  and  rich  in  yolk  ;  in  all 
others  they  are  small  and  almost  yolkless.  In  the  ovary  each 
ovum  lies  embedded  in  a  nest  of  cells,  within  a  swelling  or 
Graafian  follicle  which  eventually  bursts  and  liberates  the 
egg  cell.  In  Monotremes  the  segmentation  is  necessarily 
meroblastic,  in  other  cases  it  is  holoblastic.  As  in  Sauropsida 
there  are  two  foetal  membranes — the  amnion  and  the  allantois, 
both  of  which  share  in  forming  the  placenta  of  the  Placental 
Mammals. 

The  Monotremes  are  oviparous ;  the  Marsupials  bring 
forth  their  young  prematurely  after  a  short  gestation  ;  the 
Placental  Mammals  have  a  longer  gestation,  during  which 
the  young  are  vitally  connected  to  the  wall  of  the  uterus  by 
means  of  the  placenta. 

General  Life  of  Mammals. 

Most  Mammals  live  on  dry  land.  The  bats,  however, 
have  the  power  of  flight,  and  not  a  few  forms,  belonging  to 
diverse  orders,  are  able  to  take  long  swooping  leaps  from 
tree  to  tree.  Thus,  there  are  "  flying  phalangers,"  such  as 
Petaurus,  among  Marsupials ;  "  flying  squirrels,"  such  as 
Pteromys,  among  Rodents;  "flying  lemurs"  (Galeopithecus), 
allied  to  Insectivores.  Not  a  few  are  aquatic, — all  the 
Cetaceans,  the  two  Sirenians,  and  the  Pinniped  Carnivores, 
such  as  seals  and  walruses  ;  while  water  voles,  beavers, 
otters,  polar  bear,  and  many  others  are  also  at  home  in 
the  water.  Burrowers  are  well  represented  by  moles  and 
rabbits  ;  aboreal  forms  by  squirrels  and  monkeys. 

As  to  the  diet,  man,  most  monkeys,  pigs,  and  many  others, 
may  be  called  omnivorous ;  kangaroos,  hoofed  animals,  and 
most  rodents  are  herbivorous ;  the  Echidna,  the  ant  eaters, 
hedgehogs  and  shrews,  and  most  bats  are  insectivorous; 


638  MAMMALIA. 

most  of  the  Carnivora  are  carnivorous ;  dolphins  and  seals 
feed  chiefly  on  fishes ;  but  in  most  cases  the  diet  varies  not 
a  little  with  the  available  food  supply. 

The  struggle  for  existence  among  Mammals  is  sometimes 
keen  among  fellows  of  the  same  kind ;  thus  the  brown  rat 
(Mus  decumanus)  tends  to  drive  away  the  black  rat  (M. 
rattus],  but  stress,  due  to  over-population,  is  sometimes 
mitigated  by  migration,  as  in  the  case  of  the  lemmings.  The 
struggle  seems  to  be  keener  between  foes  of  different  kinds, 
between  carnivores  and  herbivores,  between  birds  of  prey 
and  small  mammals ;  but  combination  for  mutual  defence 
often  mitigates  the  intensity  of  the  conflict.  Teeth  and 
claws,  hoofs  and  horns  are  the  chief  weapons,  while  the 
scales  of  pangolins,  the  bony  shields  of  armadillos,  the  spines 
of  hedgehogs  and  porcupines,  and  the  thick  hide  of  the 
rhinoceros  may  be  regarded  as  protective  armature.  In 
keeping  their  foothold  some  mammals  are  helped  by  the 
harmony  between  their  colouring  and  that  of  their  surround- 
ings ;  thus,  the  white  Arctic  fox  and  hare  are  inconspicuous 
on  the  snow,  the  striped  tiger  is  hidden  in  the  jungle,  and 
many  tawny  animals  harmonise  with  the  sandy  background 
of  the  desert. 

The  majority  of  Mammals  are  gregarious,  witness  the  herds 
of  herbivores,  the  cities  of  the  prairie  dogs,  the  packs  of 
wolves,  the  schools  of  porpoises,  the  bands  of  monkeys. 
Combinations  for  attack  and  for  defence  are  common ;  senti- 
nels are  posted  and  social  conventions  are  respected ;  such 
migrations  as  those  of  the  lemming  and  reindeer  are  char- 
acteristically social.  In  the  beaver  village  and  among 
monkeys  there  is  combination  in  work,  and  their  communal 
life  seems  prophetic  of  that  sociality  which  is  distinctively 
human. 

Among  Birds,  mates  are  won  by  beauty  of  song  and 
plumage ;  Mammals  not  less  characteristically  woo  by  force. 
Rival  males  fight  with  one  another,  and  are  usually  larger 
and  stronger  than  their  mates.  The  antlers  of  male  deer, 
the  tusk  of  the  male  narwhal,  the  large  canine  teeth  of  boars 
illustrate  secondary  sexual  characters  useful  as  weapons. 
But  manes  and  beards,  bright  colours  and  odoriferous  glands 
are  often  more  developed  in  the  males  than  in  the  females, 
and  may  be  of  advantage  in  the  rough  mammalian  court- 


GENERAL  LIFE   OF  MAMMALS.  639 

ship.  At  the  breeding  season,  a  remarkable  organic  reaction 
often  affects  the  animal,  the  timid  hare  becomes  a  fierce 
combatant,  and  love  is  often  stronger  than  hunger.  The 
courtship  of  Mammals  is  usually  like  a  storm — violent  but 
passing  :  for,  after  pairing,  the  males  return  to  their  ordinary 
life,  and  the  females  become  maternal.  Some  monkeys 
are  faithfully  monogamous ;  and  exceptional  pairs,  such  as 
beavers  and  some  antelopes,  remain  constant  year  after  year; 
but  this  is  not  the  way  of  the  majority. 

The  duckmole  lays  eggs  and  brings  up  her  young  in  the 
shelter  of  the  burrow ;  the  Echidna  has  a  temporary  pouch. 
In  Marsupials  the  time  of  gestation  is  very  short,  and  there 
is  no  truly  placental  union  between  the  unborn  young  and 
the  mother.  The  new-born  Marsupials  are  very  helpless,  and 
are  in  most  cases  transferred  to  an  external  pouch  or 
marsupium,  within  which  they  are  nurtured.  In  Placental 
Mammals  the  gestation  usually  lasts  much  longer  than  in 
Marsupials, — its  duration  varying  to  some  extent  with  the 
rank  in  the  mammalian  series,  but  there  are  great  differences 
in  the  condition  of  the  young  at  birth.  "  In  those  forms," 
Professor  Flower  says,  "  which  habitually  live  in  holes,  like 
many  Rodents,  the  young  are  always  very  helpless  at  birth ; 
and  the  same  is  also  true  of  many  of  the  Carnivora,  which 
are  well  able  to  defend  their  young  from  attack.  In  the 
great  order  of  Ungulates  or  Hoofed  Mammals,  where  in  the 
majority  of  cases  defence  from  foes  depends  upon  fleetness 
of  foot,  or  upon  huge  corporeal  bulk,  the  young  are  born  in 
a  very  highly  developed  condition,  and  are  able  almost  at 
once  to  run  by  the  side  of  the  parent.  This  state  of  relative 
maturity  at  birth  reaches  its  highest  development  in  the 
Cetacea,  where  it  is  evidently  associated  with  the  peculiar 
conditions  under  which  these  animals  pass  their  existence.'7 
The  importance  of  prolonged  infancy,  as  illustrated  among 
monkeys,  should  be  recognised  in  connection  with  the  evolu- 
tion of  sympathy. 

The  maternal  sacrifice  involved  in  the  placental  union 
between  the  mother  and  her  "foetal  parasite,"  in  the  pro- 
longed gestation,  in  the  nourishment  of  the  young  on 
milk,  and  in  the  frequently  brave  defence  of  the  young 
against  attack,  has  been  rewarded  in  the  success  of  the 
mammalian  race,  and  has  been  justified  in  the  course  of 


640  MAMMALIA. 

natural  selection.  But  it  is  important  to  recognise  that  the 
maternal  sacrifice — whatever  its  origin  may  have  been — 
expresses  a  subordination  of  self-preserving  to  species-main- 
taining. Thus,  other-regarding  as  well  as  self-regarding 
activities  have  been  factors  in  evolution. 

History  of  Mammals. — As  to  the  origin  of  Mammals  we  can  only 
speculate.  There  are  some  remarkable  resemblances  between  Mono- 
tremes  and  certain  extinct  Reptilian  types,  known  as  Anomodontia  or 
Theromorpha,  and  these  again  exhibit  affinities  with  the  extinct  Laby- 
rinththodont  Amphibians.  Amphibians  and  Mammals  agree  in  having 
two  occipital  condyles,  small  quadrates,  large  squamosals,  and  in  certain 
characteristics  of  pectoral  and  pelvic  girdles.  Possibly  the  ancestral 
Mammals  and  the  Anomodont  Reptiles  diverged  from  a  common 
Amphibian  stock. 

The  oldest  known  remains  of  Mammals  are  some  fossils  from  Triassic 
rocks,  and  similar  types  have  been  found  in  Cretaceous  and  Jurassic 
beds ;  most  of  these  Mesozoic  fossils  are  but  small  pieces  of  small 
animals,  and  secure  conclusions  as  to  their  nature  are  not  readily 
reached.  The  earliest  suggest  affinities  with  Marsupials  and  Insecti- 
vora.  Many  of  the  Mesozoic  mammals  belong  to  a  group  which  has 
received  the  name  of  Multituberculata,  on  account  of  the  longitudinal 
rows  of  tubercles  on  the  back  teeth.  It  is  possible  that  these  forms,  e.g., 
PlagiaulaX)  Tritylodon,  Poly  mastodon,  should  be  ranked  beside  the 
Monotremes,  but  they  are  usually  placed  nearer  Marsupials. 

Other  Mesozoic  forms,  such  as  Dromatheritim,  Triconodon,  Amphi- 
therium,  Spalac  other  itim,  are  often  referred  to  the  Marsupial  series 
beside  opossum,  dasyure,  and  bandicoot. 

The  first  certain  remains  of  Placental  Mammals  are  found  in  Eocene 
strata,  and  give  evidence  of  the  existence  of  generalised  types  connecting 
rather  than  referable  to  the  modern  orders.  Many  are  characterised  by 
the  presence  of  three  tubercles  on  the  back  teeth,  and  of  five  digits  on 
the  limbs,  and  by  having  brains  relatively  smaller  than  those  of  their 
modern  successors. 

Among  extinct  Tertiary  types,  we  may  especially  notice  the 
ground  sloths  (e.g.,  Megatherium]  and  Glyptodonts  allied  to  the 
modern  Edentata,  the  Zeuglodonts,  sometimes  included  among 
Cetaceans,  numerous  ancestral  Ungulates,  and  the  Creodonts  allied 
to  modern  Carnivores. 

More  detailed  account  of  some  of  the  structures  of  Mammals. 
Skin. 

The  skin  consists  of  a  superficial  epidermis  derived  from 
the  outer  or  ectodermic  layer  of  the  embryo,  and  of  a  sub- 
jacent mesodermic  dermis  or  cutis. 

The  most  characteristic  modification  of  the  mammalian 
epidermis  is  the  hair.  Each  hair  arises  from  the  cornifica- 


SKIN.  641 

tion  of  an  ingrowing  epidermic  papilla,  surrounded  at  its 
base  by  a  moat-like  follicle,  and  nourished  during  growth 
by  a  vascular  projection  of  the  dermis. 

Each  hair  consists  of  a  spongy  central  part  and  a  denser  cortex,  but 
there  are  many  diversities  of  form  and  structure,  such  as  short  fur  and 
long  tresses,  the  soft  wool  of  sheep  and  the  bristles  of  pigs,  the  spines  of 
hedgehog,  porcupine,  and  Echidna,  the  cilia  of  the  eyelids  and  the  tactile 
vibrissae  of  the  lips  and  cheeks. 

It  is  generally  believed  that  the  hairs  of  Mammals  are  homologous 
with  the  feathers  of  Birds  and  the  scales  of  Reptiles,  but  Maurer 
maintains  that  the  facts  of  development  upset  the  homology  and  point 
rather  to  a  resemblance  between  hairs  and  the  sensory  papillae  of 
Amphibians.  But  this  is  still  under  discussion. 

The  hair  keeps  the  animal  dry  and  warm  ;  in  the  practically  hairless 
Cetacea  the  layer  of  fat  or  blubber  underneath  the  skin  also  serves  to 
sustain  the  temperature  of  the  body.  Like  feathers,  hairs  die  away 
and  are  cast  off,  being  replaced  by  fresh  growths.  A  few  mammals, 
such  as  the  Arctic  fox,  the  mountain  hare,  and  the  ermine,  become  white 
in  winter,  harmonising  with  the  snow.  In  the  case  of  Ross's  lemming, 
we  know  that  this  change  is  due  to  the  influence  of  the  cold,  and 
depends  in  great  part  on  the  appearance  of  gas  bubbles  inside  the 
hairs. 

That  the  colouring  is  sometimes  of  protective  advantage  we  have 
already  noticed ;  but  in  many  cases  no  utilitarian  interpretation  can  be 
read  into  the  stripes  and  markings.  Those  of  related  species  often 
form  regular  series,  and  are  superficial  outcrops  of  constitutional  changes 
hardly  to  be  analysed.  Sometimes  there  is  considerable  change  during 
the  lifetime  of  the  animal,  thus  most  young  deer  have  spots,  but  only 
the  Fallow  and  Axis  deer  retain  these  when  adult.  To  an  excess  of 
pigment  is  due  the  variation  known  as  melanism  or  blackness,  e.g.,  in 
black  wolves  and  rabbits  ;  to  a  dearth  of  pigment  albinism  is  due,  as  in 
white  mice  and  white  elephants.  In  tropical  countries  the  skin  is  some- 
times very  darkly  coloured,  as  in  Indian  cattle,  and  many  monkeys — 
especially  males — are  notable  for  the  bright  colours  of  the  bare  parts  of 
the  body. 

Among  other  tegumentary  structures  are  the  scales  which 
occur  along  with  hairs  on  the  pangolins  (Mam's)  •  the  scales 
on  the  tails  of  rats  and  beavers  and  some  other  forms ;  the 
thickened  skin-pads  or  callosities  on  the  ischia  of  apes, 
the  breast  of  camels,  the  legs  of  horses ;  the  nails,  claws,  or 
hoofs  which  ensheath  the  ends  of  the  digits  in  all  mammals 
except  Cetaceans.  Unique  is  the  armature  of  the  armadillos, 
for  it  consists  of  bony  plates  developed  in  the  dermis, 
overlaid  by  epidermic  scales.  The  median  solid  horns  of 
the  rhinoceros  are  epidermic  outgrowths,  comparable  to 
exaggerated  warts  ;  the  paired  horns  of  the  Ruminants  con- 

41 


642  MAMMALIA. 

sist  of  epidermic  sheaths  covering  outgrowths  of  the  frontal 
bones,  but  extending  far  beyond  these ;  the  antlers  of 
stags  are  outgrowths  of  the  frontal  bones,  and,  except  in  the 
reindeer,  are  cast  and  regrown  each  year,  and  possessed  by 
the  males  only. 

The  skin  of  Mammals,  unlike  that  of  Birds,  is  rich  in 
glands.  Sebaceous  glands  are  always  associated  with  the 
hair-follicles,  and  sudorific  or  sweat  glands  are  scattered 
over  the  skin. 

Specialised  glands  are  also  very  common,  especially  those  which 
secrete  some  strongly  odoriferous  stuff,  scenting  which  the  animals 
recognise  their  fellows,  their  foes,  or  their  prey.  Often  they  are  most 
developed  in  the  males,  and  their  activity  increases  at  the  pairing 
season. 

Among  the  numerous  special  glands  may  be  noted  those  which  are  con- 
nected with  a  perforated  spur  on  the  hind-legs  of  male  Monotremes,  the 
sub-orbital  glands  of  antelopes  and  deer,  the  anal  glands  of  carnivores, 
the  perineal  glands  of  the  civet,  the  preputial  glands  of  the  musk-deer 
and  beaver,  the  inter-digital  glands  of  the  sheep. 

Most  characteristic,  however,  are  the  mammary  glands, 
functional  in  female  Mammals  after  parturition.  They 
seem  to  be  specialisations  of  sebaceous  glands,  except  in 
Monotremes,  in  which  they  are  nearer  the  sudorific  type. 
They  consist  of  branching  tubes  opening  by  one  or  several 
apertures  on  the  skin.  From  the  white  blood  corpuscles  of 
the  abundant  vascular  supply,  and  from  a  degeneration  of 
the  cells  lining  the  glandular  tubes,  the  milk  is  produced.  It 
begins  to  be  produced  when  the  young  are  born,  when,  in 
Placental  Mammals,  the  demand  upon  the  mother  through 
the  placenta  has  ceased. 

In  Monotremes,  the  simple  glands,  compressed  by 
muscles,  open  by  several  pores  on  a  bare  patch  of  skin. 
This  is  depressed  into  a  slight  cup  from  which  the  young 
lick  the  milk.  In  Marsupials,  the  glands  open  by  teats  or 
mammae,  generally  hidden  within  a  marsupium  ;  and  again 
the  action  of  surrounding  muscles  forces  the  milk  into  the 
mouths  of  the  young,  which  do  not  seem  to  be  able  to  suck. 
An  anterior  prolongation  of  the  larynx  to  meet  the  posterior 
nares,  establishes  a  complete  air  passage,  and  enables  the 
young  to  continue  breathing  while  they  are  being  fed. 
"  In  the  Cetacea,  where  the  prolonged  action  of  sucking 
would  be  incompatible  with  their  subaqueous  life,  the 


DENTITION.  643 

ducts  of  the  glands  are  dilated  into  large  reservoirs,  from 
which  the  contents  are  injected  into  the  mouth  of  the  young 
animal  by  the  action  of  a  compressor  muscle."  In  all  other 
Mammals  the  young  suck  the  milk  from  the  mammae. 

Dentition. 

The  teeth  of  Mammals  are  developed  in  the  gum  or  soft 
tissue  which  covers  the  borders  of  the  premaxillae,  maxillae, 
and  mandibles.  As  in  other  animals,  they  are  in  part  of 
epidermic,  in  part  of  dermic  origin.  In  the  course  of  their 
development  their  bases  are  enclosed  in  sockets  formed  in 
the  subjacent  bones. 

In  most  teeth  there  are  three  or  four  different  kinds  of 
tissue.  The  greater  part  consists  of  dentine  or  ivory  ;  out- 
side of  this  there  is  a  layer  of  very  hard  glistening  enamel ; 
in  the  interior  there  is  a  cavity  which  in  growing  teeth  con- 
tains a  gelatinous  tissue  or  pulp,  supplied  by  blood  vessels 
and  by  branches  of  the  fifth  nerve,  and  contributing  to  the 
increase  of  the  dentine  ;  lastly,  around  the  narrowed  bases 
or  roots  of  the  tooth,  or  between  the  folds  of  the  enamel 
if  these  have  been  developed,  there  is  a  bone-like  tissue 
called  the  crusta  petrosa  or  cement. 

The  development  of  teeth  begins  with  the  formation  of 
a  dental  ridge,  an  invagination  of  the  ectodermic  epithelium. 
From  this  ridge  a  number  of  "  enamel  germs  "  are  next 
differentiated.  Beneath  each  germ  a  papilla  of  the  vas- 
cular mesodermic  dermis  is  defined  off  as  the  "  dentine 
germ."  The  crown  of  this  papilla  becomes  hard,  and 
the  ossification  proceeds  downwards  and  inwards,  while 
above  the  dentine  crown  the  enamel  begins  to  form  a  hard 
cap.  Meantime  the  tissue  around  the  base  of  the  tooth- 
papilla  becomes  differentiated  into  an  enclosing  follicle  or 
sac,  from  the  inner  layer  of  which  the  cement  is  developed. 
The  pulp  is  but  the  uncalcified  core  of  the  papilla. 

The  base  of  a  tooth  may  remain  unconstricted,  and  the  core  of  pulp 
may  persist.  Such  a  tooth  goes  on  growing,  its  growth  usually  keeping 
pace  with  the  rate  at  which  the  apex  is  worn  away  with  use,  and  it  is 
described  as  "  rootless  "  and  "  with  persistent  pulp."  The  incisors  of 
Rodents  and  of  Elephants  illustrate  this  condition. 

In  the  development  of  most  teeth,  however,  the  base  is  narrowed 
and  prolonged  into  a  root  or  several  roots  which  become  firmly  fixed 
in  the  socket.  Through  a  minute  aperture  at  the  end  of  the  root, 


644  MAMMALIA. 

blood  vessels  and  nerves  still  enter  the  pulp  cavity  and  keep  the  tooth 
alive,  but  as  the  limit  of  growth  is  reached  the  residue  of  soft  pulp  tends 
to  disappear.  Of  these  "  rooted  "  teeth  there  are  many  kinds,  differing 
in  size  and  shape,  in  the  number  of  roots,  and  in  the  period  at  which 
these  are  definitely  established.  Mammals  also  differ  not  a  little  in 
regard  to  the  period  at  which  the  teeth — usually  concealed  at  the  time 
of  birth — appear  on  the  surface  or  cut  the  gum. 

Whereas  Fishes  and  Reptiles  have  a  practically  unlimited  succession 
of  teeth,  the  succession  in  Mammals  is  practically  limited  to  two  sets, 
though  traces  of  at  least  a  third  set  have  been  seen.  It  was  until 
recently  he  custom  to  distinguish  between  monophyodont  Mammals, 
with  only  one  set  of  teeth,  and  diphyodont  Mammals  with  two  sets. 
But  more  careful  investigation  has  shown  that  there  are  no  strictly 
monophyodont  Mammals.  Even  the  baleen  whales,  which  have  no 
functional  teeth  at  all,  have  the  rudiments  of  two  sets.  In  most  cases  we 
have  to  distinguish  a  more  important  replacing  set  which  is  functional 
through  the  greater  part  of  life,  and  a  less  important  transient  first  set  the 
members  of  which,  often  being  developed  during  the  period  of  sucking, 
are  called  milk  teeth.  The  milk  teeth  may  dwindle,  as  in  seals,  before 
or  shortly  after  birth  ;  or  they  may  remain,  as  in  Ungulates,  for  a  long 
time,  being  gradually  replaced  by  the  permanent  set  ;  or  they  may 
remain  as  the  permanent  dentition,  in  Marsupials  and  Cetaceans. 

Some  recent  investigators  distinguish  four  generations  of  teeth, 
viz.  : — 

ist,  or  pre-milk  dentition,  non-functional  vestiges,  e.g.^  Myrmecobius. 

2nd,  or  milk  dentition,  generally  functional  for  a  time,  permanent  in 
Marsupials  and  toothed  Cetaceans,  usually  in  great  part  tem- 
porary. In  most  Mammals,  except  Hyrax  and  a  few  others, 
the  first  premolar  is  a  persisting  milk  tooth. 

3rd,  or  replacing  dentition,  usually  the  permanent  dentition,  rudi- 
mentary in  Marsupials  and  Cetaceans  ; 

4th  dentition,  doubtful,  in  Phoca  (?),  in  Desmodus  (?),  sometimes  in 
man  (?). 

By  a  set  or  generation  of  teeth  we  mean  those  which  differentiate 
contemporaneously,  or  almost  contemporaneously,  from  the  dental 
ridge.  It  used  to  be  supposed  that  the  replacing  teeth  develop  from 
the  milk  set,  but  both  are  derived,  as  sister  dentitions,  from  the  dental 
ridge. 

M.  F.  Woodward,  in  careful  account  of  recent  work,  says  that  con- 
sideration of  the  facts  "leads  us  to  the  belief  (i)  that  the  living 
Mammalia  show  traces  of  from  three  to  four  distinct  generations  of 
teeth,  and  consequently  (2)  that  they  are  potentially  polyphyodont ; 
(3)  that  the  first  set  is  vestigial  and  not  functional  in  any  living 
mammal ;  (4)  that  the  second,  which  is  so  important  in  the  lower 
mammals,  is  more  or  less  replaced  by  the  third  in  the  higher  forms  ; 
and  (5)  that  this  third  dentition  remains  dormant  in  the  Marsupials  and 
Cetaceans." 

Mr.  Woodward  points  out  that  the  evolution  of  the  specialised 
heterodont  dentition  of  the  Mammalia  from  the  simple  homodont  and 
polyphyodont  dentition  of  the  lower  Reptilia  "  would  necessarily  cause 
a  reduction  in  the  number  of  successional  sets  of  teeth,  due  to  an 


DENTITION.  645 

enlargement  of  one  set  and  a  consequent  abstraction  of  growth-energy 
and  material  from  the  underlying  sets.  The  specialisation  would  not 
appear  in  the  first  generation  of  teeth,  which  must  necessarily  be  of 
small  size  from  its  early  development  and  consequent  adaptation  to  the 
small  jaw  of  the  young  animal,  and  which  would,  moreover,  be  required 
for  temporary  use,  while  the  larger  and  more  complicated  dentition  was 
developing.  The  increased  size  of  the  second  set  of  teeth  might  well 
abstract  the  growth-energy  from  the  succeeding  sets  and  retard  them." 
The  diet  of  milk  would,  however,  do  away  with  the  function  of  the 
first  teeth,  while  the  third  gradually  gained  in  prominence. 
The  following  series,  after  Leche,  is  of  interest — 

(1)  In  toothed  Cetaceans,  the  persisting  dentition  is  wholly  of  the 

milk  set  (Odontoceti). 

(2)  In  Marsupials,  the  persisting  dentition  is  of  the  first  set,  except 

the  third  (or  fourth)  premolar. 

(3)  In    the    hedgehog,  the  persisting   dentition   is   mixed,   thus  the 

incisors,  canines,  and   premolars  are   partly  of  the   milk  set 
and  partly  replacing  teeth. 

(4)  In  the  great  majority  of  Mammals  the  persisting  dentition  consists 

of  replacing  teeth,  excepting  in  most  the  first  premolar,  and 
also  excepting  (according  to  many)  the  molars. 

In  a  few  Mammals,  for  instance  in  the  dolphins,  the  teeth 
are  very  uniform,  almost  all  alike  from  beginning  to  end. 
Such  a  dentition  is  called  homodont,  in  contrast  to  the 
common  heterodont  dentition,  in  which  the  teeth  are  more 
or  less  markedly  different  in  form  and  function.  It  is 
necessary  now  to  consider  these  differences. 

In  the  typical  dentition  of  Mammals  there  are  forty-four 
permanent  teeth,  eleven  on  each  side  above  and  below. 
The  eleven  on  each  of  the  upper  jaws  may  be  divided  into 
four  sets.  Most  anteriorly,  associated  with  the  premaxilla, 
are  three  simple,  single-rooted  teeth,  usually  adapted  for 
cutting  or  seizing.  These  are  called  incisors.  Posteriorly, 
there  are  crushing  or  grinding  teeth,  whose  crowns  bear 
cusps  or  cones,  or  are  variously  ridged,  and  which  have  two 
or  more  roots  associated  with  the  maxilla.  But  of  these 
grinders,  the  last  three  occur  as  one  set,  having  no  suc- 
cessors. They  are  therefore  distinguished  as  true  molars, 
from  the  four  more  anterior,  and  often  simpler  premolars, 
which  occur  in  two  sets,  the  milk  set  being  replaced  by  a 
permanent  set,  except  in  most  cases  the  first.  Finally,  the 
tooth  just  behind  the  incisors,  that  is  to  say,  immediately 
posterior  to  the  suture  between  premaxilla  and  maxilla,  is 
distinguished  as  the  canine,  and  is  often  long  and  sharp. 


646  MAMMALIA. 

There  is  still  much  uncertainty  in  regard  to  the  nature  of  the  molar 
teeth.  Thus  some  regard  them  as  resulting  from  a  fusion  of  several 
dentitions,  others  regard  them  as  milk  teeth  which  are  not  replaced, 
others  as  belonging  to  the  replacing  set.  There  seems  no  doubt  that 
traces  of  both  preceding  and  succeeding  dental  structures  have  been 
seen  associated  with  molar  teeth ;  the  question  is,  which  set  has  been 
suppressed  ? 

This  classification  of  teeth  is  in  great  part  one  of  convenience  ;  thus, 
the  distinction  between  incisors  and  grinding  teeth  is  anatomical,  that 
between  molars  and  premolars  refers  to  the  history  of  these  teeth  ;  the 
connection  between  the  teeth  and  the  subjacent  bones  is  a  secondary 
matter  ;  there  is  often  little  to  differentiate  canine  from  premolar. 
Moreover,  the  teeth  of  the  lower  jaw,  which  is  a  single  bone  on  each 
side,  cannot  be  so  certainly  classified  as  those  of  the  upper  jaw. 

No  part  of  a  Vertebrate  is  more  distinctive  than  the  skull,  and  no 
mammalian  characteristic  is  more  useful  in  diagnosis  than  the  dentition. 
It  ^convenient,  therefore,  to  have  some  notation  expressing  the  nature 
of  the  dentition.  Thus  we  use  "  dental  formulae,"  in  which  the  incisors, 
canines,  premolars,  and  molars  are  enumerated  in  order,  and  in  which 
the  teeth  of  the  upper  jaw  are  ranked  above  the  analogous  teeth  of  the 
lower  jaw.  The  typical  mammalian  dentition  already  referred  to  may 
be"expressed  as  follows  : — 

3—3  i— i  4—4  3—3      ii— ii 

Incisors ,  canines ,  premolars ,  molars = =  total  44. 

3—3  i— i  4—4  3—3      ii— ii 

or  using  initial  letters  : — 

.   3—3        i— i  4—4         3—3 

i. ,  c. ,  pm. ,  m. =  44. 

3—3        i— i          4—4         3—3 

or,  recognising  that  the  right  and  left  side  are  almost  invariably  identical. 

and  omitting  the  initial  letters  : — 3-^3  • 

3143 

We  may  cite  the  formulae  for  the  adult  dentition  of  some  repre- 
sentative mammals  : — 

5134  4134  3024  1014         3143  1133 

Opossum ,  Thylacine ,  Kangaroo ,  Wombat ,  Pig ,  Camel , 

4134  3134  1024  1014  3143  3123 

oo33  3i43  2033  3131  3142  3142  3141 

Sheep  ,  Horse  ,  Rabbit  ,  Cat  ,  Dog  ,  Bear  ,  Seal  

3133  3133  1023  3121  3143  3143  2141 

3133  2132  2133  2123 

Hedgehog ,  Marmoset ,  New  World  Monkey ,  Old  World  Monkey 

2123  2132  2133  2123 

2123 

Man . 

2123 

It  js  more  interesting,  however,  to  try  to  associate  different  kinds  of 
dentition  with  different  kinds  of  diet.  Thus,  dolphins,  which  feed  on 


DEVELOPMENT  AND  PLACENTATION.  647 

fish  and  swallow  them  whole,  have  numerous  almost  uniform,  sharp, 
recurved,  conical  teeth,  well  suited  to  take  a  firm  grasp  of  the  slippery 
and  struggling  booty.  To  a  slight  extent  the  same  piscivorous  dentition 
may  be  seen  in  seals.  In  the  more  strictly  carnivorous  mammals,  the 
incisors  are  small ;  the  canines  are  long  and  sharp,  piercing  the  prey  with 
a  deathful  grip,  while  the  back  teeth  have  more  or  less  knife-like  edges 
which  sever  flesh  and  bone.  In  typical  insectivorous  mammals  the  upper 
and  lower  incisors  meet  precisely,  ' '  so  as  readily  to  secure  small  active 
prey,  quick  to  elude  capture  but  powerless  to  resist  when  once  seized," 
while  the  crowns  of  the  molars  bear  many  sharp  points.  Herbivorous 
mammals  have  front  teeth  suited  for  cropping  the  herbage  or  gnawing 
parts  of  plants,  the  canines  are  small  or  absent,  the  molars  have  broad 
grinding  crowns  with  transverse  ridges.  In  omnivorous  mammals,  the 
incisors  are  suited  for  cutting,  the  canines  are  often  formidable  weapons 
in  the  male  sex,  the  molars  have  crowns  raised  into  rounded  tubercles. 

It  is  likely  that  the  most  primitive  type  of  mammalian  tooth  was  a 
simple  cone,  such  as  may  be  seen  in  toothed  whales.  In  some  of  the 
extinct  mammals,  e.g.^  Triconodon,  the  tooth  is  a  main  cone  with  two 
lateral  cusps,  and  this  type  leads  to  what  is  called  the  tritubercular 
tooth,  in  which  the  crown  bears  three  cusps  disposed  in  a  triangle. 
From  this  tritubercular  type  most  of  the  more  complex  forms  of  teeth 
may  be  derived.  But  it  remains  doubtful  whether  the  tritubercular  type 
is  the  result  of  the  fusion  of  three  cones,  or  the  budding  of  one. 

Development  and  Placentation. 

The  ova  of  placental  mammals  are  small ;  even  those  of 
the  Whales  are  "  no  larger  than  fern  seed."  They  are  formed 
from  germinal  epithelium,  the  cells  of  which  grow  inwards 
in  clustered  masses  into  the  connective  tissue  or  stroma  of 
the  ovary.  In  each  cluster  one  cell  predominates  over  its 
neighbours;  it  becomes  an  ovum ;  the  others  invest  and 
nourish  it,  and  are  called  follicle  cells. 

In  the  middle  of  each  clump  or  Graafian  follicle,  a  cavity 
is  formed  containing  fluid,  and  into  this  cavity  the  follicle 
cells  immediately  surrounding  the  ovum  project  as  what  is 
called  the  discus  proligerus. 

When  mature  the  ovum  protrudes  on  the  surface  of  the 
ovary,  and  is  liberated  by  the  bursting  of  the  Graafian 
follicle.  Some  blood,  which  fills  up  the  empty  follicle, 
degenerates  into  what  is  called  the  corpus  luteum. 

The  spermatozoa  are  formed  from  germinal  epithelium  in 
the  testes.  The  primitive  male  cells  or  spermatogonia  give 
rise  by  division  to  daughter  cells  or  spermatocytes,  which 
with  or  without  further  division  form  spermatozoa. 

The  homologue  of  the  ovum  is  the  spermatogonium  or 


648 


MAMMALIA. 


mother-sperm-cell,  but  the  physiological  equivalent  of  the 
ovum  is  the  spermatozoon. 

No    one   has    succeeded   in    satisfactorily  observing    an 
extrusion  of  polar  bodies  in  the  maturation  of  the  mam- 
malian  ovum,  but  analog- 
ous processes  occur  at  an 
early  stage. 

The  ovum  having  burst 
from  the  ovary  is  imme- 
diately caught  by  the  fim- 
briated  mouth  of  the  Fal- 
lopian tube  and  begins  to 
pass  down  the  oviduct. 
There  it  is  met  by  ascend- 
ing spermatozoa,  received 
by  the  female  as  the  result 
of  sexual  union,  and  is 
fertilised.  One  of  the 
spermatozoa  enters  the 
ovum,  and  sperm  nucleus 
unites  with  ovum  nucleus 
in  an  intimate  and  orderly 
manner.  It  is  interesting 
to  remember  that  it  was 
only  in  1843  that  the  union 
of  spermatozoon  and  ovum 
was  for  the  first  time  de- 
tected by  Martin  Barry, 
and  in  the  case  of  the 
rabbit. 

The  Connection  between 
Embryo  and  Mother. — (a) 
The  lowest  Mammals,  the 
Duckmole  ( Ornithorhyn- 
chus)  and  the  Porcupine 
Ant  Eater  (Echidna)  re- 


FIG.  224. — Segmentation  of  Rab- 
bit's ovum.    (After  VAN  BENEDEN.  ) 

semble     Birds    and    most 

Reptiles  in  bringing  forth 

their  young  as  eggs,  i.e.,  in 

being  oviparous.     The  eggs  are  large,  with  a  considerable 

quantity  of  yolk,  and  after  fertilisation  divide  partially,  /.*., 


e.c.,  External  cells  (epiblast) ;  i.e.,  in- 
ternal cells  (hypoblast) ;  b.v.,  blastodermic 
vesicle. 


DEVELOPMENT  AND  PLACENTATION. 


649 


exhibit  meroblastic  segmentation  like  the  eggs  of  Birds 
and  Reptiles.  The  tunic  formed  round  about  them  in  the 
Graafian  follicles  of  the  ovary  consists  as  in  Birds  and 

Reptiles  of  a  single  layer 
of  cells.  As  they  develop 
they  are  unattached  to  the 
walls  of  the  oviducts. 
u.  Y/  I  They  are  laid  in  a  nest 

by  the  Duckmole;   in  the 

•••y*^J5fe'-r£PL  Echidna  they  are  hatched 

jf  \  in  a  slight,  periodically  de- 

/•'  veloped,  external  pouch. 

I  2  (b)    In   the  •  Marsupials, 

Jr  the     connection     between 

mother  and  offspring  has 
become  closer.  The  em- 
bryo is  born  alive,  though 
prematurely,  after  a  short 
uterine  life,  during  which, 
however,  it  is  either  not 
vitally  attached  at  all  to 
the  uterus,  or  only  to  a 
slight  degree  by  villi  from 
the  yolk  sac.  In  the  opos- 
sum, it  lies  surrounded  by 
a  quantity  of  nutritive  al- 
buminoid material.  Here 
it  may  be  recalled  that  in 
two  Elasmobranch  fishes 
and  in  two  lizards,  there 
is  a  connection  between 
the  yolk  sac  of  the  embryo 
and  the  wall  of  the  ovi- 
duct. As  we  shall  see, 
there  is  a  preliminary  yolk 
sac  placenta  in  three  orders 
of  Placentals. 

(c)  In  all  the  other  Mam- 
mals, the  maternal  sacrifice 


B.v 


FIG.  225.  —  Development  of 
Hedgehog.  Three  early  stages. 
(After  HUBRECHT.) 

I.  Shows  internal  vesicle  of  hypoblast, 
the  disc  and  external  sheath  of  epiblast.  II. 
Shows  villi  arising  from  trophoblast  ;  the 
disc  of  formative  epiblast  (Ep.}  ;  the  blasto- 
dermic  vesicle  (B.v.)  III.  A  more  ad- 
vanced stage,  TV.,  trophoblast  ;  Ep.,  di.sc 
of  formative  epiblast  ;  B.v.,  blastodermic 
vesicle  ;  H.,  hypoblast. 


prior  to  birth  is  greater,  for  a  close  connection  is  established 
between  the  embryo  and  the  wall  of  the  uterus,  by  means 


650 


MAMMALIA. 


of  a  special  adaptation — the  placenta.  This,  in  rough 
physiological  language,  is  a  double  vascular  sponge,  partly 
embryonic,  partly  maternal,  by  means  of  which  the  blood 
of  the  mother  nourishes  and  purifies  that  of  the  embryo. 

As  many  of  the  most  fundamental  structural  and  func- 
tional problems  in  connection  with  placentation  are  still 
being  investigated,  it  is  impossible  to  discuss  even  the  lead- 
ing questions  with  defmiteness  and  certainty.  The  authority 
here  followed  is  Hubrecht,  in  his  study  of  the  placentation 
of  the  hedgehog,  which  is  at  once  a  simple  and  a  central 
type. 

First,  then,   let  us  seek  to    define   the   embryonic   and 
maternal  structures  which  are  associated  with  placentation. 
(i)  At  a  very  early  stage,  the  divided  ovum  of  the  hedgehog 
consists  of  a  sac  of  cells, 
an    outer    layer,    epiblastic 
or    ectodermic,     enclosing 
another       aggregate  —  the 
future    inner    layer,    endo- 
derm    or    hypoblast    (Fig. 
226,  I.).     (2)  The  epiblast 
divides  into  an  embryonic 
disc  which   will   form    the 
epidermis,  nervous  system, 
&c.,    of   the   embryo,    and 
an  external  layer,  the  wall 

of    the   embryonic   sac   or        FlG>   22(5._Two  stages   in  seg. 
blastocyst,   with   which   the     mented  ovum  of  Hedgehog.     (After 
disc    retains  a    slight  con-     HUBRECHT.) 
nection    until    the    protec-  E^  Epibiast;  //,.,  Hypoblast. 

tive     amnion     is    formed. 

In  the  outer  epiblastic  wall  lacunae  develop,  which  are 
bathed  by  the  maternal  blood,  and  the  pillars  of  tissue 
between  the  lacunae  grow  out  into  villi,  which  aid  in 
this  earliest  connection  between  mother  and  offspring. 
Long  before  any. vascular  area  or  foetal  placenta  is  devel- 
oped, the  outer  epiblastic  wall  has  the  above  nutritive  func- 
tion, and  deserves  its  name  of  trophoblast  (Fig.  225,  Tr.). 
(3)  The  hypoblast  or  inner  mass,  which  is  at  first  a  solid 
aggregate  of  cells  (Fig.  224,  i.c.\  becomes  a  sac,  as  a  morula 
may  become  a  blastosphere.  The  upper  part  of  this  sac 


DEVELOPMENT  AND  PLACENTATION.  651 

forms  the  lining  of  the  incipient  gut,  while  the  lower  por- 
tion, following  the  contour  of  the  blastocyst  wall,  becomes 
the  yolkless  yolk  sac  or  umbilical  vesicle.  Its  connection  with 
the  upper  part  is  narrowed  into  a  canal — the  vitelline  duct, 
which  is  part  of  the  "  umbilical  cord,"  entering  the  embryo 
at  the  future  navel.  (4)  Between  the  epiblast  and  the  hypo- 
blast  of  the  embryo,  the  mesoblast  develops,  splitting  into 
an  outer,  parietal,  or  somatic,  and  an  inner,  visceral,  or 
splanchnic  layer.  The  cavity  between  these  is  the  incipient 
body  cavity.  A  double  fold  of  somatic  mesoblast,  carrying 
with  it  a  single  sheet  of  epiblast,  rises  up  round  about  the 
embryo,  arching  over  it  to  form  the  amnion.  Over  the 
embryo  the  folds  of  amnion  meet  in  a  cupola,  and  the  inner 
layers  of  the  double  fold  unite  to  form  the  "amnion  proper," 
while  the  outer  layers  also  unite  to  form  a  layer  lying  inter- 
nally to  the  epiblastic  blastocyst  wall, — and  termed  by  Sir 
William  Turner  the  subzonal  membrane.  The  folds  of 
amnion  are  continued,  as  the  diagram  shows,  ventrally  as 
well  as  dorsally,  so  that  the  subzonal  membrane  surrounds 
the  embryo  beneath  the  blastocyst  wall,  while  a  splanchnic 
layer  of  mesoblast  grows  round  about  the  hypoblastic  yolk 
sac.  The  space  between  the  two  layers  of  mesoblast,  which 
are  shortly  termed  somatopleure  and  splanchnopleure,  is 
obviously  continuous  with  the  body  cavity  of  the  embryo. 
The  epiblastic  outer  wall  or  trophoblast,  and  the  mesoblastic 
subzonal  membrane,  are  included  in  Hubrecht's  term — 
diplotrophoblast.  (5)  From  the  hind  wall  of  the  gut  there 
grows  out  a  hypoblastic  sac,  the  allantois,  insinuating  itself 
and  spreading  out  in  the  space  between  the  two  layers  of 
mesoblast.  As  an  outgrowth  of  the  gut,  homologous  with 
the  bladder  of  the  frog,  the  allantois  is  of  course  lined  by 
hypoblast  or  endoderm,  but  it  is  covered  externally  by  a 
layer  of  mesoblast,  which  it  bears  with  it  as  it  grows.  In  all 
placental  mammals  the  allantois,  which  becomes  richly 
vascular,  unites  with  the  subzonal  membrane,  and  therefore 
with  the  external  epiblast  as  well,  to  form  the  foetal  part  of 
the  placenta,  with  outgrowing  vascular  processes  or  villi 
which  fit  into  corresponding  depressions  or  crypts  on  the  wall 
of  the  uterus.  [To  the  mesoblastic  wall  of  the  allantois,  plus 
the  subzonal  membrane,  the  term  "  chorion  "  is  sometimes 
applied,  but  as  the  word  has  been  used  in  many  different 


652 


MAMMALIA. 


senses,  its  abandonment,  except  perhaps  in  human  embryo- 
logy, is  almost  imperative.]  The  complex  union  of  allan- 
tois  with  diplotrophoblast, 
Hubrecht  calls  the  allan- 
toidean  trophoblast.  (6)  But 
in  the  hedgehog,  rabbit,  and 
some  other  types,  there  is  a 
mode  of  embryonic  nutrition 
between  that  attained  by  the 
epiblastic  trophoblast  and  that 
effected  by  the  final  placenta. 
The  wall  of  the  yolk  sac, 
hypoblastic  internally,  meso- 
blastic  externally,  unites  with 
the  subzonal  membrane,  and 
becomes  the  seat  of  villous 
processes,  which  through  the 
external  epiblast  are  connected 
with  the  uterine  wall.  Thus 
is  formed  what  Hubrecht  calls  ' 
an  omphaloidean  trophoblast. 
Neither  omphaloidean  nor 
allantoic  villi  ever  directly 
interlock  with  maternal  tissue, 
but  always  through  the  agency 
of  the  external  epiblastic 
trophoblast. 

(7)  It  is  now  time  to  turn  for 
a  little  to  the  maternal  tissue. 

FIG.  227. — Development  of  Foetal 
Membranes.  (After  HERTWIG.) 

Uppermost  figure  shows  up-growth  and 
down-growth  of  amnion  folds.  E.,  em- 
bryo; a.f.,  amnion  fold;  «i.,  amnion 
proper;  #2.,  subzonal  membrane;^-.,  the 
gut ;  y.t  umbilical  vesicle  or  yolk  sac. 
The  dotted  line  represents  mesoderm,  the 
dark,  hypoblast.  The  second  figure  shows 
origin  of  allantois,  and  the  amnion  folds 
have  met.  The  third  figure  shows  increase 
of  allantois  (#/.)  ;  the  dwindling  yolk  sac 
(y-s.) ;  a.c.,  amniotic  cavity;  s.z.m.,  sub- 
zonal  membrane.  The  fourth  figure 
shows  the  embryo  apart  from  its  mem- 
branes ;  in.,  mouth;  a.,  anus.  Note 
umbilical  connection  with  yolk  sac. 


DEVELOPMENT  AND  PLACENTATION.  653 

The  embryo  lay  at  first  in  a  groove  of  the  uterine  wall, 
moored  by  the  preliminary  blastocyst  villi,  which  are  as  it 
were  pathfinders  for  those  subsequently  developed  from 
yolk  sac  and  allantoic  regions.  At  the  point  of  attachment, 
the  mucous  lining  of  the  uterus  ceases  to  be  glandular,  and 
becomes  much  more  vascular.  As  the  embryo  becomes 
fixed,  the  blastocyst  almost  eating  its  way  in,  the  outer 
epithelium  degenerates  and  disappears ;  below  this  the  outer 
layer  of  the  mucous  membrane  becomes  spongy  and  exhibits 
unique  blood  spaces,  forming  what  Hubrecht  calls  the  tro- 
phospongia;  below  this  there  is  the  vascular  and  vitally 
active  remainder  of  the  mucosa,  less  modified  than  the 


FIG.  228. — Diagram  of  Foetal  Membranes.     (After 
TURNER.) 

£,  Embryo  ;  ff,  gut  lined  by  hypoblast  dotted  ;  the  dark  is  meso- 
blast  ;  UV,  umbilical  vesicle  or  yolk  sac;  AC,  amniotic  cavity; 
ant,  amnion  proper  ;  sz>,  sub-zonal  membrane  ;  AL,C,  allantoic 
cavity  ;  al.,  allantois  ;  J./.,  may  be  here  taken  to  represent  the  early 
epiblastic  trophoblast. 

above  mentioned  sponge ;  below  this  again,  there  are  the 
muscular  and  other  elements  of  the  uterine  wall,  with  which 
we  are  not  now  concerned.  The  most  important  fact  to 
emphasise  is,  that  the  maternal  blood  in  the  spaces  of  the 
spongy  outer  layer  of  the  mucous  membrane  directly  bathes 
the  foetal  tissue  represented  by  the  trophoblast.  By  the 
activity  of  the  trophoblast  cells,  the  nutritive  and  respiratory 
advantages  of  the  maternal  blood  are  secured  for  the  villi  of 


654  MAMMALIA. 

the  allantois  and  yolk  sac.  It  ought  also  to  be  mentioned 
that  mainly  by  a  folding  of  the  uterine  wall,  the  hedgehog 
embryo  is  virtually  enclosed  in  a  maternal  sheath,  homo- 
logous with  a  fold  called  the  decidua  reflexa  in  human 
embryology,  and  analogous  with  a  similar  capsule  in  the 
rabbit. 

To  sum  up  : — 

(1)  At  an  early  stage,  a  wall  of  epiblast  encloses  an  aggre- 

gate of  hypoblast  (Figs.  224,  225,  I.,  226). 

(2)  The  epiblast  divides  into  an  embryonic  disc  and  an 

outer  blastocyst  wall,  with  fixing  and  nutritive  func- 
tions,— the  trophoblast  (Figs.  225,  I.  and  II.). 

(3)  The  hypoblast  becomes  a  sac,   of  which  the  upper 

portion  lines  the  gut,  while  the  lower  part  forms  the 
yolk  sac  (Fig.  225,  III.). 

(4)  The  mesoblast  divides  into  somatic  and  splanchnic 

layers ;  a  double  fold  of  the  somatic  layer  (along  with 
a  slight  sheet  of  epiblast)  forms  the  amnion,  of  which 
the  outer  limbs  unite  as  the  subzonal  membrane,  and 
form  along  with  the  external  epiblast — the  diplotro- 
phoblast.  The  splanchnic  layer  of  the  mesoblast  is 
continued  round  the  yolk  sac  (Fig.  227). 

(5)  The  allantois  grows  out  from  the  hind  region  of  the 

gut,  being  lined  internally  by  hypoblast,  externally  by 
splanchnic  mesoblast.  The  allantois  plus  the  diplo- 
trophoblast  always  forms  the  true  placenta  (Fig. 
228). 

(6)  Part  of  the  yolk-sac  wall,  uniting  with  the  diplotro- 

phoblast,  also  forms  an  efficient  but  temporary 
placenta. 

(7)  At  the  area  of  fixing  the  uterine  epithelium  degenerates, 

the  glands  disappear,  vascularity  increases.  The 
outer  part  of  the  modified  mucous  membrane  (or 
decidua)  becomes  a  spongy  tissue,  with  spaces  filled 
with  maternal  blood.  This  maternal  blood  bathes 
the  trophoblast,  which  is  intermediate  between  it  and 
the  placental  villi. 

The  three  modes  of  embryonic  nutrition  are  as  follows : — 
(a)  At  first  the  maternal  blood  bathes  the  lacunae  in  the 
epiblasttc  outer  wall — the  trophoblast  with  its  pre- 
liminary path-finding  villi. 


CLASSIFICATION  OF  PLACENTATION. 


655 


(b)  An  efficient  yolk  sac  placenta  functions  for  a  time, 

but  decreases  and  shrivels  as  the  final  allantoidean 
placenta  develops.  The  maternal  blood  in  the  spaces 
of  the  outer  layer  of  the  mucous  layer  of  the  uterus 
bathes  the  trophoblast.  Thus  it  comes  into  indirect 
connection  with  the  vascular  villi  from  the  region 
where  the  yolk-sac  wall  unites  with  the  diplotropho- 
blast.  This  yolk  sac  placenta  is  found  in  Insectivora, 
Chiroptera,  and  Rodents. 

(c)  The   final  placenta   is   allantoidean,    it   replaces   the 

yolk-sac  placenta,  if  there  be  one.  In  Insectivora 
Chiroptera,  and  Rodentia,  and  probably  in  other 
cases,  the  trophoblast  is  always  intermediate  between 
the  maternal  blood  and  the  villi,  and  is  the  only 
intervening  tissue. 


Caducous 

or 

Deciduate. 

(Vascular 

parts  of 

maternal 

placenta 

come 

away 

at  birth). 


THE  CUSTOMARY  CLASSIFICATION  OF  PLACENTATION 
IS  AS  FOLLOWS  : — 

Meta-DiscoidaL — Villi  at  first  scattered  a.re\Homo  and 
restricted  to  a  disc.         j  Monkeys. 

The  maternal  mucous  membrane  forms  a 
capsule  around  embryo  (decidua  reflexa, 
also  seen  in  hedgehog). 


Non-Caducous 

or 

Indeciduate. 
(Maternal 

part  of 

placenta  does 

not  come  away 

at  birth). 


Discoidal. — Villi   on   a    circular  I  \ 

cake  like  disc          \  Insectlvora  and  Chiroptera. 
(Most  Edentata. 

^Carnivora. 

Elephants  and  Hyrax. 
Orycteropus  and  Dasypus 

among  Edentata. 
Dugong  (in  whole  or  in  great 

part  non-deciduate). 

Cotyledonary. — Villi  in  patches.         Ruminants. 


Zonary. — Villi  on  a  partial 
or  complete  girdle- 
round  the  embryo. 


{Lemurs. 
Most   Ungulates   except 
Ruminants. 
Cetacea. 
Manis  among  Edentata. 


[TABLE. 


656 


MAMMALIA. 


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THE   RABBIT  AS  A    TYPE   OF  MAMMALS.         657 

While  Sir  William  Turner,  "  the  grand  master  of  placental 
research,"  in  his  arrangement  of  placentas,  allots  the  lowest 
place  to  such  diffuse  forms  as  that  of  the  pig,  passes  thence 
to  the  cotyledonary  of  Ruminants,  thence  to  the  zonary 
of  Carnivores,  and  finally  to  the  discoidal  of  monkeys  and 
man,  others  maintain  that  the  discoidal,  as  illustrated  in  the 
Insectivora,  is  the  most  primitive  type. 

To  avoid  confusion  it  may  be  better,  as  Hubrecht  sug- 
gests, to  revert  to  the  old  terms  caducous  and  non-caducous, 
instead  of  deciduate  and  indeciduate,  for  all  placental  mam- 
mals have  a  "  decidua,"  i.e.,  a  specially  active  region  of  the 


FIG.  229. — View  of  Embryo  with  its  foetal  membranes. 
(After  KENNEL.) 

am.,  Amnion  ;  d.,  dwindled  yolk  sac  ;  al.t  allantois  ;  a/.i,  sub- 
zonal  membrane  ;  z.,  villi. 

mucous  membrane  of  the  uterus  to  which  the  embryo  is 
attached.  Moreover,  the  distinction  between  deciduate  and 
indeciduate  is  one  of  degree,  for  no  sharp  line  can  be 
drawn  between  the  two  types. 

THE  RABBIT  as  a  type  of  Mammals. 

The  rabbit  (Lepus  cuniculus)  is  a  familiar  representative  of 
the  Rodent  order,  to  which  rats  and  mice,  voles  and  beavers, 
42 


658  MAMMALIA. 

lemmings  and  marmots  also  belong.  Like  the  hare  (Lepus 
timidus)  and  other  species  of  the  same  genus,  and  like  the 
Picas  or  tailless  hares  (Lagomys)^  the  rabbit  has  two  pairs  of 
incisors  in  the  upper  jaw,  while  other  Rodents  have  a  single 
pair.  Therefore  the  genera  Lepus  and  Lagomys  are  ranked 
in  the  sub -order  Duplicidentata,  in  contrast  to  all  other 
Rodents  which  form  the  sub-order  Simplicidentata. 

With  the  rabbit's  mode  of  life  all  are  familiar.  It  is  herbi- 
vorous, and  often  leaves  softer  food  for  the  succulent  bark 
of  young  trees  ;  it  is  gregarious  and  a  burrower ;  it  is  very 
prolific,  often  breeding  four  to  eight  times  in  a  year.  It  is 
said  to  live,  in  normal  conditions,  seven  or  eight  years.  The 
rabbit  seems  to  have  had  its  original  home  in  the  western 
Mediterranean  region,  but  it  has  spread  widely  throughout 
Europe,  and  is  now  abundant  in  countries,  such  as  Scotland 
and  Ireland,  in  which  not  many  years  ago  it  was  quite  rare. 
Introduced  into  Australia  and  New  Zealand  it  has  multiplied 
exceedingly,  and  has  become  a  scourge.  There  are  many 
varieties  of  rabbit,  some  in  isolated  regions  perhaps  illustra- 
ting the  effect  of  segregation  in  fostering  divergent  types. 
According  to  Darwin,  the  rabbits  introduced  early  in  the 
fifteenth  century  into  Porto  Santo,  an  island  near  Madeira, 
are  now  represented  by  a  dwarf  race  of  about  half  the  normal 
size,  and  these  are  said  to  be  incapable  of  breeding  with  the 
ordinary  forms.  But  the  varieties  with  which  we  are  familiar 
in  the  breeds  of  tame  rabbits,  illustrate  variation  under 
domestication  and  the  efficacy  of  artificial  selection. 

External  Appearance. 

The  head  bears  long  external  ears  which  are  freely  mov- 
able. The  black  patch  at  the  tip  of  the  ears  in  the  hare 
is  either  absent  or  very  small  in  the  wild  rabbit.  This 
external  ear  is  characteristic  of  most  Mammals,  and  collects 
the  sound  like  an  ear  trumpet.  In  the  rabbit  it  is  longi- 
tudinally folded,  thin  and  soft  towards  its  tip,  firm  and 
cartilaginous  at  its  base.  The  large  eyes  have  eyelids 
with  few  eyelashes,  and  a  third  eyelid  or  nictitating  mem- 
brane— a  white  fold  of  skin — lies  in  the  anterior  corner. 
This  third  eyelid,  which  also  occurs  in  Reptiles  and  Birds, 
is  present  in  most  Mammals,  and  is  of  use  in  cleaning  the 
cornea.  It  is  absent  in  Cetaceans,  where  the  front  of  the 


SKIN  AND  MUSCLES.  659 

eye  is  bathed  by  the  water,  and  it  is  rudimentary. in  man 
and  monkeys  where  its  absence  is  compensated  for  by  the 
habitual  winking  of  the  upper  eyelid.  The  nostrils  are  two 
slits  at  the  end  of  the  snout,  and  are  connected  with  the 
mouth  by  a  "  hare  lip  "  cleft  in  the  middle  of  the  upper  lip. 
In  front  of  the  mouth  are  seen  the  chisel-edged  incisors,  a 
pair  on  the  mandibles,  and  two  pairs  on  the  premaxillae, — 
the  smaller  pair  hidden  behind  the  larger  pair.  The  first 
milk  incisors  above  and  below  never  cut  the  gum,  but  are 
absorbed  before  birth  ;  the  second  milk  incisors  above 
(there  are  none  below)  are  functional,  but  are  shed  about 
the  third  week  of  extra-uterine  life ;  the  same  is  true  of 
the  milk  premolars.  Into  the  toothless  gap  or  diastema 
between  the  front  and  back  teeth,  the  hairy  skin  of  the  lips 
projects  into  the  mouth.  On  the  sides  of  the  snout,  and 
about  the  eyes,  there  are  tactile  hairs  or  vibrissae. 

The  plump  trunk  is  separated  from  the  head  by  a 
short  neck.  The  tail  is  very  short,  but  in  the  scampering 
wild  rabbit  it  is  conspicuous  as  a  white  tuft,  which  some 
naturalists  interpret  as  a  directive  signal.  Beneath  the  base 
of  the  tail  the  food  canal  ends,  and  beside  the  anus  are  the 
openings  of  the  perineal  glands,  whose  secretion  has  a  charac- 
teristic odour.  In  front  of  the  anus  is  the  urinogenital 
aperture, — in  the  male  at  the  end  of  an  ensheathed  penis, 
in  the  female  a  slit  or  vulva,  with  an  anterior  process  or 
clitoris — the  homologue  of  the  penis.  Beside  the  penis  in 
the  male  lie  the  scrotal  sacs,  into  which  the  testes  descend 
when  the  rabbit  becomes  sexually  mature.  Along  the 
ventral  surface  of  the  thorax  and  abdomen  in  the  female 
there  are  four  or  five  pairs  of  small  teats  or  mammae. 

The  limbs  have  clawed  digits,  five  on  the  fore  feet,  four 
on  the  hind  feet ;  they  are  very  hairy. 

Skin  and  Muscles. 

The  skin  is  thickly  covered  with  hair,  and  has  the  usual 
sebaceous  and  sudorific  glands,  besides  special  glands,  such 
as  the  perineal  glands  beside  the  anus,  the  glands  of  the 
eyelids,  the  lachrymal  glands,  and  the  mammary  glands 
developed  in  the  females.  Between  the  skin  and  the 
subjacent  muscles  there  is  a  layer  of  fatty  tissue,  known  as 
the  panniculus  adiposus ;  it  is  present  in  all  Mammals 


660  MAMMALIA. 

except  the  common  hare,  and  forms  the  blubber  of  whales 
and  seals.  Beneath  the  skin  is  a  thin  sheet  of  muscle  (the 
panniculus  carnosus),  and  when  this  is  removed  with  the 
skin,  many  of  the  muscles  of  head  and  neck,  limbs  and 
trunk  are  disclosed.  [The  student  who  wishes  to  study 
these,  and  to  compare  them  with  their  homologues  in  man, 
will  find  practical  directions  in  Parker's  ZootomyJ\ 

The  Skeleton.  t 

The  bones,  like  those  of  other  Vertebrates,  are  developed 
either  as  replacements  of  pre-existent  cartilages,  or  indepen- 
dent of  any  such  preformations,  but  in  all  cases  through 
the  agency  of  active  periosteal  membranes.  By  themselves, 
however,  must  be  ranked  little  sesamoid  bones,  which  are 
developed  within  tendons  and  near  joints,  notably,  for 
instance,  the  patella  or  knee  pan.  There  is  no  bony  exo- 
skeleton  in  any  mammals  except  the  armadillos,  unless  we 
rank  the  teeth,  which  develop  in  connection  with  the  skin 
of  the  jaws,  as  in  a  sense  exoskeletal.  The  vertebral  centra 
of  Mammals,  except  in  Monotremes  and  Sirenians,  have 
distinct  terminal  epiphyses,  and  the  same  distinctness  of 
ossification  is  seen  in  many  of  the  larger  bones. 

Vertebral  Column. 

The  vertebrae  may  be  grouped  as  cervical  (seven  in 
number),  thoracic  (with  ribs),  lumbar  (without  ribs),  sacral 
(fused  to  support  the  pelvis),  and  caudal.  The  faces  of  the 
centra  are  more  or  less  flat,  and  between  adjacent  vertebrae 
there  are  inter-vertebral  discs  of  fibro-cartilage. 

The  first  vertebra  or  atlas  is  ring-like,  its  neural  canal 
being  very  large,  its  centrum  unrepresented  except  by  the 
odontoid  process  which  fuses  to  the  second  vertebra.  The 
ring  is  divided  transversely  by  a  ligament,  through  the  upper 
part  the  spinal  cord  passes,  into  the  lower  the  odontoid 
process  projects.  The  transverse  processes  are  very  broad  ; 
the  articular  surfaces  for  the  two  condyles  of  the  skull  are 
large  and  deep. 

The  second  vertebra  or  axis  has  a  broad  flat  centrum  pro- 
duced in  front  in  the  odontoid  process.  The  neural  spine 
forms  a  prominent  crest,  the  transverse  processes  are  small, 
the  anterior  articular  surfaces  are  large. 


THE  SKULL.  661 

A  typical  lumbar  vertebra  will  show  the  centrum  and  its 
epiphyses,  the  neural  arch  and  neural  spine,  the  transverse 
processes,  the  anterior  and  posterior  articular  processes  or 
zygapophyses,  the  median  ventral  hypapophysis,  the  small 
anapophyses  from  the  neural  arch  below  the  posterior 
zygapophyses,  below  the  anapophyses  the  posterior  inter- 
vertebral  notches — passages  through  which  the  spinal  nerves 
pass  out,  and  anteriorly  a  similar  pair  of  notches.  There  are 
twelve  or  thirteen  pairs  of  ribs  which  support  the  wall  of  the 
thorax,  and  aid  in  the  mechanism  of  respiration.  The  first 
seven  pairs  articulate  with  the  breast  bone,  the  eighth  and 
ninth  are  connected  to  the  ribs  in  front,  the  others  are  free. 
Any  one  of  the  first  seven  or  more  typical  ribs  consists  of 
two  parts,  a  vertebral  portion  articulating  with  a  vertebra,  an 
imperfectly  ossified  sternal  portion  connecting  the  end  of 
the  vertebral  portion  with  the  sternum.  Each  of  the  first 
nine  ribs  has  a  double  head — the  capitulum  articulating 
with  the  centrum  of  the  corresponding  vertebra,  and  partly 
with  that  of  the  one  in  front,  the  tubercle  articulating  with 
the  transverse  process  of  the  corresponding  vertebra.  The 
posterior  ribs  have  no  tubercles,  and  the  capitular  articula- 
tions are  restricted  to  the  corresponding  vertebrae. 

The  sternum  is  a  narrow  jointed  plate,  with  a  large  keeled 
praesternum  or  manubrium,  then  five  segments  composing 
the  mesosternum,  then  a  posterior  xiphisternum  ending  in 
cartilage. 

The  Skull. 

The  skull  consists,  as  in  all  the  higher  Vertebrates,  of  two 
sets  of  bones, — cartilage-bones  preformed  in  the  cartilage  of 
the  original  gristly  brain-box  and  its  associated  arches, 
membrane  bones  developing  in  the  investing  membrane  and 
not  preformed  in  cartilage.  (The  names  of  the  membrane 
bones  are  printed  in  italics.) 

We  have  already  noticed  the  chief  characteristics  of  the 
mammalian  skull,  such  as  the  usual  persistence  of  sutures, 
the  two  condyles,  the  bony  palate,  the  fusion  of  the  periotic 
bones,  the  articulation  of  the  mandible  with  the  squamosal, 
the  fusion  of  the  parts  of  each  ramus  of  the  mandible  into 
a  single  bone  in  the  adult,  and  the  three  ossicles  of  the 
ear. 


662 


MAMMALIA. 


In  studying  the  skull  it  is  convenient  to  consider  the  bones  in  groups. 

On  the  posterior  surface  of  the  skull,  the  foramen  magnum,  through 
which  the  spinal  cord  issues  from  the  cranial  cavity,  is  bounded  by  the 
basi-occipital  beneath,  the  ex-occipital  on  the  sides,  the  supra-occipital 
above.  The  ex-occipitals  form  most  of  the  occipital  condyles,  but  the 
basi-occipital  contributes  a  small  part.  In  many  mammals  the  ex- 
occipitals  alone  form  the  condyles.  From  each  ex-occipital  a  par-occi- 
pital process  descends  and  is  applied  to  the  tympanic  bulla — a  dilatation 
at  the  base  of  the  tympanic  bone  which  protects  the  external  auditory  tube. 

Along  the  roof  of  the  skull  from  behind  forwards  lie  the  supra-occipi- 
tal, the  parietals,  the  frontals,  and  the  nasals.  Between  the  supra- 
occipital  and  the  parietals  there  is  a  small  interparietal. 

On  the  very  front  of  the  skull  are  the  premaxillce  bearing  the  incisor 
teeth.  Behind  each  premaxilla  is  a  maxilla,  bearing  the  premolars  and 
molars,  behind  this,  along  the  zygomatic  or  temporal  arch  projecting 


Pax. 


FIG.  230.— Side  view  of  Rabbit's  skull. 

Pmx.,  Premaxilla;  No..,  nasal;  Fr.,  frontal;  Pa.,  parietal;  Sp., 
squamosal ;  S.O.,  supra-occipital;  Per.,  periotic  ;  T.,  tympanic; 
P.O.,  par-occipital  process. 

beneath  the  orbit  is  ttiejtigal  or  malar  which  unites  posteriorly  with  the 
squamosal.  This  zygomatic  arch  bridges  over  the  deep  temporal  fossa 
behind  the  orbit,  and  serves  for  the  insertion  of  muscles,  and  its 
"  squamoso-maxillary "  structure  occurs  outside  of  Mammalia  in  the 
Anomodont  reptiles  only.  The  squamosals  form  a  great  part  of  the 
posterior  side  walls  of  the  skull,  and  articulate  with  the  parietals, 
frontals,  orbitosphenoids,  and  alisphenoids.  At  the  posterior  end  of 
the  zygomatic  arch  is  the  longitudinally  elongated  glenoicl  cavity  in 
which  the  mandible  moves  backwards  and  forwards. 

In  connection  with  the  floor  of  the  skull  and  the  roof  of  the  mouth, 
there  lie  from  behind  forwards  the  following  components  : — the  median 


THE  SKULL. 


663 


basi-occipital ;  the  median  basisphenoid  which  lodges  the  pituitary  body 
in  a  dorsal  depression  called  the  sella  turcica ;  the  paired  alisphenoids 
fused  to  the  sides  of  the  basisphenoid  ;  the  median  presphenoid  which 
forms  the  lower  margin  of  the  optic  foramen  between  the  two  orbits ; 
the  paired  orbitosphenoids,  fused  to  the  presphenoid,  sutured  to  the 
alisphenoids  and  squamosals,  and  surrounding  the  optic  foramen  ;  the 
vertical  pterygoids  attached  at  the  junction  of  basisphenoid  and  alisphen- 
oids ;  the  partly  vertical  palatines,  united  above  to  the  presphenoid  and 
behind  to  the  pterygoids  and  alisphenoids,  separating  the  posterior  nasal 
passages  from  the  orbits,  and  uniting  in  front  to  form  the  posterior  part 
of  the  bony  palate  ;  the  median  vertical  mesethmoid  cartilage  extending 
in  front  of  the  presphenoid,  separating  the  two  nasal  cavities,  posteriorly 

ossified  and  expanded  into  the  sieve- 
like  cribriform  plates  through  the 
apertures  of  which  the  branches  of 
the  olfactory  nerves  pass  to  the 
nose ;  the  paired  vomers  along  the 
ventral  edge  of  the  mesethmoid ; 
and  lastly,  the  anterior  bony  palate 
formed  from  inward  extensions  of 
maxilla  and  premaxillce. 

Wedged  in  between  the  occipitals, 
the  squamosals^  and  the  bones  of 
the  basisphenoid  region,  there  is  on 
each  side  a  periotic  bone  surround- 
ing the  internal  ear.  It  ossifies  from 
three  centres  in  the  cartilaginous 
auditory  capsule,  and  consists  of  a 
dense  petrous  portion  enclosing  the 
essential  part  of  the  ear  and  a  more 
external  porous  mastoid  portion 
which  is  produced  downwards  into 
a  mastoid  process  in  front  of  the 
paroccipital  process.  From  each 
periotic  a  tympanic  bone  extends 
outwards,  swollen  basally  into  a  tym- 
panic bulla  in  which  the  tympanum 
or  drum  of  the  ear  is  stretched,  and 
continued  around  the  external  audi- 
tory meatus.  From  an  aperture  be- 
tween the  tympanic  and  the  periotic 
the  Eustachian  tube  passes  to  the 
pharynx.  Stretching  from  the  tympanum  to  the  fenestra  ovalis  of  the 
inner  ear  is  the  chain  of  minute  ear  ossicles,  the  three  links  of  which 
— malleus,  incus,  and  stapes — possibly  correspond  respectively  to  the 
articular,  the  quadrate,  and  hyo-mandibular  or  columella  of  most  other 
Vertebrates. 

The  orbits  are  bounded  anteriorly  by  the  lachrymals  and  the  maxilla, 
and  above  by  the  frontals.  The  interorbital  septum  is  formed  above 
and  behind  by  the  orbitosphenoids,  below  by  the  presphenoid. 

Associated  with  the  olfactory  chambers,  are  the  nasals  above,  the 


Na. 


Pmx. 


FIG.  231.  —  Dorsal  view  of 
Rabbit's  skull. 

S.O.,  Top  of  supra-occipital  ;  //., 
interparietal  ;  T.,  tympanic  ;  Pa., 
parietal ;  Sq.,  squamosal  ;  Fr.,  frontal ; 
/.  jugal  ;  Na.,  nasal  ;  Pmx.,  pre- 
maxilla. 


664  MAMMALIA. 

vomers  beneath,  the  mesethmoid  in  the  median  line,  while  internally 
there  are  several  thin  scroll -like  turbinal  bones. 

The  lower  jaw  or  mandible  consists  in  adult  life  of  a  single  bone  or 
ramus  on  each  side,  but  this  is  formed  around  Meckel's  cartilage  from 
several  centres  of  ossification.  Its  condyle  works  on  the  squaw o sal. 

The  hyoid  lies  between  the  rami  of  the  mandible,  in  the  back  of 
the  mouth,  and  consists  of  a  median  "  body,"  and  two  pairs  of  horns  or 
cornua  extending  backwards. 

The  Appendicular  Skeleton  consists  of  the  bones  of  the 
limbs  and  the  girdles. 

The      pectoral      girdle,  ^rf'1'  • 

which    supports    the    fore 

limbs  and  is  itself  attached  '"'f 

by  muscles  and  ligaments 
to   the   vertebral    column, 

virtually   consists    of    one  yj  m*m  ^tr 

bone  —  the    scapula  —  on 

each     side.       For    in    all  /;///.>'. 

Mammals    except    Mono-          !^  ,  , 

tremes,       the       coracoid,  ^  j;  m.j 

though  a  distinct  ossifica-  rr 

tion,    forms    only  a    small         4>:  jjp    /  ^ 

(epicoracoid)    process    on 
the  anterior  margin  of  the 

glenoid    cavity    in    which  .s>/ 

the  head  of  the  humerus  VJ(|  f~"f  -    7^ 

works.      The    last  of   the  iw 

metacoracoid    is    seen    in 
Monotremes.         The      cla-  FlG.    232.  —  Under    surface    of 

vicle  is  also  much  reduced      Rabbit's  skull. 

in   the  rabbit,   being  only        ^  Front  incisors .  ^  small  posterior 

about     an     inch     in    length        incisors;  j>mx.>  premaxilla;  mx.,  maxilla; 

,  i        j  T       •          Pm-  3>  ^ird  premolar ;  m.  3,  third  molar ; 

and     Very     Slender.         it     IS        /.,  jugal;yr.,  supra-orbital  ridge  of  frontal 

a  membrane  bone,  and  lies     ^Tg^^^™^^ 

in     the     ligament     between         tympanic  bulla ;  b.o.,   basi-occipital ;   6.s., 

the   scapula  and  the  ster-      une.sphe 
num.         The      triangular 

scapula  has  a  prominent  external  ridge  or  spine,  continued 
ventrally  into  an  acromion  with  a  long  metacromion  pro- 
cess. The  scapula  is  usually  strong,  and  the  clavicle 
is  usually  present  in  mammals  which  grasp  or  climb  or 
burrow. 


THE  APPENDICULAR  SKELETON. 


665 


•>Sc. 


The  fore  limb  consists  of  an  upper  arm  or  humerus,  a 
fore  arm  of  two  bones — the  radius  and  the  ulna,  a  wrist  or 

carpus,  five  palm  bones 
or  metacarpals,  and  five 
digits  with  joints  or 
phalanges. 

The  head  of  the  humerus 
works  in  the  glenoid  cavity 
formed  by  the  scapula  and  the 
coracoid  process. 

When  the  arm  of  a  mam- 
mal is  directed  outwards  at 
right  angles  to  the  body,  with 


FIG.  233. — Rabbit's  fore  leg. 

Sc.,  Scapula;  cor.,  coracoid  process;  ac., 
acromion  ;  //.,  humerus  ;  R.,  radius  ;  6'.,  ulna  ; 
C.t  carpal  region  ;  M.C.,  metacarpal  region. 

the  palm  vertical  and  the  thumb  upper- 
most, the  thumb  and  the  radius  are  in 
a  preaxial  position,  the  little  finger  and 
the   ulna  are   in   a   postaxial    position. 
But  in  the  normal  position  of  the  limb 
in     most     mammals, 
the    radius    and    the 
ulna    cross    one    an- 
other in  the  fore  arm, 
so   that   the   preaxial 
radius  is  external   at 
the  upper  end,  inter- 
nal at  the  lower  end. 

The  typical  mam- 
malian wrist  or  car- 
pus consists  of  two 

rows  or  bones,  with  a  central  bone  between  the   two  rows. 
rabbit  all  the  bones — nine  in  number — are  present,  viz  : — 
Ulnare  or  Cuneiform.      Intermedium  or  Lunar.      Radial e  or  Scaphoid. 
Centrale. 

Carpale  5  and  4  Carpale  3     Carpal  e  2  Carpale  I 

or  or  or  or 

Unciform.  Magnum.    Trapezoid.  Trapezium. 


C. 


FIG.  234. — Rabbit's  hind  leg. 

Fe.,  Femur  ;  Tr.,  third  trochanter  ;  E£.,  epiphysis 
at  head,  of  tibia  (7V).;  Ft.,  incomplete  fibula;  C., 
calcaneum  ;  A.,  astragalus  ;  int.,  metatarsals. 


In  the 


666  MAMMALIA. 

In  Mammals  the  fourth  and  fifth  carpals  are  always  fused  ;  the  cen- 
trale  is  often  absent.  In  the  tendons  of  the  flexor  muscles  there  are 
often  two  sesamoid  bones,  of  which  the  ulnar  is  called  the  pisiform. 

In  the  rabbit  there  are  five  metacarpal  bones  and  five  digits,  each 
with  three  phalanges  except  the  thumb  or  pollex  which  has  but  two. 

The  pelvic  girdle  is  articulated  to  the  backbone,  and  bears 
externally  a  cup-like  socket  or  acetabulum  in  which  the 
head  of  the  thigh  bone  works.  Each  half  of  the  girdle — 
forming  what  is  called  the  innominate  bone — really  consists 
of  three  bones  which  meet  in  the  acetabulum.  The  dorsal 
bone  or  ilium,  which  corresponds  to  the  scapula,  articulates 
with  the  sacral  vertebrae  ;  the  pubis — the  anterior  of  the  two 
lower  bones — unites  with  its  fellow  on  the  opposite  side  in 
the  pubic  symphysis  ;  the  two  ischia,  which  correspond  to 
the  coracoids,  extend  backwards,  separated  from  the  pubes 
by  the  large  obturator  foramen,  and  expand  into  posterior 
tuberosities.  The  ischia  of  mammals  may  touch  one  another 
ventrally,  but  do  not  fuse  in  a  symphysis  ;  the  pubic  sym- 
physis is  almost  invariably  present.  Only  in  Cetacea  and 
Sirenia  is  the  pelvis  markedly  rudimentary. 

The  hind  leg  consists  of  a  thigh  or  femur,  a  lower  leg  with 
two  bones — the  tibia  and  the  fibula,  an  ankle  or  tarsus,  the 
sole  bones  or  metatarsals,  the  toes  with  several  joints  or 
phalanges. 

The  head  of  the  femur  works  in  the  acetabulum  of  the  pelvis.  Near 
the  head  are  several  processes  or  trochanters,  serving  for  the  insertion  of 
muscles  ;  in  the  rabbit  there  are  three — the  great  trochanter,  the  lesser 
trochanter,  and  the  third  trochanter. 

In  front  of  the  knee  there  is  a  sesamoid  bone — the  knee-pan  or  patella 
— and  posteriorly  there  are  smaller  fabellse. 

In  the  lower  leg,  the  tibia,  which  corresponds  to  the  radius,  is  pre- 
axial,  and  in  the  normal  position  interior ;  the  fibula,  which  corresponds 
to  the  ulna,  is  postaxial,  and  in  the  normal  position  exterior.  In  the 
rabbit  the  fibula  is  slender,  and  is  fused  distally  with  the  tibia. 

In  the  mammalian  tarsus  there  are  two  rows  of  bones,  and  a  central 
bone  interposed  between  the  two  rows  on  the  inner  or  tibial  side. 
Calcaneum  Astragalus 

or  Fibulare.  (  =  Intermedium  and  Tibiale). 

Centrale 
or  Navicular. 

Tarsale  5  and  4  Tarsale  3  Tarsale  2  Tarsale  I 

=  Cuboid.  or  or  or 

External  Middle  Internal 

Cuneiform.        Cuneiform.        Cuneiform. 
In  the  rabbit  the  first  tarsal  and  the  corresponding  toe  or  hallux  are 


NERVOUS  SYSTEM. 


667 


wanting,     There  are  thus  only  four  metatarsals  and  digits.     Each  digit 
has  four  phalanges. 

Nervous  System. 

The  brain  has  the  usual  five  parts — cerebral  hemispheres, 
optic  thalami,  optic  lobes,  cerebellum,  and  medulla  oblongata, 
but  the  cerebral  hemispheres  cover  the  next  two  parts,  and 
the  cerebellum  conceals  the  medulla.  Of  the  brain  mem- 
branes, the  dura  mater  lines  the  cranial  cavity,  projecting 
longitudinally  between  the  cerebral  hemispheres,  and  trans- 
versely between  the  latter  and  the  cerebellum,  while  the 


./iff/ 


w... 

FIG.    236.  —  Under    surface    of 
Rabbit's  brain.     (After  KRAUSE.) 

olf.l.)  Olfactory  lobes  ;  <?./.,  olfactory 
tract  \f.L,  frontal  lobe  ;  ch.,  optic  chiasma  ; 
i.e.,  infundibulum  ;  c.m.  corpus  mammil- 
lare  ;  3,  root  of  oculomotor  ;  4,  root  of 
pathetic  ;  5,  root  of  trigeminal ;  6,  root  of 
abducens  ;  7-8,  roots  of  facial  and  auditory  ; 
/"7.,  flocculi  of  cerebellum  ;  //.,  i2th  or 
hypoglossal  nerve  ;  10,  roots  of  vagus ;  9, 
the  line  runs  in  front  of  the  root  of  the 
glossopharyngeal  to  the  root  of  6  \  p>v.t 
pons  Varolii ;  /./.,  temporal  lobe. 

vascular  pia  mater  invests  the  brain  closely.  There  are  the 
usual  twelve  pairs  of  cranial  nerves.  The  spinal  cord  gives 
off  the  usual  spinal  nerves,  and  there  is  a  sympathetic  system 
as  in  most  other  Vertebrates. 


FIG.  235. — Dorsal  view  of 
Rabbit's  brain,  with  most  of 
cerebellum  cut  away.  (After 
KRAUSE.) 

olf.l.)  Olfactory  lobes  ;  cJi., 
cerebral  hemispheres ;  o.l.,  optic 
lobes;  4.7'.,  fourth  ventricle  (ex- 
posed); s.c.,  spinal  cord;  10,  root 
of  vagus;  Ct>.,  lobe  of  cerebellum. 


668  MAMMALIA. 

The  cerebral  hemispheres  of  the  rabbit  are  very  slightly  convoluted, 
and  they  leave  the  cerebellum  quite  uncovered.  They  are  connected 
transversely  by  a  broad  bridge — the  corpus  callosum — and  beneath  this 
there  is  a  longitudinal  band  of  fibres — the  fornix.  The  corpus  callosum 
is  readily  disclosed  by  gently  separating  the  hemispheres.  The  outer 
wall  and  floor  of  the  anterior  part  of  the  cavity  or  ventricle  of  each  hemi- 
sphere is  formed  by  a  thick  mass,  called  the  corpus  striatum,  and  the 
internal  cavity  is  lessened  by  a  prominent  convex  ridge,  called  the  hippo- 
campus major.  The  ventricles  of  the  cerebrum  communicate  with  the 
third  ventricle,  between  the  optic  thalami,  by  a  small  aperture,  called 
the  foramen  of  Munro.  In  front  of  the  hemispheres  two  club-shaped 
olfactory  lobes  project.  The  thin  cortical  layer  of  the  cerebrum  consists 
of  grey  (ganglionic)  matter,  and  so  does  the  thick  corpus  striatum,  while 
the  central  part  consists  of  white  matter  (nerve  fibres). 

The  thalamencephalon  is  entirely  hidden,  but  gives  origin  as  usual  to 
the  dorsal  epiphysis,  ending  in  a  pineal  body,  which  lies  on  the  surface 
between  the  cerebrum  and  cerebellum,  and  to  the  ventral  infundibulum, 
at  the  end  of  which  the  pituitary  body  lies,  lodged  in  a  fossa  of  the  basi- 
sphenoid.  Immediately  in  front  of  the  infundibulum  the  optic  nerves 
cross  in  a  chiasma,  from  which  optic  tracts  can  be  traced  to  the  optic 
lobes.  Immediately  behind  the  infundibulum  lies  a  rounded  elevation, 
called  the  mamillary  body.  Anteriorly  on  the  ventral  surface  of  each 
side  of  the  thalamencephalon  there  is  a  rounded  swelling,  called  the 
corpus  geniculatum.  The  roof  of  the  third  ventricle  is  formed  by  a  thin 
membrane  or  velum,  with  a  plexus  of  blood  vessels.  In  the  anterior 
wall  of  the  third  ventricle  lies  the  small  anterior  commissure,  across  the 
third  ventricle  the  large  middle  commissure  runs,  in  the  roof  of  the  hind 
part  of  the  ventricle  lies  a  small  posterior  commissure. 

The  optic  lobes  are  fourfold — corpora  quadrigemina.  They  are  almost 
quite  covered  by  the  cerebrum.  Between  them  runs  the  iter  connecting 
the  third  ventricle  and  the  fourth.  The  floor  of  this  passage  is  formed  by 
the  thick  crura  cerebri,  which  connect  the  medulla  with  the  cerebrum. 

The  cerebellum  is  divided  into  a  median  and  two  lateral  lobes,  and  is 
marked  by  numerous  folds,  mostly  transverse.  The  two  sides  are  con- 
nected ventrally  by  the  pons  Varolii,  lying  across  the  anterior  ventral 
surface  of  the  medulla. 

The  medulla  oblongata  lies  beneath  and  behind  the  cerebellum,  and 
is  continued  into  the  spinal  cord.  The  cavity  of  the  fourth  ventricle  is 
roofed  by  a  thin  membrane  or  velum,  above  which  lies  the  cerebellum. 
On  the  ventral  surface  the  medulla  is  marked  by  a  deep  fissure,  bordered 
by  two  narrow  bands  or  ventral  pyramids. 

The  spinal  cord  presents  its  usual  appearance,  with  its  dorsal  sensory 
nerve  roots  with  ganglia,  its  ventral  motor  nerve  roots  apparently  with- 
out ganglia,  and  the  spinal  nerves  formed  from  the  union  of  these.  The 
ganglia  of  the  adjacent  sympathetic  system  perhaps  belong  to  the  ventral 
roots  of  the  spinal  nerves. 

A  large  number  of  nerves  pass  down  the  neck.  Of  these  the  follow- 
ing are  most  important : — 

(l)  The  eleventh  cranial  nerve  or  spinal  accessory,  leaving  the  skull 
with  the  ninth  and  tenth,  and  distributed  to  the  muscles  of  the 
neck. 


ALIMENTARY  SYSTEM.  669 

(2)  The  twelfth  cranial  nerve  or  hypoglossal,  lying  at  first  close  to  the 

ninth,  tenth,  and  eleventh,  turning,  however,  to  the  muscles  of  the 
tongue. 

(3)  The  tenth  cranial  nerve,  the  pneumogastric  or  vagus,  lies  outside 

the  carotid  artery,  and  gives  off  a  superior  laryngeal  to  the  larynx 
with  a  depressor  branch  to  the  heart,  an  inferior  or  recurrent 
laryngeal  which  loops  round  the  subclavian  artery  and  runs 
forward  to  the  larynx,  and  other  nerves  to  the  heart,  lungs,  and 
gullet. 

(4)  The  cervical  part  of  the  sympathetic,  lying  alongside  of  the  trachea, 

with  two  ganglia. 

(5)  The  great  auricular,  a  branch  of  the  third  spinal  nerve,  running  to 

the  outer  ear. 

(6)  The  phrenic  nerve,  a  branch  of  the  fourth  cervical  nerve,  with  a 

branch  from  the  fifth  and  sometimes  from  the  sixth,  runs  along 
the  backbone  to  the  diaphragm. 

For  details  as  to  these  nerves,  the  student  should  consult  the  practical 
manuals  of  Marshall  and  Hurst  and  of  Parker. 

As  to  the  sense  organs  little  need  be  said,  for  their  general  structure  is 
like  that  of  other  Vertebrates,  while  the  detailed  peculiarities  are  beyond 
our  present  scope. 

The  third  eyelid,  present  in  all  mammals  except  the  Cetaceans  and 
the  Primates,  is  well  developed.  The  lachrymal  gland  (absent  in 
Cetacea)  lies  under  the  upper  lid,  and  the  lids  are  kept  moist  by  the 
secretion  of  Harderian  and  Meibomian  glands.  The  external  ear  or 
pinna  is  conspicuously  large.  The  cochlea  of  the  inner  ear  is  large  and 
spirally  twisted.  The  nostrils  are  externally  connected  with  the  mouth 
by  a  characteristic  cleft  lip.  The  tongue  bears  numerous  papillae  with 
taste  bulbs.  The  long  hairs  or  vibrissoe  on  the  snout  are  tactile. 

Alimentary  System. 

In  connection  with  the  cavity  of  the  mouth  we  notice  the 
characteristic  dentition,  the  hairy  pad  of  skin  intruded  in 
the  gap  between  incisors  and  premolars,  the  long  and 
nairow,  in  part  bony,  palate  separating  the  nasal  from  the 
buccal  cavity,  the  muscular  tongue  with  its  taste  papillae, 
the  glottis  which  leads  into  the  windpipe,  and  the  bilobed 
flap  or  epiglottis  which  guards  the  opening,  the  paired 
apertures  of  the  Eustachian  tubes  opening  into  the  posterior 
nasal  passage,  the  end  of  this  passage  above  the  glottis, 
and  the  beginning  of  the  pharynx.  Less  obvious  are  the 
organs  of  Jacobson,  paired  tubular  bodies  lying  enclosed  in 
cartilage  in  the  front  of  the  nasal  chamber,  and  communi- 
cating on  the  one  hand  with  the  nostrils,  and  on  the  other 
hand  with  the  mouth  by  two  naso-palatine  canals  which  open 
a  little  way  behind  the  posterior  incisors.  Opening  into  the 


670  MAMMALIA. 

mouth  and  bearing  the  salivary  juice,  whose  ferment  alters 
the  starchy  parts  of  the  food,  are  the  ducts  of  four  pairs  of 
salivary  glands.  The  parotid,  which  is  largest,  lies  between 
the  external  ear  chamber  and  the  angle  of  the  mandible ; 
the  infra-orbital  lies  below  and  in  front  of  the  eye ;  the  sub- 
maxillary  lies  between  the  angles  of  the  mandible ;  the 
sub-lingual  lies  along  the  inner  side  of  each  ramus  of  the 
mandible. 

The  pharynx  passes  into  the  gullet,  and  that  leads  through 
the  diaphragm  to  the  expanded  stomach,  which  is  dilated  at 
its  upper  or  cardiac  end,  and  narrows  to  the  curved  pyloric 
end.  Partly  covering  the  stomach  is  the  large  liver.  The 
first  portion  of  the  intestine,  which  is  called  the  duodenum, 
receives  the  bile  duct,  and  has  the  pancreas  in  its  folds. 
Then  follows  the  much  coiled 
small  intestine  measuring  many 
feet  in  length.  The  lower  end 
of  the  small  intestine  is  ex- 
panded into  a  sacculus  rotun- 
dus.  Here  the  large  caecum 
—  a  blind  diverticulum  —  is 
given  off;  it  ends  in  a  finger- 
like  vermiform  appendix.  Its 
proximal  end  is  continuous 
with  the  colon  or  first  part 
of  the  large  intestine,  the  be-  .  FIG.  237.— Diagram  of  caecum 
ginning  of  which  is  much  sac-  m  Rabblt 

CUlated.         The    large     intestine  J.z'.,  Small  intestine  ;  s.r.,  sacculus 

,         ,  rotundus  ;   col. ,  sacculated  colon  ;  c., 

narrows    into    the    long    rectum        cwmm ;».«.,  venniform  appendix. 

in  which  lie  little  faecal  pellets. 

On  the  last  two  inches  of  the  rectum  there  are  paired 
yellowish  glands.  Beside  the  anus  are  two  perineal  sacs  of 
skin,  into  which  open  the  ducts  of  the  perineal  glands, 
whose  secretion  has  a  characteristic  and  strong  odour. 

The  liver  is  attached  to  the  diaphragm  by  a  fold  of  peri- 
toneum— the  glistening  membrane  which  lines  the  abdo- 
minal cavity.  In  the  liver  there  are  five  lobes.  From  these 
lobes  the  bile  is  collected  by  hepatic  ducts  into  a  common 
bile  duct,  which  is  also  connected  to  the  gall  bladder  by  the 
cystic  duct. 

The  very  diffuse  pancreas  lies  in  the  mesentery  of  the 


VASCULAR  SYSTEM. 


671 


duodenal  loop.     Its  secretion  is  gathered  by  several  tubes 
into  the  pancreatic  duct  which  opens  into  the  duodenum. 

The  mesentery  which  supports  the  alimentary  canal,  is  a 
double  layer  of  peritoneum  reflected  from  the  dorsal  abdo- 
minal wall. 

The  dark-red  spleen  (of  importance  in  connection  with  the 
blood),  lies  behind  the  stomach.  In  the  mesentery,  not  far 
from  the  top  of  the  right  kidney,  lie  a  pair  of  cceliac  ganglia, 

which  receive  nerves  from 
the  thoracic  sympathetic  sys- 
tern,  and  give  off  branches 
to  the  gut. 

Vascular  System. 

The  four-chambered  heart 
lies  in  the  thoracic  cavity 
between  the  lungs.  It  is 
surrounded  by  a  thin  peri- 
cardium, and  immediately 
in  front  of  it  there  lies  the 
soft  thymus,  which  is  larger 
in  the  young  than  in  the 
adult  animal. 

By  two  superior  venae 
cavse,  and  by  the  inferior 
vena  cava,  the  venous  blood 
collected  from  the  body 
enters  the  right  auricle. 
Thence  the  blood  passes 
into  the  right  ventricle 
through  a  crescentic  open- 
ing, bordered  by  a  threefold 
(tricuspid)  membranous  valve  (worked  by  chordae  tendineae 
attached  to  papillary  muscles  projecting  from  the  wall  of  the 
ventricle). 

The  right  ventricle  is  not  so  muscular  as  the  left,  which 
it  partly  surrounds.  By  its  contraction  the  blood  is  driven 
into  the  pulmonary  trunk,  whose  orifice  is  guarded  by  three 
semilunar  valves.  During  contraction,  the  tricuspid  valves 
are  pressed  together,  so  that  no  regurgitation  into  the  right 
auricle  can  take  place. 


FIG.  238. — Duodenum  of  Rabbit. 
(From  KRAUSE,  in  part  after 
CLAUDE  BERNARD.) 

P.,  Pyloric  end  of  stomach  ;  g-.fi.,  gall 
bladder  with  bile  duct  and  hepatic  ducts  ; 
p.d.,  pancreatic  duct. 


672 


MAMMALIA. 


The  pulmonary  trunk  divides  into  two  pulmonary  arteries, 
which   branch  into  capillaries   on   the  walls   of  the  lungs. 
There  the  red  blood  corpuscles  gain  oxygen,  and  the  blood 
is  freed  from  much  of  the  car- 
bonic  acid    gas   which    it   has 
borne   away  from   the   tissues. 
The  purified  blood  returns  to 
the    heart    by   two   pulmonary 
veins,    which    unite    as    they 
enter  the  left  auricle. 

FIG.  239. — Circulatory  system  of 
the  Rabbit.  (In  part  after  Professors 
PARKER  and  KRAUSE.) 

(a)  Letters  to  right— 

e.c.  External  carotid. 
i.e.  Internal  carotid. 
e.j.  External  jugular. 
scl.a.  Subclavian  artery. 
scl.v.  Subclavian  vein. 
p.a.  Pulmonary  artery  (cut  short). 
p.v.  Pulmonary  vein. 
L.A.  Left  auricle. 
L.V.  Left  ventricle. 
d.ao.  Dorsal  aorta. 
h.v.  Hepatic  veins. 
c.  Coeliac  artery. 
a.m.  Anterior  mesenteric. 
s.r.b.  Supra-renal  body. 
l.r.a.  Left  renal  artery. 
l.r.v.  Left  renal  vein. 

K.  Kidney. 

p.m.  Posterior  mesenteric  artery. 
spin.  Spermatic  artery  and  vein. 
c.il.a.  Common  iliac  artery. 
(/;)  Letters  to  left— 

p.f.  and  a.f.  Posterior  and  anterior 

facial. 

e.j.  External  jugular  vein. 
i.j.  Internal  jugular. 
R.Scl.  Right  subclavian  artery. 
6".  V.C.  Superior  vena  cava. 
R.A.  Right  auricle. 
R.  V.  Right  Ventricle. 
/.  V.C.  Inferior  vena  cava. 
r.r.a.  Right  renal  artery. 
r.r.v.  Right  renal  vein. 
s.r.b.  Supra-renal  body. 
spm.  Spermatic  artery  and  vein. 
/./.  llio-lumbar  vein. 
f.v.  Femoral  vein. 
i.il.v.  Internal  iliac  veins. 

From  the  left  auricle,  the  pure  blood  passes  into  the  left 
ventricle   through   a   funnel -like   opening,   bordered   by   a 


VASCULAR  SYSTEM.  673 

(mitral)  valve  with  two  membranous  flaps,  with  chordae 
tendineae  and  musculi  papillares  as  on  the  right  side,  but 
the  muscles  here  are  larger. 

The  left  ventricle  receives  the  pure  blood  and  drives  it  to 
the  body.  During  contraction,  the  mitral  valve  is  closed,  so 
that  no  blood  can  flow  back  into  the  auricle.  The  blood 
leaves  the  left  ventricle  by  an  aortic  trunk,  whose  base  is 
guarded  by  three  semilunar  valves,  just  above  which  coronary 
arteries  arise  from  the  aortic  trunk  and  supply  the  heart 
itself. 

The  aortic  trunk  bends  over  to  the  left,  and  passes  back- 
ward under  the  backbone,  dividing  near  the  pelvis  into  two 
common  iliac  arteries,  which  supply  the  hind  legs  and  pos- 
terior parts.  The  arteries  given  off  near  the  heart  and  in 
the  abdominal  region  may  be  grouped  as  follows  : — 

The  aortic  trunk 

gives  off  the  innominate  artery, 

which  divides  into  (a)  the  right  subclavian,  continued  as  the 
brachial  to  the  fore  limb,  but  giving 
off  the  vertebral  to  the  spinal  cord 
and  brain,  and  the  internal  mam- 
mary to  the  ventral  wall  of  the 
thorax  : 

(b]  the  right  carotid,  running  along  the 

trachea,  dividing  into  the  right 
internal  carotid  to  the  brain,  and 
the  right  external  carotid  to  the 
head  and  face  : 

(c)  the  left  carotid,  with  a  similar  course : 
thereafter  the  aorta  gives  off 

the  left  subclavian  artery,  with  branches  like  the  right, 
the  cceliac  artery  to  the  liver,  stomach,  and  spleen, 
the  anterior  mesenteric  to  the  pancreas  and  intestine, 
the  renal  arteries  to  the  kidneys, 

the  spermatic  or  ovarian  arteries  to  the  reproductive  organs, 
the  posterior  mesenteric  to  the  rectum, 
the  lumbar  arteries  to  the  posterior  body  walls. 

The  aorta  is  continued  terminally  in  the  median  sacral  artery  to  the 
tail,  and  laterally  in  the  common  iliacs  which  form  the  femorals  of 
the  hind  legs,  and  give  off  in  the  abdomen  several  branches  to  the 
abdominal  walls,  the  pelvic  cavity,  the  bladder,  and  the  uterus. 

The  Venous  System. — The  two  superior  venae  cavce  bring  blood  from 
the  head,  neck,  thorax,  and  fore  limbs.  Each  is  formed  from  the 
union  of 

a  subclavian  from  the  shoulder  and  fore  limb, 
an  external  jugular  from  the  face  and  ear, 
an  internal  jugular  from  the  brain, 

43 


674  MAMMALIA. 

an  anterior  intercostal  from  the  spaces  between  the  anterior 

ribs, 

an  internal  mammary  from  the  ventral  wall  of  the  thorax  ; 
and  the  right  superior  vena  cava  also  receives  an  azygos  cardinal  vein, 
which  runs   along  the   mid-dorsal   line   and   collects   blood   from   the 
posterior  intercostal  spaces. 

The  inferior  vena  cava  is  a  large  median  vein  lying  beside  the  aorta 
beneath  the  backbone.  Anteriorly  it  is  embedded  in  the  liver,  and  re- 
ceives the  hepatic  veins.  Thence  it  passes  through  the  diaphragm  into 
the  right  auricle.  Posteriorly  the  inferior  vena  cava  has  the  following 
components  : — 

internal  iliacs  from  the  back  of  the  thighs,  forming  by  their  union 

the  beginning  of  the  inferior  vena  cava ; 
femoral  veins  from  the  inner  borders  of  the  thighs,  continued  into 

external  iliacs  which  open  into  the  inferior  vena  cava ; 
paired  ilio-lumbars  from  the  posterior  abdominal  walls  ; 
spermatic  or  ovarian  veins  from  the  reproductive  organs ; 
renal  veins  from  the  kidneys. 
There  is  no  renal  portal  system. 

The  food  which  has  been  digested — rendered  soluble  and  diffusible — 
passes  from  the  food  canal  into  the  vascular  system  by  two  paths  : — 

(a)  All  except  the  fatty  material  is  absorbed  by  veins  from  the  stomach 

and  intestine.  These  unite  in  a  main  trunk  the  portal  vein.  The 
components  of  the  portal  vein  are — the  lieno-gastric  from  the 
stomach  (and  also  from  the  spleen),  the  duodenal  from  the  duo- 
denum (and  also  from  the  pancreas),  the  anterior  mesenteric  from 
the  intestine,  the  posterior  mesenteric  from  the  rectum.  The 
portal  vein  breaks  up  into  branches  in  the  liver,  whence  the 
modified  blood  passes  by  hepatic  veins  into  the  inferior  vena 
cava. 

(b]  The  fat  passes   through   the   intestinal  villi   into   the   lymphatic 

vessels,  which  combine  to  form  a  thoracic  duct  which  runs  for- 
ward, and  opens  into  the  left  subclavian  vein  at  its  junction  with 
the  left  external  jugular.  Here  and  there  lie  lymphatic  glands. 

Respiratory  System. 

The  lungs  are  pink,  spongy  bodies,  lying  in  the  thorax, 
connected  to  the  exterior  by  the  bronchial  tubes  and  the 
trachea,  and  to  the  heart  by  blood  vessels.  The  pleural 
membrane  which  invests  the  surface  of  the  lungs  is  reflected 
from  the  sides  of  the  thoracic  cavity.  When  the  lungs 
expand,  the  pleural  cavity — between  the  two  folds  of  pleural 
membrane — is  almost  obliterated.  The  thoracic  cavity  is 
separated  from  the  abdominal  cavity  by  a  partly  muscular 
diaphragm,  which  is  supplied  by  two  phrenic  nerves,  arising 
from  the  fourth  cervical  spinal  nerves.  By  its  contraction 
the  diaphragm  alters  the  size  of  the  thoracic  cavity,  and 


EXCRETORY  SYSTEM. 


675 


thus  shares  in  the  mechanism  of  respiration.  At  the  top  of 
the  trachea  lies  the  complex  larynx,  the  seat  of  the  voice  in 
mammals. 

Anteriorly  the  larynx  is  supported  on  its  sides  and  beneath  by  the 
thyroid  cartilage,  behind  this  lies  the  ring-like  cricoid,  dorsally  to  the 
cricoid  are  two  small  triangular  arytenoids. 

Within  the  larynx  there  are  stretched  membranous  bands — the  vocal 
cords.  Beside  the  larynx  is  the  paired  thyroid  gland. 


cl 


e.t. 


b.o 


f.b. 


-tr 


T 

pi. 


\ 

\   smx 
s.L 


epg 


FIG.  240. — Vertical  section  through  Rabbit's  head. 
(From  a  section,  with  help  from  PARKER'S  Zootomy 
and  KRAUSE.) 

pinx.,  Premaxilla  with  incisors  ;  m.e.,  part  of  mesethmoid  parti- 
tion ;  t.b.,  maxillary  turbinals  ;  e.t.,  ethmoidal  turbinal ;  m.e.,  part 
of  mesethmoid  ;  olf.L,  olfactory  lobe  of  cerebrum  ;  ps.,  presphenoid  ; 
c.c.,  position  of  corpus  callosum  ;  bs.,  basisphenoid  with  depression 
for  pituitary  body  ;  cb.,  cerebellum  ;  b.o.,  basi-occipital ;  s.c.,  spinal 
cord  ;  n.p.,  nasal  passage  ;  g~.,  gullet ;  tr. ,  trachea  ;  epg.,  epiglottis  ; 
smx.)  sub-maxillary  salivary  gland  ;  s.L,  sublingual  salivary  gland  ; 
T.,  tongue;^/.,  transverse  portion  of  palatine;  inn.,  anterior  end 
of  mandible. 

Excretory  System. 

The  excretory  system  includes  the  blood  filtering  kidneys, 
their  ducts  the  ureters,   and  a  reservoir  or  bladder,    into 


676  MAMMALIA. 

which  these  open.  The  kidneys  and  their  ducts  are  formed 
from  the  metanephros  and  metanephric  ducts  of  the  em- 
bryo. The  bladder  arises  as  a  diverticulum  from  the  hind 
end  of  the  gut,  being  in  fact  a  remnant  of  the  intra-em- 
bryonic  part  of  the  allantois.  It  loses  its  connection  with 
the  gut,  and  the  ureters  which  originally  opened  into  the 
rectum  follow  the  bladder  and  open  into  it. 

The  kidneys  are  dark-red  ovoid  bodies  lying  on  the  dorsal 
wall  of  the  abdomen ;  the  one  on  the  left  is  further  down 
than  that  on  the  right,  because  of  the  position  of  the  stomach 
on  the  left  side.  When  a  kidney  is  dissected,  a  marked  dif- 
ference is  seen  between  the  superficial  cortical  part  and  the 
deeper  medullary  substance.  On  papillae  or  pyramids  in  the 
very  centre,  the  coiled  excretory  tubules  open,  and  empty 
the  water  and  waste  products  into  the  "  pelvis  "  or  mouth 
of  the  ureter. 

The  ureters  run  backward  along  the  dorsal  wall  of  the 
abdomen,  and  open  into  the  bladder,  a  thin-walled  sac  lying 
in  front  of  the  pelvic  girdle. 

In  front  of  each  kidney  lies  a  yellow  suprarenal  body  of 
doubtful  physiological  significance. 

Reproductive  Organs. 

(a)  Male. — The  testes  arise  on  the  dorsal  abdominal  wall 
near  the  kidney,  but  as  the  rabbit  becomes  sexually  mature, 
they  are  loosened  from  their  original  attachment,  and  pass 
out  on  the  ventral  surface,  as  if  by  a  normal  rupture,  into 
the  scrotal  sacs.  A  spermatic  cord,  consisting  of  an  artery,  a 
vein,  and  a  little  connective  tissue,  runs  from  the  abdomen 
to  the  testis. 

The  testis  is  attached  to  the  base  of  the  scrotal  sac,  and 
is  bordered  by  a  mass  of  convoluted  tubes — the  epididymis 
— consisting  of  the  caput  epididymis  anteriorly,  the  larger 
cauda  epididymis  posteriorly,  and  a  narrow  band  between 
them,  The  cauda  epididymis  is  connected  to  the  scrotal 
sac  by  a  short  cord  or  gubernaculum. 

Through  the  tubes  of  the  epididymis  (the  modified  meso- 
nephros)  the  spermatozoa  developed  in  the  testis  are  col- 
lected into  the  vas  deferens  (the  modified  Wolffian  duct), 
which  arises  from  the  cauda  epididymis,  ascends  to  the 
abdomen,  extends  round  to  the  dorsal  surface  of  the  neck  of 


REPRODUCTIVE   ORGANS. 


677 


the  bladder,  and  there  opens  beside  its  fellow  into  a  median 
sac  called  the  uterus  masculinus.  In  many  Mammals  paired 
diverticula,  known  as  seminal  vesicles,  are  connected  with 


K 


U.G 


FIG.  241. — Urinogenital  organs 
of  Male  Rabbit. 

K.,  Kidney  ;  £/.,  ureter  ;  BL, 
bladder;  T.,  testis  ;  s.c.,  spermatic 
cord  ;  cp.ep.,  caput  epididymis  ; 
Ca.ep.,  cauda  epididymis  ;  Sc.,  scro- 
tal  sac  ; pr.,  prostates  ;  e.g.,  Cowper's 
glands  ;  p.g.,  perineal  glands  ;  (Jr., 
urethra  ;  c.c.,  corpus  cavernosum  ; 
P.,  penis. 

the  ends  of  the  vasa  deferentia,  but  they  are  not  developed 
in  the  rabbit. 


FIG.  242. — Urinogenital  organs 
of  Female  Rabbit. 

K.,  Kidney  ;  U.,  ureter  ;  O.,  ovary  ; 
F.t.,  Fallopian  tube  ;  O.d.,  oviduct  ; 
Ut.,  uterus  ;  V.,  vagina  ;  BL,  bladder  ; 
Ye.,  vestibule  ;  U.G.,  Urinogenital 
aperture;  A.,  anus.  Bladder  and 
vestibule  are  cut  open. 


678  MAMMALIA. 

The  uterus  masculinus  is  the  homologue  of  the  vagina  in 
the  female,  and  seems  to  arise  from  the  Miillerian  ducts. 
It  opens  into  the  urethra,  which  runs  backwards  from  the 
bladder,  and  the  urinogenital  canal  thus  formed  is  continued 
through  the  penis. 

Beside  the  uterus  masculinus  and  the  vasa  deferentia, 
there  are  lobed  prostate  glands  opening  by  several  ducts 
into  the  urinogenital  canal.  Behind  the  prostate,  on  the 
dorsal  wall  of  the  urinogenital  canal,  lie  two  Cowper's 
glands. 

The  penis  projects  in  front  of  the  anus  behind  the  pubic 
symphysis,  has  vascular  dorsal  walls  (corpus  spongiosum), 
stiff  ventral  walls  (corpora  cavernosa),  and  is  invested  by  a 
loose  sheath  of  skin — the  prepuce.  At  the  side  of  the  penis 
lie  two  perineal  glands. 

(fr)  Female. — The  ovaries  are  small  oval  bodies  about  three 
quarters  of  an  inch  in  length,  attached  behind  the  kidneys 
to  the  dorsal  abdominal  wall,  exhibiting  on  their  surface 
several  clear  projections  or  Graafian  follicles,  each  of  which 
encloses  an  ovum. 

The  ova,  when  mature,  burst  from  the  ovaries,  and  are 
caught  by  the  adjacent  anterior  openings  of  the  oviducts. 
The  oviducts  are  modified  Miillerian  ducts,  differentiated 
into  three  regions.  The  anterior  portion  or  Fallopian  tube 
is  narrow,  slightly  convoluted,  with  a  funnel-shaped,  fimbri- 
ated  mouth  lying  close  to  the  ovary.  The  median  portion 
or  uterus  is  the  region  in  which  the  fertilised  ova  become 
attached  and  develop.  In  the  rabbit,  the  uterine  regions  of 
the  two  oviducts  are  distinct,  forming  what  is  called  a  double 
uterus.  In  most  cases  the  uterine  regions  of  the  two  ovi- 
ducts coalesce,  forming  a  bicornuate  or  a  single  uterus, 
according  to  the  completeness  of  the  fusion.  In  all  mam- 
mals above  Marsupials,  the  posterior  parts  of  the  two  ovi- 
ducts unite  in  a  median  tube — the  vagina. 

The  vagina  unites  with  the  neck  of  the  bladder,  and  forms 
the  wide  but  short  urinogenital  canal  or  vestibule,  which 
opens  at  the  vulva  ventral  to  the  anus.  On  the  ventral  wall 
of  the  vestibule  lies  the  clitoris,  a  small  rod-like  body — the 
homologue  of  the  penis.  On  the  dorsal  wall  lie  two  small 
Cowper's  glands,  and  there  are  also  perineal  glands  as  in 
the  male. 


SURVEY  OF  THE   ORDERS  OF  MAMMALIA.       679 

The  development  of  the  fertilised  ovum  is  in  most  respects 
like  that  of  the  hedgehog  (pp.  648-56).  In  the  guinea-pig 
and  some  other  Rodents,  out  not  in  the  rabbit,  there  is  a 
remarkable  inversion  of  the  germinal  layers. 

There  is  in  the  rabbit,  as  in  all  Rodents,  a  provisional 
yolk  sac  placenta.  The  allantoic  placenta  is  discoidal  and 
deciduate. 

SYSTEMATIC  SURVEY  OF  THE  ORDERS  OF  MAMMALIA. 

I.  Sub-Class  —  PROTOTHERIA  or  ORNITHODELPHIA  —  Order 

Monotremata. 

II.         „         —  METATHERIA  or  DIDELPHIA  —  'Order  Mar- 
supialia. 

III.  „  —  EUTHERIA    Or     MONODELPHIA    Or    PLACEN- 

TALIA. 

Orders  of  EUTHERIA. 

1.  Edentata. 

2.  Sirenia. 

3.  Ungulata. 


Hyracoidea. 
Proboscidea. 
Extinct  sub-orders. 

4.  Cetacea. 

Mystacoceti  —  baleen  cetaceans. 
Archceoceti  —  (extinct  types). 
Odontoceti  —  toothed  cetaceans. 

5.  Rodentia. 

Simplicidentata. 
Duplicidentata. 

6.  Carnivora. 

Carnivora  Vera. 
Pinnipedia. 
Creodonta  (extinct). 

7.  Insectivora. 

Insectivora  Vera. 
Dermoptera. 

8.  Chiroptera. 

Megachiroptera. 
Microchiroptera. 

9.  Lemuroidea.      }=Primates. 
10.  Anthropoidea.  j 


680 


MAMMALIA. 


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PROTOTHERIA. 
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CHARACTERS   OF  PROTOTHERIA. 


68 1 


e.co. 


Sub-class  PROTOTHERIA  (Syn.  ORNITHODELPHIA), 
Order  Monotremata. 

This  sub-class  includes  the  duckmole  (Ornithorhynchus 
anatinus],  the  spiny  ant  eater  (Echidna  aculeata),  and  a 
third  form  resembling  Echidna,  but  often  referred  to  a 
distinct  genus  as  Proechidna.  These  are  the  lowest  Mam- 
mals, and  exhibit  affinities  with  Sauropsida,  and  perhaps 
even  with  Amphibia.  It  need  hardly  be  said  that  they 
have  no  special  affinities  with  Birds. 

General  Characters  of  Prototheria. 

The  duckmole  is  found  in  the  rivers  of  Australia  and 
Tasmania ;  Echidna,  in  Australia,  Tasmania,  and  New 

Guinea  ;      Proechidna      in 
New  Guinea. 

In  Ornithorhynchus,  the 
skin  is  covered  with  soft 
fur;  in  Echidna  and  Pro- 
echidna,  there  are  spines 
among  the  hairs.  The 
mammary  glands  in  the 
female  Ornithorhynchus 
open  on  a  flat  patch  ;  in 
Echidna,  in  a  depressed 
area  around  which  a  tem- 
porary pouch  seems  to  be 
developed.  There  are  no 
distinct  mammae. 

The  vertebral  centra  bear 
no  epiphyses.  The  skull  is 
smooth  and  polished  as  in  Birds,  for  the  sutures  disappear. 
The  rami  of  the  lower  jaw  do  not  unite  in  front,  and  have 
no  ascending  process.  In  Ornithorhynchus,  there  are.  true 
mammalian  teeth,  but  only  in  the  young ;  in  Echidna,  none 
are  present.  Cervical  ribs  remain  distinct  for  a  time  at 
least ;  the  odontoid  process  of  the  second  vertebra  is  long 
and  not  fused  to  the  centrum.  The  (meta-)  coracoids  reach 
the  praesternum,  there  are  also  large  epicoracoids  and  a 
T-shaped  interclavicle,  the  whole  girdle  resembling  that  of 
Lizards.  An  interclavicle  is,  however,  recognisable  in  he 


FIG.  243. — Pectoral  girdle  of 
Echidna.  (From  Edinburgh 
Museum  of  Science  and  Art. ) 

Sc.,  Scapula;  cL,  clavicle;  i.cl.,  inter- 
clavicle ;  co.,  metacoracoid  ;  e.co.,  epicora- 
coid  ;  st.,  sternum. 


682 


MAMMALIA. 


embryos  of  some  Placentals  also.  In  Ornithorhynchus, 
the  ischia  form  a  long  ventral  symphysis ;  in  Echidna,  the 
acetabulum  socket  for  the  femur  is  incompletely  ossified  as 
in  Birds ;  the  pubes  bear  epipubic  bones,  as  in  Marsupials. 
On  the  side  of  the  tarsus,  in  the  duckmole,  there  is  a  spur 
perforated  by  the  duct  of  a  gland.  This  spur  persists  in  the 
males,  is  rudimentary  in  the  females.  The  male  Echidna 
has  a  similar  but  smaller  spur. 

The  brain  is  smooth,  the  cerebellum  is  not  covered  by 
the  cerebrum,  there  is  a  large  anterior  commissure  and  the 
corpus  callosum  is  rudimentary,  or,  according  to  Symington, 
absent. 

The  food  canal  ends  in  a  cloaca. 

The  right  auriculo-ventricular  valve  in  Ornithorhynchus 
is  partly  muscular  as  in  Birds, 
while  in  other  Mammals  it  is 
membranous  and  is  worked  by 
papillary  muscles  attached  to  it 
by  tendon-like  cords  (chordae 
tendineae).  The  temperature  of 
the  body  is  said  to  be  about 
25-28°  C. 

The  ureters  open,  not  into  the 
bladder,  but  into  the  urinogenital 
canal. 

The  testes  remain  in  the  abdo- 
men. The  left  ovary  is  larger 
than  the  right,  as  in  Birds.  The 
vasa  deferentia  open  separately 
into  the  urinogenital  canal.  So  in 
the  female  do  the  oviducts,  and  these  have  no  fringed 
fimbriated  apertures  nor  distinct  uterine  region.  The  penis 
is  attached  to  the  ventral  wall  of  the  cloaca,  and  its  canal  is 
not  continuous  with  the  urinogenital  canal. 

The  ova  are  large,  with  abundant  yolk,  and  undergo 
meroblastic  segmentation.  The  Prototheria  are  oviparous. 

The  duckmole,  duck-billed  platypus,  or  water  mole,  lives  beside  lakes 
and  rivers.  It  swims  by  means  of  its  fore  limbs,  which  are  webbed  as 
well  as  clawed  ;  it  grubs  for  aquatic  insects,  crustaceans,  and  worms  in 
the  mud  at  the  bottom  of  the  water.  It  collects  small  animals  in  its 
cheek  pouches,  and  chews  them  at  leisure  with  its  eight  horny  jaw 


FIG.  244.— Pelvis  of  Ech- 
idna. (From  Edinburgh 
Museum  of  Science  and  Art.) 

S,  sacrum  ;  Ep,  epipubic  bones  ; 
Ac,  acetabulum  ;  o.f,  obturator 
foramen  between  ischium  and 
pubis  (/.)• 


CHARACTERS  OF  PROTOTHERIA. 


683 


plates.  It  makes  long  burrows  in  the  banks,  often  with  two  openings, 
one  above,  one  under  the  water.  The  animal  is  shy,  and  dives  swiftly 
when  alarmed.  When  about  to  sleep,  it  rolls  itself  into  a  ball.  In  the 
recesses  of  the  burrows  the  eggs  are  laid,  two  at  a  time.  The  egg 
measures  about  three  quarters  of  an  inch  in  length,  and  is  enclosed  in  a 
"strong,  flexible,  white  shell,"  through  which  the  young  animal  has  to 
break  its  way. 

The  full  grown  duckmole  measures  from  eighteen  to  twenty  inches  in 
length  ;  the  male  slightly  exceeds  his  mate.  The  fur  is  short  and  soft, 
dark  brown  above,  lighter  beneath.  The  jaws  are  flattened  like  the  bill 
of  a  duck,  and  covered  with  naked  skin,  which  forms  a  soft,  sensitive, 
collar  around  the  region  where  the  bill  joins  the  skull.  The  eyes  are 
very  small ;  there  is  no  external  ear  flap  or  pinna  ;  the  nostrils  lie  near 
the  end  of  the  upper  part  of  the  bill.  The  tail  is  short  and  flat. 

Horny  plates,  two  on  each  jaw  above  and  below,  serve  as  teeth  in  the 
adult.  True  teeth,  three  on  each  jaw  above  and  below,  are  calcified,  but 


-v.d. 


FIG.  245.— Urinogenital  or- 
gans of  Male  Duckmole.  (After 
OWEN.) 

BL,  Bladder;  u.,  ureter;  v.d.,  vas 
deferens  ;  r.^  rectum  ;  £•/.,  gland  ;  cL, 
cloaca  ;/.,  penis  ;  u.g.c.,  urine-genital 
canal. 


FIG.  246. — Urinogenital  or- 
gans of  Female  Duckmole. 
(After  OWEN.) 

Ov,  ovary  ;  od,  oviduct ;  ut,  ' '  uter- 
ine" region;  u,  ureter;  r,  rectum; 
uv,  bladder  ;  ug,  Urinogenital  sinus ; 
cl,  cloaca. 


soon  worn  away  and  shed.  The  digits  are  clawed  and  connected  by  a 
web  which  is  best  developed  on  the  fore  limbs.  The  spur  borne  on  the 
heel  seems  to  be  sometimes  used  as  a  weapon,  and  as  it  persistsjpnly  in 
the  males,  is  perhaps  useful  in  contests  between  rivals. 

Echidna  and  Proechidna  live  in  rocky  regions,  are  mainly  nocturnal 
in  habit,  and  burrow  rapidly,  legs  foremost.  They  feed  on  ants,  which 
are  caught  on  the  rapidly  mobile,  slender,  viscid  tongue.  No  traces 
of  teeth  have  as  yet  been  seen. 


684  MAMMALIA. 

Strong  spines  occur  thickly  in  Echidna,  more  sparsely  in  Proechidna 
among  the  hairs.  The  snout  is  prolonged  into  a  slender  tube.  The 
limbs  bear  five  toes,  two  of  which  in  Proechidna  are  often  without  claws 
and  somewhat  rudimentary.  In  Echidna,  the  eggs  seem  to  be  hatched 
in  a  temporarily  developed  pouch. 

Sub-class  METATHERIA  or  UIDELPHIA, — Order  Marsupialia. 

With  the  exception  of  the  N.  American  opossums,  all 
the  Marsupials  now  alive  are  natives  of  Australasia.  But 
fossil  remains  found  in  Europe  and  America  show  that  they 
once  had  a  wide  range.  As  there  are  no  higher  mammals 
indisputably  indigenous  to  Australasia,  it  seems  as  if  the 
insulation  of  that  region  had  occurred  after  the  Marsupials 
had  gained  possession,  but  before  higher  mammalian  com- 
petitors had  arrived.  Thus  saved  and  insulated,  the 


FIG.  247. — Lower  jaw  of  Kangaroo. 
a.,  Inflected  angle  ;  /.,  single  incisor. 

Marsupials   have   developed   in  many  different   directions, 
the  families  included  in  this  order  being  very  diverse. 

General  Characters  of  Marsupials. 

The  brain  is  less  developed  than  in  placental  Mammals, 
for  the  convolutions  are  simple  or  absent,  the  anterior  com- 
missure is  large,  the  corpus  callosum  is  small  or  even  absent. 
In  the  skeleton  there  are  several  peculiarities  ;  thus  the 
angle  of  the  lower  jaw  is  more  or  less  inflected,  except  in 
the  genus  Tarsipes ;  there  are  generally  two  epipubic  or 
"  marsupial "  bones  in  front  of  the  pubic  symphysis  ;  there 
are  more  incisors  above  than  below  (except  in  the  wombat), 
and  the  number  of  incisors  sometimes  exceeds  three  on 
each  side. 


CHARACTERS  OF  MARSUPIALS.  685 

The  functional  teeth  are  those  of  the  milk  set,  with  the 
probable  exception  of  the  third  (or  rather  fourth)  premolar. 
It  may  be,  however,  that  this  successional  premolar  is  a 
retarded  milk  tooth,  intermediate  in  position  between  pm.  2 
and  pm.  3.  In  living  Marsupials,  there  seems  to  be  a 
suppression  of  what,  in  typical  placentals,  would  be  called 
the  second  premolar. 

A  common  sphincter  muscle  surrounds  the  anus  and  the 
urinogenital  aperture,  and  in  the  females  (except  kangaroos) 
the  anus  lies  so  much  within  the  urinogenital  sinus  that  the 
arrangement  maybe  described  as  cloacal.  The  scrotal  sac 
containing  the  testes  lies  in  front  of  the  penis.  The  genital 
ducts  of  the  females  are  often  separate  throughout,  so  that 
there  are  two  uteri  and  two  vaginae.  But  the  bent  proximal 
parts  of  the  vaginae  sometimes  fuse  and  form  a  caecum, 
which,  according  to  the  degree  of  fusion,  may  be  a  single 
tube  or  divided  by  a  partition.  Moreover,  in  Bennett's 
kangaroo,  the  caecum  opens  independently  into  the  cloaca 
between  the  apertures  of  the  distal  portions  of  the  vaginae. 

The  allantois  may  be  small,  and  may  fail  to  reach  the 
subzonal  membrane.  In  no  case  is  there  an  allantoic 
placenta.  The  yolk-sac  is  large  and  adheres  to  a  portion 
of  the  subzonal  membrane.  From  this  region  in  some 
cases,  e.g.,  opossums,  non-vascular  villi  are  given  off,  which 
enter  into  close  connection  with  the  glands  of  the  uterine 
wall.  The  embryo  is  also  attached  to  the  uterus  by 
amoeboid  processes  from  the  subzonal  membrane. 

The  gestation  is  short,  only  lasting  a  fortnight  in  the 
opossum,  about  five  weeks  in  the  kangaroo,  whereas,  that  of 
the  mare,  for  instance,  is  about  eleven  months.  Except  in 
some  opossums,  there  is  a  marsupial  pouch,  usually  with  a 
forward  directed  aperture.  Within  this  pouch  are  the  teats, 
and  here  the  delicate  young  are  nurtured  after  birth.  As 
they  are  unable  to  suck,  the  milk  is  forced  down  their 
throat,  the  mammary  gland  being  compressed  by  the 
cremaster  muscle  which  covers  it.  Vague  vestiges  of  a 
marsupium  are  said  to  be  visible  in  some  Placentals. 

Families  of  Marsupials. 

A.  POLYPROTODONTIA.  "  Incisors  numerous,  small,  subequal 
Canines  larger  than  the  incisors.  Molars  with  sharp  cusps." 


686  MAMMALIA. 

Family,  Didelphyidse  : — American  opossums,  distributed  from  the 
United  States  to  Patagonia,  arboreal  in  habit,  usually  carnivorous 
or  insectivorous  in  diet.  The  limbs  have  five  digits  with  claws, 
the  hallux  is  opposable.  The  tail  is  generally  long  and  often 
prehensile.  The  stomach  is  simple,  the  caecum  small.  The 
pouch  is  generally  absent,  but  the  young  are  often  carried  on 
the  back  of  the  mother,  their  tails  coiled  round  hers.  Dentition, 
5134 

4J34 

Examples  : — The  Virginian  or  crab-eating  opossum  (Didelphys 
marsupialis})  with  a  pouch  ;  the  woolly  opossum  (D.  lanigera}; 
the  aquatic  Yapock  (Chironectes]  which  feeds  on  fish  and 
smaller  water  animals. 

Family  Dasyuridse  : — Carnivorous  or  insectivorous  Marsupials.     The 

limbs  have  clawed  digits,  five  in  front,  four  or  five  behind.     The 

canines  are  generally  large.     The  stomach  is  simple,  there  is  no 

csecum. 

Examples  : — The  Tasmanian  wolf  (Thylacinus],  of  dog-like  form, 

4134 
dentition, >    and  the  Dasyure  {Dasyurus},  civet-like,  den- 

3J34 
4124 

tition, ;  are  specialised  as  carnivores.     The  members  of  the 

3^24 

genus  Phascogale  are  small  and  insectivorous.  The  banded 
ant  eater  (Myrmecobius]  of  W.  and  S.  Australia,  a  somewhat 
squirrel-like  animal,  has  a  long  thread-like  protrusible  tongue 

4135  or  6 
and  more  teeth  than  any  other  Marsupial  — 

3135  or  6 

Family  Peramelidse  : — The  burrowing  bandicoots,  all  small  in  size, 
insectivorous  or  omnivorous  in  diet.  In  the  fore  feet,  two  or 
three  of  the  middle  toes  are  well  developed  and  clawed,  the 
others  being  rudimentary  ;  in  the  hind  feet,  the  hallux  is  small 
or  absent,  the  second  and  third  toes  are  very  slender  and  united 
in  the  same  fold  of  skin,  the  fourth  toe  is  very  large,  the  fifth 
smaller, — the  whole  foot  suggesting  that  of  the  kangaroo.  The 
stomach  is  simple,  the  csecum  not  large.  Clavicles  are  absent. 

4  or  5 134 

Dentition,  —    — • 
3    J34 

Examples  : — The  true  bandicoot  (Perameles}  ;  the  native  rabbit 
(Peragale  lagotis]  ;  the  rat-like  Chceropus. 

B.    DIPROTODONTIA.       "  Incisors    usually  - ;    the   first    large   and 

i 

cutting  ;  the  upper  canines  generally,  the  lower  canines  always 
small  or  absent ;  the  molars  with  bluntly  tuberculated  or  trans- 
versely ridged  crowns." 

Family     Phascolomyidce : — The    Wombats,    terrestrial,    vegetarian, 
nocturnal  Marsupials,  somewhat  bear-like  in  appearance.     The 
1014 

dentition  is  rodent-like, ,  the  teeth  have  persistent  pulps,  the 

1014 
incisors  are  chisel-edged,  there  being  no  enamel  except  in  front. 


FAMILIES  OF  MARSUPIALS.  687 

In  the  embryo,  however,  there  are  four  upper  incisors,  of  which  the 
first  persists,  and  five  lower  incisors,  of  which  the  third  persists. 
The  fore  feet  have  five  distinct  toes  with  strong  nails  :  the  hind 
feet  have  a  small  nailless  hallux,  the  second,  third,  and  fourth 
toes  partly  united  by  skin,  the  fifth  distinct.  The  tail  is  very 
short.  The  stomach  is  simple,  the  caecum  very  short. 

There  is  but  one  genus — Pkcacoloniys^  with  three  species. 
Family  Phalangeridse  : — Small  woolly  arboreal  nocturnal  Marsupials, 
with  vegetarian  or  mixed  diet.  The  fore  feet  have  five  distinct 
toes  ;  the  hind  feet  have  a  large,  nailless,  opposable  hallux,  the 
second  and  third  toes  are  narrow  and  bound  together  by  skin,  the 
fourth  and  fifth  free.  The  tail  is  generally  long  and  prehensile. 
The  stomach  is  simple,  the  caecum  usually  large.  Average 

3>   i>  2-3,  3-4 

dental  formula, • 

I,    O,    O-2,   3-4 

Examples  : — The  grey  Cuscus  (Phalanger  orientalis)  ;  Tarsipes,  a 
small  mouse-like  animal  which  feeds  on  honey,  and  is  remark- 
able in  having  no  inflection  of  the  angle  of  the  mandible  and 
no  caecum  ;  the  flying  phalangers  (Petaurus],  with  a  parachute 
of  skin  extending  from  the  little  finger  to  the  ankle  ;  the 
Koala  or  "  native  bear  "  (Phascolarctos  cinereus],  a  relatively 
large  form  about  two  feet  in  length.  An  extinct  form,  Thy- 
lacoleo,  of  the  late  Tertiary  period  of  Australia,  is  interesting 
in  its  extraordinary  dentition,  the  functional  teeth  being  reduced 
to  large  front  incisors  and  third  premolars,  both  adapted  for 
sharp  cutting. 
Family  Macropodidce : — Kangaroos,  herbivorous  terrestrial  Marsupials. 

3>  O-T>  2>  4 

Dentition, •       The  incisors  are  sharp  and  suited  for 

I,      O,      2,     4 

cropping  herbage.  The  hind  legs  are  usually  larger  than  the 
fore  legs,  and  the  animals  move  by  leaps. 

Examples : — The  true  Kangaroos,  e.g.,  Macropus ;  the  rat-kangaroos 
or  potoroos  (Potorous] ;  the  genus  Hypsipryinnodon,  with  a  foot 
approaching  that  of  the  Phalangers. 

The  true  Kangaroos,  belonging  to  the  genus  Macropus,  include  the 
largest  living  Marsupials,  but  within  the  genus  there  is  much  difference 
in  size. 

The  grey  Kangaroo  (M.  giganteus]  lives  on  the  grassy  plains  of  Eastern 
Australia  and  Tasmania,  and  is  as  tall  as  a  man  ;  the  Wallabies,  at  home 
in  the  bush,  are  smaller,  and  some  are  no  bigger  than  rabbits. 

The  hind  limbs  seem  disproportionately  long,  and  are  well  suited  for 
rapid  bounding.  The  long  tail,  carried  horizontally,  helps  to  balance 
the  stooping  body  as  the  animal  leaps,  and  it  gives  additional  stability 
to  the  erect  pose.  The  fore  limbs  sometimes  come  to  the  ground  when 
the  animal  is  feeding,  and  in  the  largest  species  they  are  strong  enough 
to  throttle  a  man. 

The  fore  limbs  bear  five  clawed  digits,  the  hind  feet  have  only  four. 
The  hallux  is  absent ;  the  fourth  toe  is  very  long ;  the  fifth  is  about  half 
as  large  ;  the  third  and  second  are  too  slender  to  be  useful  for  more  than 
scratching,  and  are  bound  together  by  the  skin  (syndactylous).  The 


688 


MAMMALIA. 


length  of  the  hind  limb  is  due  to  the  tibia  and  fibula,  and  to  the  foot. 
The  clavicles  and  fore  arm  are  well  developed.  The  epipubic  or  mar- 
supial bones  are  large. 

The   Kangaroos  feed  on  herbage,  and   are  often   hunted  down  on 
account  of  the  damage  which  they  do  to  pas- 
tures and  crops.     The  sharp  incisors  are  suited 
for  cropping  the  grass  and  herbs,  which  the 
ridged  and  tuberculated  molars  crush. 

As  the  Kangaroos  are  exclusively  herbi- 
vorous, it  is  not  surprising  to  find  that  the 
stomach  is  large  and  complex,  with  numerous 
saccules  on  its  walls.  The  whole  gut  is  long, 
and  there  is  a  well-developed  caecum. 

Among  extinct  Marsupials  there  were  some 
gigantic  forms,  notably  Diprotodon  auslralis, 
as  large  as  a  Rhinoceros.  It  is  likely  that 
many  of  the  early  Mesozoic  mammals  were 
Marsupials.  The  discovery  of  extinct  Thy- 
lacine-like  marsupials  in  the  lower  Tertiaries 
of  Patagonia  suggests  an  ancient  land  connec- 
tion between  Australia  and  the  southern  ex- 
tremity of  America.  No  other  fossil  Mar- 
supials of  Australian  type  are  known  in  other 
parts  of  the  world. 

A  remarkable  new  type  of  Marsupial  (Noto- 
ryctes  typhlops)  has  been  recently  discovered 
by  Dr.  E.  C.  Stirling.  Four  or  five  cervical 
vertebrae  are  fused,  there  is  a  keeled  sternum, 
and  a  bird-like  pelvis.  The  eyes  are  rudi- 
mentary and  beneath  the  skin.  The  marsupial 
bones  are  small  nodules.  There  is  a  functional 
pouch.  The  animal  is  a  rapid  burrower.  In 
its  mole-like  characters  it  is  a  good  illustration  of  "  convergence,"  i.e., 
the  appearance  of  similar  structures  in  forms  not  nearly  related,  appar- 
ently in  response  to  similar  conditions  of  life. 

Most  palaeontologists  recognise,  besides  Marsupials  and  Monotremes, 
another  order  of  non-placental  Mammals,  —  the  Multi-tuberculata,  — 
wholly  extinct. 


FIG.  248. — Foot  of 
young  Kangaroo. 

2-3,  Small  syndactylous 
toes ;  4,  large  fourth  toe  ; 
5,  fifth  toe. 


Sub-class  EUTHERIA — Order  i.     EDENTATA. 

This  order  includes  five  very  distinct  families  with  living 
representatives — the  sloths,  the  ant  eaters,  the  armadillos, 
the  pangolins,  and  the  aard-varks.  The  first  three  families 
are  found  in  the  New  World,  the  last  two  in  the  Old  World. 

Functional  teeth  are  absent  in  several,  but  the  ant  eaters  (Myrme- 
cophagidse)  are  the  only  forms  which  still  appear  strictly  edentulous. 
When  present  the  teeth  are  uniform,  usually  simple,  without  roots, 
and  with  persistent  pulp.  They  are  never  developed  on  the  fore 


FAMILIES   OF  EDENTATA.  689 

part  of  the  mouth,  and  they  have  not  more  than  hints  of  enamel.  Till 
recently  the  dentition  was  described  as  monophyodont,  but  we  now 
know  that  there  is  evidence  of  two  sets  in  Tatusia,  Orycteropus, 
Dasypus,  and  others.  It  is  the  milk  set  which  disappears. 

The  modern  Edentata  are  specialised  survivors  of  a  wan- 
ing order,  whose  extinct  representatives  seem  to  have  been 
larger  and  more  primitive.  The  modern  forms  usually  have 
protective  peculiarities  of  structure  and  habit  which  secure 
their  persistence.  Thus  some  are  arboreal,  others  are  bur- 
rowers,  and  many  are  covered  with  strong  armature  of  bone 
or  of  horn.  It  is  interesting  to  observe  how  very  varied  the 
nature  of  the  placenta  is  : — 

a  dome-shaped  disc  (deciduate)  in  the  sloths, 
dome-shaped    or    discoidal    (deciduate)    in    the    ant 

eaters, 

discoidal  (deciduate)  in  the  armadillos, 
diffuse  (non-deciduate)  in  the  pangolins, 
zonary  (deciduate)  in  the  aard-varks. 

Families  of  Edentata. 

i.  Bradypodidae — Sloths. — The  three-toed  sloths  (Bradypus)  and  the 
two-toed  sloths  (Chol&pus]  are  restricted  to  the  forests  of  South 
and  Central  America.  They  are  the  most  arboreal  of  mammals, 
passing  their  whole  life  among  the  branches,  to  which  they 
hang,  and  along  which  they  move  back  downwards.  They  are 
solitary,  nocturnal,  vegetarian  animals,  sluggish,  as  their  name 
suggests,  and  with  a  very  firm  grip  of  life.  Their  shaggy  hides 
harmonise  with  the  mosses  and  lichens  on  the  branches,  and 
the  protective  resemblance  is  increased  by  the  presence  of  a 
green  alga  on  the  hair.  Their  food  consists  of  leaves  and 
shoots  and  fruits. 

The  body  is  covered  with  coarse  shaggy  hair  ;  the  head  is  rounded, 
and  bears  very  small  external  ears ;  the  fore  limbs  are  longer  than  the 
hind  limbs,  and  the  two  or  three  digits  are  bound  together  by  skin,  and 
have  long  claws  ;  the  tail  is  rudimentary. 

Concerning  the  skeleton  we  may  note  the  —  rootless,  unenamelled 

teeth,  the  incomplete  zygomatic  arch  with  a  descending  process  from 
the  jugal,  the  presence  of  clavicles,  the  rod-like  appearance  of 
the  embryonic  stapes,  the  occurrence  of  nine  cervical  vertebrae  in 
Bradypus,  of  six  in  Cholcepus.  The  adult  Bradypus  has  a  distinct 
coracoid  or  epicoracoid. 

As  in  most  herbivorous  animals,  the  stomach  is  complex,  but  there  is 
no  caecum.     In  the  limbs  the  main  blood  vessels  break  up  into  numerous 
parallel  branches.     The  uterus  is  simple,  the  vagina  seems  to  be  origin- 
44 


690  MAMMALIA. 

ally  divided  by  a  median  partition,  the  placenta  is  a  deciduate  dome- 
shaped  disc.     One  young  one  is  born  at  a  time.         • 

2.  MegatheriidcS  or  Ground  Sloths — extinct  forms  of  large  size,  inter- 

mediate between  the  sloths  and  the  ant  eaters.  Their  remains 
are  found  in  Pleistocene  deposits  in  N.  and  S.  America. 
Megatherium  exceeded  the  Rhinoceros  in  size. 

3.  MyrmecophagidcE — the  Ant  Eaters,  hairy  animals,  without  even 

traces  of  teeth,  with  long  thread-like  protrusible  tongues,  viscid 
with  the  secretion  of  greatly  enlarged  submaxillary  glands.  One 
form,  Myrmecophaga  jubata,  is  terrestrial,  the  others,  belonging 
to  the  genera  Tamandua  and  Cycloturus^  are  arboreal.  All 
feed  on  insects.  All  are  Neotropical.  The  skull  is  long  ;  the 
third  finger  is  greatly  developed,  the  others  are  small ;  the  pes 
has  four  or  five  almost  equal  clawed  toes ;  the  clavicles  are 
rudimentary  ;  the  tail  is  long  and  sometimes  prehensile.  The 
brain  is  well  convoluted.  The  uterus  is  simple.  The  placenta 
is  dome-like  or  discoidal. 

4.  Dasypodidae — the   Armadillos,    all   S.  American   except    Tatusia 

novemcincta,  which  extends  as  far  north  as  Texas.  They  are 
nocturnal,  omnivorous  animals,  able  to  run  and  burrow  rapidly. 
They  are  unique  among  living  mammals  in  having  a  dermal 
armature  of  bony  scutes  united  into  shields  and  rings,  and 
covered  by  horny  epidermis.  The  teeth  are  numerous,  simple, 
and  of  persistent  growth.  Clavicles  are  well-developed.  The 
digits  have  strong  claws  or  nails.  The  brain  has  large  olfactory 
lobes ;  the  cerebral  hemispheres  have  few  convolutions.  The 
tongue  is  long  and  protrusible,  and  the  submaxillary  glands 
are  large.  The  stomach  is  simple.  The  uterus  is  simple. 
The  placenta  is  discoidal  and  deciduate. 

Examples  : — Dasypus^  Chlamydophorus^  Tatusia. 

5.  Glyptodontidoe — extinct  Pleistocene  types,  mostly  S.  American,  but 

represented  in  Mexico  and  Texas.  The  body  was  often  huge, 
and  was  covered  by  a  solid  carapace  of  great  strength,  "  Why 
such  a  form  as  the  Glyptodon  should  have  failed  to  keep  his 
ground  is  a  great  mystery  ;  nature  seems  to  have  built  him,  as 
Rome  was  built,  for  eternity."  (W.  K.  Parker.) 

6.  Manidse — the  Ethiopian  and  Oriental  Pangolins,  covered  dorsally 

with  overlapping  horny  scales.  They  are  terrestrial,  burrowing 
animals,  but  sometimes  climb  trees.  They  usually  feed  on  ter- 
mites. Teeth  are  rudimentary,  the  tongue  is  long  and  pro- 
trusible. The  uterus  is  bicornuate,  the  placenta  diffuse  and 
non-deciduate.  There  is  one  extant  genus  Manis. 

7.  Orycteropidae — the    Ethiopian    Aard-varks,    represented    by   two 

species  of  Orycteropus,  ranging  from  S.  Africa  to  Egypt. 
They  are  shy,  nocturnal  animals,  living  in  burrows,  feeding 
on  termites.  There  are  numerous  complex  teeth.  The  skin 
bears  scanty  bristles.  The  mouth  is  tubular,  and  the  tongue 
is  narrow  and  protrusible.  The  digits  bear  nails  suited  for 
diggmg-  The  uterus  is  bicornuate,  the  placenta  broadly 
zonary. 


SEA    COWS.  691 

Order  2.  SIRENIA — Sea  Cows. 

A  small  moribund  order  of  sluggish,  aquatic,  vegetarian 
Mammals,  in  no  direct  way  connected  with  Cetaceans, 
possibly  related  to  Ungulates,  but  certainly  primitive.  There 
are  two  living  genera,  Halicore  (Dugong),  and  Manatus 
(Manatee),  and  one  recently  exterminated  (Rhytina). 

The  Sirenia  are  sluggish  animals,  with  massive  heavy  bones, 
a  plump  body,  some  oil,  and  sparse  hair.  They  are  aquatic, 
with  fish-like  form,  no  trace  of  hind  limbs,  flipper-like  fore 
limbs,  no  external  ear,  valved  nostrils,  networks  in  the 
arteries  (useful  in  prolonged  immersion).  Th,ey  are  herbi- 
vorous, and  like  others  of  similar  habit  have  a  chambered 
stomach,  a  long  intestine,  and  a  caecum. 

They  are  primitive,  and  with  this  fact  may  be  associated 
the  abdominal  testes,  the  absence  of  distinct  epiphyses  on 
the  vertebrae  (as  in  Prototheria),  and  the  small,  rather 
smooth  brain. 

The  body  is  fish-like,  the  head  rounded,  the  skin  is 
thick  and  tough,  with  sparse  bristles,  mostly  about  the 
mouth. 

The  paddle-shaped  fore  limbs  have,  at  most,  rudimentary 
nails  ;  there  are  no  hind  limbs.  The  skull  is  not  like  that 
of  Cetaceans.  The  nasals  are,  at  most,  rudimentary.  There 
are  no  canine  teeth.  There  are  chevron  bones  below  the 
tail.  There  are  no  clavicles.  The  pelvis  is  rudimentary, 
and  there  is  no  sacrum. 

The  brain  is  small  and  has  few  convolutions.  The  nostrils 
are  valved,  and  lie  at  the  end  of  the  snout.  The  small 
eyes  have  imperfect  eyelids,  but  have  a  nictitating  mem- 
brane. There  are  no  external  ears.  In  the  mouth  there 
are  horny  crushing  plates.  The  stomach  is  chambered, 
and  there  is  a  caecum.  The  ventricles  are  separated  by  a 
cleft.  There  are  retia  mirabilia  on  the  arteries.  The  testes 
are  abdominal  in  position.  The  uterus  is  bicornuate.  Two 
teats  lie  behind  the  arm  pits.  The  placenta  of  the  dugong 
is  zonary,  wholly  or  in  great  part  non-deciduate.  The 
placenta  of  the  manatee  has  not  yet  been  investigated. 
The  food  consists  of  Algae  and  estuarine  plants. 


692  MAMMALIA. 

MANATEE  (Manatus}.  DUGONG  (Halicore). 

Neck  vertebras  reduced  to  s\x.  The  usual  seven  neck  vertebras. 

Abortive  incisors  (|)  in  both  sexes.         Two  tusk-like  incisors  persist  in  the 

the  male. 

Molars   (Li)    six   or   so   at   a   time,     Molars  (4  or  «.}   2  or  3  at  a  time), 
uniform,  with  square  enamelled  primitive,  with  persistent  pulps 

crowns,  and  tuberculated  trans-  and  no  enamel, 

verse  grinding  ridges. 

Premaxillas  almost  straight.  Premaxillas  crooked  downwards. 

Tail  rounded.  Deeply  notched  tail. 

Rudimentary  nails  on  fingers.  Nailless  digits. 

Caecum  divided.  Thick  and  single  caecum. 

M.    australis   and   M.    senegalensis     H.    tabernaculi,    E.    African    coast 
live  in  the  mouths  of  great  rivers  and  Red  Sea ;  H.  dugong,  Indian 

which    flow    into     the    tropical  and    Pacific    Oceans,  eastward 

Atlantic.  from  the  home  of  the  last  species 

to  the  Philippines;  H.  australis, 
E.  and  N.  Australia. 

The  genus  Rhytina  was  toothless,  with  a  slightly  crooked  snout,  small 
head  and  arms,  and  thick  naked  skin.  Steller's  Sea  Cow  (R.  stelleri], 
the  only  known  species,  from  the  North  Pacific,  seems  to  have  been 
exterminated  in  the  last  century. 

The  order  was  once  much  larger.  Fossil  forms  occur  in  Tertiary  strata. 
The  most  important  is  Halitherittm,  a  less  specialised  Sirenian  than 
those  still  extant,  e.g.,  with  traces  of  hind  limbs. 

Order  3.    UNGULATA. 

Hoofed  Animals — Artiodactyla,  Perissodactyla,  Hyra- 
coidea,  Proboscidea,  and  extinct  sub-orders. 

This  large  and  somewhat  heterogeneous  order  includes 
pigs,  hippopotamus,  camels,  cattle,  deer,  tapirs,  rhinoceros, 
horses,  hyrax,  elephants,  and  some  other  distinct  types. 

They  are  terrestrial,  and  for  the  most  part  herbivorous 
animals.  Their  digits  generally  end  in  hoofs  or  at  least  in 
broad  nails.  In  the  adults  of  the  modern  types  there  are  no 
clavicles.  The  teeth  are  diverse,  the  milk  set  in  part 
persistent  until  the  animal  attains  maturity. 

Ungulata  Vera : — ARTIODACTYLA  and  PERISSODACTYLA. 

In  these  typical  Ungulates,  the  feet  are  never  plantigrade. 
In  modern  types  there  are  never  more  than  four  functional 
toes.  The  os  magnum  of  the  carpus  articulates  freely  with 
the  scaphoid.  The  brain  is  well  convoluted.  The  testes 
descend  into  a  scrotum.  The  uterus  is  bicornuate.  The 
placenta  is  non-deciduate,and  either  diffuse  or  cotyledonary. 


EVEN-TOED    UNGULATES. 


693 


ARTIODACTYLA — PIGS,  CAMELS, 
CHEVROTAINS,  and  RUMINANTS. 


PERISSODACTYLA — TAPIRS, 
RHINOCEROS,  HORSES. 


The  third  and  fourth  digits  of  each  foot 
are  equally  developed,  and  the  line 
halving  the  foot  runs  between  them. 

The  premolars  and  molars  are  usually 
different. 

There  are  nineteen  dorso-lumbar  verte- 
brae. 

The  femur  has  no  third  trochanter. 

The  astragalus  has  always  equal  articular 
facets  for  the  navicular  and  for  the 
cuboid.  The  calcaneum  has  an  arti- 
cular facet  for  the  fibula. 

The  stomach  tends  to  be  complex,  and  the 
caecum  is  small. 

The  mammae  are  few  and  inguinal,  or 
numerous  and  abdominal. 

The  placenta  is  diffuse  or  cotyledonary. 


The  third  digit  occupies  the  middle  of 
the  foot,  is  largest,  and  is  symmetrical 
on  itself,  so  that  the  line  halving  the 
foot  bisects  the  third  digit. 

The  premolars  resemble  the  molars. 

There  are  almost  always  twenty-three 
dorso-lumbar  vertebrae. 

The  femur  has  a  third  trochanter. 

The  astragalus  has  a  large  facet  for  the 
navicular,  a  small  facet  for  the  cu- 
boid. The  calcaneum  does  not  arti- 
culate with  the  lower  end  of  the 
fibula. 

The  stomach  is  always  simple,  and  the 
caecum  is  large. 

The  mammae  are  always  inguinal. 

The  placenta  is  always  diffuse. 


Sub-Order  ARTIODACTYLA — Even-toed  Ungulates. 

Pigs  and  Hippopotamus  (Suina),  Camels  (Tylopoda), 
Chevrotains  (Tragulina),  and  Ruminants  (Pecora)  like 
Cattle  and  Deer. 

The  general  characters  of  this  sub-order  have  been  stated 
above  in  contrast  to  those  of  Perissodactyla.  The  equal 
development  of  the  third  and  fourth  digits,  the  fact  that  the 
premolars  have  a  single  lobe  while  the  molars  have  two,  the 
nature  of  the  tarsal  bones,  the  tendency  that  the  stomach 
has  to  be  complex  (as  in  Camels  and  Ruminants)  are  im- 
portant characteristics.  There  are  others  of  less  obvious 
importance,  such  as  the  absence  of  the  alisphenoid  canal, 
which  in  Perissodactyla  encloses  the  external  carotid  artery 
as  it  passes  along  the  alisphenoid. 

There  are  primitive  extinct  Artiodactyla  which  connect 
the  four  modern  groups — Suina,  Tylopoda,  Tragulina,  and 
Pecora.  Thus  they  unite  the  bunodont  types,  such  as  pigs, 
with  cone-like  tubercles  on  the  crowns  of  the  molars,  and 
the  selenodont  types,  such  as  cattle,  with  the  tubercles 
expanded  from  before  backwards  and  curved  in  crescents. 

Group  i.  Suina — Hippopotamus,  Pigs,  and  Peccaries.  The  molars 
are  bunodont ;  the  third  and  fourth  metacarpals  and  metatarsals  are  not 
completely  fused  as  "cannon  bones." 

Hippopotamidne  : — Huge  African  mammals,  included  in  the  single 
genus  Hippopotamus.  They  spend  the  day  in  the  rivers  and 
lakes,  swimming  and  diving  well,  but  usually  remaining  con- 


694 


MAMMALIA. 


cealed.  At  night  they  come  on  land  and  browse  on  grass  and 
herbage.  The  skin  is  extremely  thick,  with  a  few  hairs  restricted  to 
the  snout,  head,  neck,  and  tail.  There  are  four  toes  on  each  foot, 
all  reaching  the  ground.  The  rootless  incisors  continue  growing  ; 

2-3,  M3 
so  do  the  large  curved  canines  ;    the  dental  formula  is  — 

i-3>  143 

The  stomach  has  three  chambers  ;  there  is  no  coecum. 
Suidse  : — The  Old  World  boars  and  pigs,  characterised  by  the  mobile 
snout  and  terminal  nostrils.     There  are  four  well-developed  digits 
on  the  narrow  feet,  but  the  second  and  fifth  do  not  reach  the 


FIG.  249.— Foot  of  Ox. 

a.,  Astragalus  ;  c.,  os  calcis  ; 
m.t.t  cannon  bone  (fused  third 
and  fourth  metatarsals)  ;  ph., 
phalanges. 


FIG.  250. — Fore  leg  of  Pig. 

/i.,  Humerus  ;  r.,  radius  ;  u.,  ulna; 
£.,  scaphoid  ;  /.,  lunar  ;  c.,  cuneiform  ; 
t.,  trapezoid  ;  ;;/.,  os  magnum  ;  «., 
unciform  ;  2-5,  digits. 


ground  in  walking.  The  incisors  are  rooted,  the  upper  canine 
curves  outwards  or  upwards.  The  stomach  is  almost  simple,  but 
has  more  or  less  of  a  cardiac  pouch  ;  there  is  a  caecum. 

3*43  2I23 

Examples  : — Stis, ;    Babirusa  — ,  the  male  with   remarkable 

3J43  3*23 

canines,   the    upper  pair   growing  upwards   from    their    base 


EVEN-TOED    UNGULATES.  695 

through  the  skin,  arching  backwards  As  far  as  the  forehead, 
and  sometimes  forwards  and  downwards  again,  the  lower  pair 
with  a  more  or  less  parallel  course  ;   Phacockatrus^  the  wart 
hog. 
Dicotylidre  : — The  New  World   Peccaries  (Dicotyles\  with  a  snout 

like  that  of  pigs,  with  four  toes  on  the  fore  feet,  and  three  behind. 

The  incisors  are  rooted,  the  upper  canines  are   directed   down- 

2133 
wards,  the  dental  formula  is •  The  stomach  is  complex,  and 

3133 
there  is  a  caecum. 

Group  2. — Tylopoda,  comprising  the  family  Camelidae — the  Camels  of 
the  Old  World  and  the  Llamas  of  S.  America.  The  limbs  are 
long,  with  only  the  third  and  fourth  digits  developed  ;  the  two 
metacarpals  and  metatarsals  are  united  for  the  greater  part  of 
their  length,  but  there  is  a  deep  distal  cleft ;  the  tips  of  the  digits 


FIG.  251. — Side  view  of  Sheep's  skull,  with  roots  of 
back  teeth  exposed.  (From  Edinburgh  Museum  of  Science 
and  Art.) 

f,  Frontal ;  «,  nasal ;  pm,  premaxilla  ;  m,  maxilla  ;  /,  jugal ;  sq, 
squamosal  ;  /,  lachrymal. 

have  very  incomplete  hoofs,  and  the  animals  walk  on  a  broad  pad 
of  skin  surrounding  the  middle  phalanges.  The  femur  is  long 
and  vertical,  and  the  knee  is  low  down.  Of  the  three  upper 
incisors  only  one  persists  in  adult  life,  as  an  isolated  sharp  tooth, 
those  of  the  lower  jaw  are  long  and  slope  forwards.  There  are 
canines  both  above  and  below.  The  molars  are  selenodont.  The 
animals  are  ruminant  in  habit,  and  the  stomach  is  divided  into 
three  chambers,  of  which  the  first  two  have  on  their  walls 
remarkable  pouches,  which  can  be  filled  with  fluid,  and  closed 
by  sphincter  muscles.  The  Camelidge  are  unique  among  Mam- 
mals in  having  oval  instead  of  circular  red  blood  corpuscles. 
The  placenta  is  diffuse. 


696  MAMMALIA. 

"33 

Examples: — Camelus, ,  the  Arabian   camel   (C.  dromedarius] 

3T23 

has  a  dorsal  hump  of  fat,  the  Bactrian  camel  (C.  bactrianus] 

1123 

has   two    humps.      The   genus   Auchenia ,    includes   the 

3123 

llama,  alpaca,  huanaco,  and  vicugna  of  S.  America,  smaller 
forms  than  the  camels,  and  without  humps. 

Group  3. — Tragulina,  comprising  the  family  Tragulidse  or  Chevrotains. 
These  are  small  animals,  u  intermediate  in  their  structure  between 
the  Deer,  the  Camels,  and  the  Pigs."  There  are  four  complete 
toes  on  each  foot,  but  the  second  and  fifth  are  slender ;  the  third 
and  fourth  metacarpals  and  metatarsals  are  fused  in  Tragzilus, 
free  in  the  other  genus  Dorcatherium  ;  the  fibula  is  complete. 
There  are  no  upper  incisors,  the  upper  canines  are  long  and 
pointed  especially  in  the  males,  the  lower  canines  are  like 

OI33 

incisors,  the  dental  formula  is .      The  Chevrotains  ruminate, 

. 3T33 
and  the  stomach  is  divided  into  three  chambers.     The  placenta 


FIG.  252. — Stomach  of  Sheep.     (From  LEUNIS.) 

a,  (Esophagus  ;  c,  rumen  or  paunch  ;  d,  reticulum  or  honeycomb 
bag  ;  e,  psalterium  or  many-plies  \f,  abomasum  or  reed  ;  l>,  beginning 
of  duodenum. 

is  diffuse.     The  Chevrotains  are  often  confusedly  associated  with 
the  musk  deer  (Moschus)  with  which  they  have  no  special  affinities. 
Species  of  Tragulus  (smallest  among  living  Ungulates)  occur  in  Indo- 
Malaya,  India,  and  Ceylon  ;  one  species  of  Dorcatherium ,  of  aquatic  pig- 
like  habits,  is  found  on  the  west  coast  of  Africa. 

Group  4. — Pecora  or  Cotylophora — the  true  Ruminants,  including  deer, 
giraffes,  cattle,  and  sheep.  Only  the  third  and  fourth  digits  are 
complete,  the  fused  third  and  fourth  metacarpals  and  metatarsals 
form  "  cannon  bones."  In  the  embryos  of  ox  and  sheep,  the 
second  and  fifth  metacarpals  and  metatarsals  are  also  represented ; 
the  second  metacarpal  and  fifth  metatarsal  are  unstable  and  soon 
disappear  ;  small  traces  of  the  fifth  metacarpal  and  second 
metatarsal  persist.  Paired  outgrowths  of  the  frontal  bones  are 
common,  capped  with  horny  sheaths  in  the  Bovidre,  deciduous 


EVEN- TO  ED    UNGULATES. 


697 


and  restricted  to  the  males  in  almost  all  Cervidae.  There  are  no 
upper  incisors,  and  rarely  upper  canines  ;  there  are  three  pairs  of 
lower  incisors  which  bite  against  the  hardened  gum  above,  and 
the  lower  canine  resembles  and  is  in  the  same  series  as  the 

0033 

incisors ;    the  typical  dentition  is  .     The   stomach  has  four 

3133 

distinct  compartments,  a  psalterium  or  many-plies  in  addition  to 
the  three  which  are  present  in  Camels  and  Chevrotains.  The 
placenta  is  cotyledonary,  the  villi  occurring  on  a  number  of 
distinct  patches. 

The  process  of  rumination  or  chewing  the  cud  cannot  be  understood 
without  considering  the  complex  stomach.  It  is  divided  into  four 
chambers,  the  paunch  or  rumen,  the  honeycomb  bag  or  reticulum,  the 
many-plies  or  psalterium,  the  reed  or  abo- 
masum.  The  swallowed  food  passes  into 
the  capacious  paunch,  the  walls  of  which 
are  beset  with  close  set  villi  resembling 
velvet  pile.  After  the  food  has  been 
softened  in  the  paunch,  it  is  regurgitated 
into  the  mouth  where  it  is  chewed  over 
again  and  mixed  with  more  saliva.  Swal- 
lowed a  second  time  the  food  passes  not 
into  the  paunch,  but  along  a  muscular 
groove  on  the  upper  wall  of  the  globular 
honeycomb  bag  into  the  third  chamber  or 
many-plies.  The  honeycomb  bag  owes  its 
name  to  the  hexagonal  pattern  formed  by 
the  mucous  membrane  on  its  walls.  The 
many-plies  or  psalterium  is  a  filter,  its 
lining  membrane  being  raised  into  numer- 
ous leaf-like  folds  covered  with  papillae. 
Along  these  the  food  passes  to  the  reed, 
which  secretes  the  gastric  juice. 

Cervidae — the  widely  distributed  deer, 
absent  only  from  the  Ethiopian  and 
Australian  regions.  The  second 
and  fifth  digits  are  usually  repre- 
sented, often  along  with  the  distal 
parts  of  the  corresponding  meta- 
carpals  and  metatarsals.  The  upper 
canines  are  usually  present  in  both 
sexes.  The  horns,  if  present,  are 
antlers,  confined  to  the  males  and 
deciduous,  except  in  the  reindeer, 
where  they  are  possessed  by  both 
sexes  and  are  permanent.  They 
are  outgrowths  of  the  frontal  bones, 

are  covered  during  growth  by  vascular  skin — the  velvet — and 
attain  each  year  to  a  certain  limit  of  growth.  After  the  breeding 
season  the  blood  supply  ceases,  the  velvet  dies  off,  and  an 


FIG.  253. — Side  view 
of  Calf's  fore  leg. 

h. ,  Distal  end  of  humerus  ; 
u. ,  olecranon.  process  of  ulna  ; 
r.,  radius  ;  me. 3-4,  metacar- 
pals  3  and  4  fused  to  form 
cannon  bone  ;  we.  5,  fifth 
metcarpal ;  n. ,  nodule. 


698  MAMMALIA. 

annular  absorption  occurs  near  the  base.  Then  the  antlers  are 
shed,  leaving  a  stump,  from  which  a  fresh  but  larger  growth 
takes  place  in  the  next  year.  The  earliest  (Lower  Miocene) 
deer  had  no  antlers,  thus  resembling  young  stags  of  the  first 
year ;  the  Middle  Miocene  deer  had  simple  antlers  with  not 
more  than  two  branches,  thus  resembling  two-year-old  stags. 
Thus  there  is  a  parallelism  between  the  history  of  the  race  and 
the  individual  development. 

Examples: — Cervtts,  most  Old  World  deer;  Rangifer,  the  reindeer; 
A  Ices,  the  elk  or  moose  ;  Capreolus,  the  roe  deer  ;  Hydropotes, 
the  water  deer,  without  antlers  ;  Moschus,  the  musk  deer,  with- 
out antlers,  with  long  sharp  upper  canines  in  the  males,  with 
large  musk  glands. 

Girafiidse,  represented  solely  by  the  giraffe  (Giraffa  camelopardalis),  a 
tall  Ethiopian  animal,  notable  for  its  enormously  elongated  cervi- 
cal vertebrae,  and  for  its  long  limbs.  It  is  gregarious  in  its  habits, 
and  feeds  on  the  leaves  of  trees.  The  lateral  digits  are  entirely 

absent.     The  dental  formula  is  — 3.     On  both  sexes  there  are  on 

.  3T33 

the  forehead  short  erect  prominences,  over  the  union  of  parietals 
and  frontals,  which  arise  from  two  distinct  centres  of  ossification, 
but  afterwards  fuse  with  the  skull.  In  front  of  these  there  is 
median  protuberance. 

Antilocapridcg,  represented  solely  by  the  prongbuck  (Antilocapra 
americand),  a  North  American  animal,  with  most  of  the  char- 
acteristics of  Bovidse,  but  with  deciduous  and  branched  horns. 

Bovidoe,  the  hollow-horned  Ruminants,  widely  distributed  throughout 
the  world,  but  without  indigenous  representatives  in  Australia, 
South  or  Central  America.  The  second  and  fifth  digits  may  be 
completely  absent,  but  are  often  represented  by  minute  hoofs  and 
supporting  nodules  of  bone.  The  frontal  appendages,  if  present, 
consist  of  a  solid  bony  core  growing  from  the  frontal,  and  a  much 
longer  sheath  of  horn,  which  grows  at  the  base  as  it  is  worn  away 
at  the  tip.  They  are  not  deciduous,  and  are  usually  present  in 
both  sexes,  though  larger  in  the  males. 
Examples  : — Antilope,  GazeZla,  Capra,  Ovis,  Bos. 

Sub-Order  PERISSODACTYLA. 
Horses,  Tapirs,  Rhinoceros,  and  their  extinct  Allies. 

The  middle  or  third  digit  of  fore  and  hind  feet  is  larger 
than  the  others,  and  symmetrical  on  itself.  It  may  be  the 
only  complete  digit,  as  in  the  horse,  or  it  may  be  accom- 
panied by  a  second  and  a  fourth,  and  in  the  fore  foot  of 
Tapirs  and  some  extinct  forms,  by  a  fifth  digit.  No  modern 
forms  have  any  trace  of  a  first  digit.  The  astragalus 
has  a  pulley-like  surface  above  for  articulation  with  the 
tibia ;  its  distal  surface  is  flattened  and  unites  to  a  much 


PERISSODACTYLA. 


699 


greater  extent  with  the   navicular  than  with   the   cuboid. 
The  last-named  bone   is   of  less   importance  than  in  the 


//"-.•;    I    [ 


FIG.  254. — Side  view  of 
lower  part  of  Pony's  fore 
leg.  (From  Edinburgh 
Museum  of  Science  and 
Art.) 

h,  Distal  end  of  humerus;  u, 
olecranon  process  of  ulna  ;  r, 
radius  ;  sc,  scaphoid  ;  /,  lunar  ; 
c,  cuneiform ;  m,  os  magnum  ; 
un,  unciform  ;  p,  pisiform  ;  me.  4, 
splint  of  fourth  metacarpal ;  me.  3, 
third  metacarpal ;  s,  sesamoid  ; 
i,  2,  3,  phalanges  of  third  digit. 


mt.  J. 


ph  3. 


cub. 


FIG.  255. — Side  view  of  ankle 
and  foot  of  Horse.  (From  Edin- 
burgh Museum  of  Science  and 
Art.) 

«.,  Astragalus;  c.,  calcaneum  ;  «., 
navicular;  e.c.,  external  cuneiform;  cub., 
cuboid ;  mt.  3,  third  metatarsal ;  int.  4, 
splint  of  fourth  metatarsal ;  s.,  sesamoid  ; 
ph.  1-3,  phalanges  of  third  digit. 

Artiodactyla.     The  calcaneum  does  not  articulate  with  the 
lower  or  distal  extremity  of  the  fibula.     The  femur  has  a 


700  MAMMALIA. 

third  trochanter  or  process  for  the  insertion  of  muscles. 
There  are  usually  twenty-three  dorso-lumbar  vertebrae. 

As  to  the  dentition,  the  premolars  and  molars  form  a 
continuous  series,  with  broad  transversely  ridged  crowns, 
the  last  premolars  often  very  like  the  molars. 

The  stomach  is  simple,  the  caecum  is  large,  there  is  no 
gall  bladder. 

The  mammae  are  inguinal ;  the  placenta  is  diffuse  and 
non-deciduate. 

Families  of  Perissodactyla. 

Family  Tapirid^e.     In  the  Tapirs  ( Tapirus\  there  are  four  digits  in 
the  manus,  but  the  third  finger  is  still  practically  median,  as  the 

F  P 


\ 

pp- 


FIG.  256. — Side  view  of  Horse's  skull,  roots  of  back 
teeth  exposed.  (From  Edinburgh  Museum  of  Science 
and  Art.) 

P,  Parietal ;  F,  frontal ;  n,  nasal ;  pm,  premaxilla ;  m,  maxilla ; 
j,  jugal  ;  /,  lachrymal ;  s<?,  squamosal ;  //,  paroccipital  process ;  c, 
canine  ;  C,  condyle. 

fifth  digit  scarcely  reaches  the  ground.     The  hind  foot  has  three 

3T43 

digits.      The  dentition  of  the  genus  is •      The   orbit   and 

3133 

temporal  fossa  are  continuous.  The  nose  and  upper  lip  form  a 
short  proboscis.  The  thick  skin  has  but  scanty  hair.  In  habit, 
the  Tapirs  are  shy  and  nocturnal,  fond  of  forests  and  water, 
feeding  on  tender  shoots  and  leaves.  The  distribution  is  some- 
what remarkable,  for  four  species  live  in  Central  and  South 
America,  while  a  fifth  is  Malayan.  The  genus  was  once 
widespread,  it  has  survived  in  these  two  widely  separated 
regions. 

Family  Equidae.  In  the  modern  horses  (Eqnus),  there  is  on  each 
foot  one  functional  digit — the  third,  with  splints  representing  the 
metacarpals  and  metatarsals  of  the  second  and  fourth.  Professor 


FAMILIES   OF  PER1SSODACTYLA. 


701 


Cossar  Ewart  has  recently  found  in  the  embryo  of  the  horse  the 
rudiments  of  the  three  phalanges  of  the  second  and  fourth  digits. 
The  vestigial  phalanges  of  these  digits  subsequently  fuse  with 
one  another  and  with  the  respective  metacarpals  or  metatarsals, 
forming  "buttons"  at  the  end  of  the  splints.  The  ulna  and 

3143 

fibula  are  incomplete.     The  dentition  is >  but  the  first  pre- 

3J43 

molar  is  rudimentary.  The  orbit  is  completely  surrounded  by 
bone. 

The  modern  horses  are  connected  by  a  very  complete  series  of  forms 
with  ancestral  Eocene  types.  The  progress  shows  an  increase  of  size, 
a  diminution  in  the  number  of  digits,  an  increased  folding  of  the  back 
teeth,  and  other  differentiations.  The  Eocene  Phenacodus  is  regarded 


FIG.  257. — Feet  of  Horse  and  its  progenitors. 
(From  NEUMAYR.) 

i.  Palaeotherium  ;  2.  Anchitherium  ;  3.  Hippotherium  ;  4.  Equus. 

by  some  as  near  the  origin  of  the  stock,  it  had  five  complete  digits  on 
each  foot  ;  Hyracotherium  and  Systemodon  had  only  four  functional 
digits  in  the  manus ;  Anchitherium  from  the  Miocene,  an  animal  about 
the  size  of  a  sheep,  had  three  digits,  or  three  and  a  rudiment: 
Hippotherium  and  Protohippus  from  the  Pliocene,  were  as  large  as 
donkeys,  and  show  a  marked  diminution  of  the  second  and  fourth 
digits  ;  finally,  in  the  Pleistocene,  the  modern  forms  appeared. 

The  living  species  are  the  horses  (Equus  caballus],  apparently  origin- 
ating in  Asia,  domesticated  in  prehistoric  times,  artificially  selected  into 
many  breeds,  sometimes  reverting  to  wildness  as  in  the  case  of  those 
imported  into  America  and  Australia  by  European  settlers  ;  the  wild 
horse  of  Central  Asia  (E.  prezevalskii] ;  the  donkey  (E.  asimis]  of 


702  MAMMALIA. 

African  origin,  the  wild  asses  of  Africa  and  Asia;  the  striped  African 

species — the  zebras  and  the  (exterminated)  quagga. 

Family  Rhinocerotidce.  There  is  now  but  one  genus  Rhinoceros, 
species  of  which  occur  in  Africa  and  in  some  parts  of  India  and 
Indo- Malaya.  They  are  large  heavy  Ungulates,  shy  and  noc- 
turnal, fond  of  wallowing  in  water  or  mud,  feeding  on  herbage, 
shoots,  and  leaves.  The  skin  is  very  thick,  with  scanty  hair. 
One  or  two  median  horns  grow  as  huge  warts  from  the  snout 
and  forehead.  The  dentition  is  very  variable,  but  the  back 

teeth are  almost  uniform,  there  are  no  upper  canines,  but 

sometimes  a  large  lower  pair,  there  are  a  few  incisors,  but  these 
are  often  small  and  deciduous. 

There  are  several  entirely  extinct  families  of  Perissodactyla,  such  as 
Lophiodontidae  (Eocene),  e.g.,  Lophiodon,  Hyracotherium, 

Systemodon, — a   family  perhaps   ancestral   to   most  of  the 

modern  Perissodactyla. 
Palseotheriidoe    (Eocene    to    Miocene),   e.g.,  Palcsotherium   and 

A  nch  itherium. 
Other  remarkable  types — 

Ldmbdotherium,  Ckalicotherium,  Tit  another ium,  of  elephantine 

size,    and    the    specialised  Macrauchcnia — are   referred   to 

distinct  families. 

Sub-Order  HYRACOIDEA. 

Small  Rodent-like  Ungulates,  represented  by  one  genus 
— Hyrax  or  Procavia,  living  in  rocky  regions  and  on  trees 
in  Africa  and  Syria.  The  species  (14)  are  adept  climbers. 

The  upper  incisors  have  persistent  pulps  and  are  curved 
as  in  Rodents,  but  they  are  sharply  pointed,  not  chisel-edged. 
The  outer  lower  incisors  are  straight  and  have  trilobed 
crowns.  There  are  no  canines  in  the  second  set,  but  the 
upper  milk  canine  sometimes  persists  ;  and  there  is  a  wide 
space  between  incisors  and  premolars.  The  back  teeth  are 
very  uniform  and  like  those  of  Perissodactyla.  The  milk 

dentition  is  -— ,  the  permanent  is  — — .     Hyrax  is  one  of  the 

214  2043 

few  Mammals  in  which  the  first  premolar  is  a  replacing 
tooth. 

In  the  fore  feet,  the  thumb  is  rudimentary,  the  little  finger 
is  smaller  than  the  median  three,  which  are  almost  equal. 
In  the  hind  feet,  which  are  like  miniatures  of  those  of  the 
rhinoceros,  the  hallux  is  absent,  and  the  fifth  toe  is  rudi- 
mentary. There  are  no  clavicles.  The  tail  is  very  short. 

The  brain  is  like  that  of  Ungulates.      The  stomach  is 


PROBOSC1DEA.  703 

divided  into  two  parts  by  a  constriction.  In  addition  to  the 
short  but  broad  caecum,  there  are  two  supplemental  caeca 
lower  down  on  the  intestine.  The  testes  are  abdominal. 
Of  the  mammae,  four  are  on  the  groin  and  two  are  axillary. 
The  placenta  is  zonary  as  in  the  Proboscidea  and  Carnivora. 
No  extinct  forms  are  known. 

Sub-Order  PROBOSCIDEA. 

This  sub-order  is  now  represented  by  two  species  of 
elephant  (Elephas).  They  occupy  a  somewhat  isolated 
position,  though  distinctly  Ungulates.  As  regards  skull, 
proboscis,  and  teeth  they  are  highly  specialised,  but  their 
limbs  are  of  a  generalised  type. 

The  elephants  are  confined  to  the  Ethiopian  and  Oriental 
regions.  They  feed  on  leaves,  young  branches,  and  herbage. 
By  means  of  the  mobile  proboscis  they  gather  their  food, 
and  they  drink  by  filling  the  proboscis  and  then  ejecting  the 
water  into  the  mouth. 

The  proboscis  is  a  muscular  extension  of  the  nose,  and 
bears  the  nostrils  at  its  tip. 

The  skin  is  strong  and  the  hair  somewhat  scanty. 

In  the  limbs,  radius  and  ulna,  tibia  and  fibula,  are  quite 
distinct;  the  radius  and  ulna  are  fixed  in  a  crossed  position; 
owing  to  the  length  of  the  humerus,  and  yet  more  of  the 
femur,  elbow  and  knee  are  lower  than  usual ;  the  carpal  and 
tarsal  bones  have  flat  surfaces ;  the  feet  are  broad  and  bear 
five  hoofed  toes  embedded  in  a  common  integument.  There 
are  no  clavicles. 

The  skull  is  very  large,  being  adapted  to  support  the 
proboscis  and  tusks,  and  to  afford  a  broad  insertion  for  the 
large  muscles.  In  most  of  the  bones  there  is  during  growth 
an  extraordinary  development  of  air  spaces,  which  com- 
municate with  the  nasal  passages.  The  nasal  bones  are  very 
short;  the  zygomatic  arch  (formed  anteriorly  by  the  maxilla, 
medianly  by  the  small  jugal)  is  slender  and  straight.  The 
neck  is  very  short. 

The  dentition  is  unique.  The  two  upper  incisors  or  tusks 
are  mainly  composed  of  solid  ivory;  the  enamel  is  restricted 
to  the  apex  and  soon  wears  off.  As  the  tusks  grow,  their 
roots  sink  through  the  premaxillae  into  the  maxillae.  There 
are  no  canines  nor  premolars.  The  molars  are  very  large, 


704  MAMMALIA. 

and  the  enamel  is  very  much  plaited,  forming  a  series  of 
transverse  ridges  enclosing  the  dentine,  and  united  to  one 
another  by  cement.  Thus  on  the  worn  tooth  there  are 
numerous  successive  layers  of  enamel,  dentine,  and  cement. 
Extinct  forms  show  transitions  between  this  complex  type 
and  the  horse's  tooth.  In  a  lifetime  there  may  be  six  molar 
teeth  on  each  side  of  each  jaw,  but  of  these  only  one,  or 
portions  of  two,  can  find  space  at  a  time.  The  series 
gradually  moves  forward  as  the  front  parts  are  worn  away 
and  cast  out. 

The  brain  is  highly  developed. 

The  stomach  is  simple,  and  there  is  a  large  caecum. 

There  are  two  superior  venae  cavae  entering  the  right 
auricle. 

The  testes  remain  abdominal  in  position. 

There  are  two  pectoral  mammae;  the  uterus  is  bicornuate; 
the  placenta  is  non-deciduate  and  zonary. 

106 
Elephas,  — ,  now  represented  by  the  Indian  Elephant  (£.  indicus), 

with  parallel  folds  of  enamel  on  the  molars  and  ears  of  moderate  size, 
and  the  African  Elephant  (E.  africamis],  with  lozenge -shaped  folds  of 
enamel  and  very  large  ears. 

The  mammoth  (E.  primigenius]  belonged  to  the  Pleistocene  period, 
and  had  a  wide  geographical  range,  occurring  for  instance  in  Britain. 

The  genus  Mastodon  is  represented  by  fossil  remains  in  Miocene, 
Pliocene,  and  even  in  Pleistocene  strata,  in  Europe,  India,  and  America. 
The  molar  teeth  show  transitions  between  those  of  elephants  and  those 
of  other  Ungulates. 

In  Dinotherium ,  found  in  Miocene  and  Pliocene  strata  in  Europe  and 
Asia,  the  lower  jaw  bore  an  enormous  pair  of  tusks  projecting  vertically 
downwards,  and  all  the  back  teeth  seem  to  have  been  in  use  at  the 
same  time. 

SEVERAL  EXTINCT  SUB-ORDERS. 

Although  we  cannot  describe  the  following  remarkable  types,  it  is 
important  to  notice  their  existence,  for  they  serve  to  impress  us  with  the 
original  connectedness  of  what  are  now  separate  orders. 

The  huge  Amblypoda,  found  in  Eocene  formations  in  W.  America, 
had  three  pairs  of  remarkable  protuberances  on  the  top  of  the  skull,  no 
'upper  incisors,  large  upper  canines,  especially  in  the  males,  and  six  back 
teeth.  Example —  Uintatherium  ;  the  genus  Coryphodon  may  also  be 
related. 

Cope  includes  a  number  of  generalised  Eocene  Ungulates  under  the 
title  Condylarthra.  Some  seem  ancestral  to  the  Perissodactyla  and 
Artiodactyla ;  some  suggest  a  union  of  ancestral  Ungulates  and  ancestral 
Carnivores.  The  genus  Periptychus  may  be  regarded  as  an  ancestral 


CRT  ACE  A. 


705 


Bunodont,  and  Phenacodus  as  near  the  origin  of  the  horse  stock.  But 
Phenacodus  is  so  generalised  that  Cope  has  suggested  affinities  between 
it  and  not  only  Ungulates,  but  also  Carnivores  and  Lemurs. 

The  tertiary  strata  of  S.  America  have  yielded  a  number  of  strange 
types,  e.g.)  Toxodon^  Nesodon,  and  Typotherium,  ranked  in  the  sub- 
orders Toxodontia  and  Typotheria. 

From  the  Eocene  of  N.  America,  Marsh  has  disentombed  a  group  of 
animals  which  he  calls  Tillodontia,  e.g.,  Tillotherium,  which  seem  to 
combine  the  characters  of  the  Ungulata,  Rodentia,  and  Carnivora. 

Order  4. — CETACEA. 

The  Cetaceans,  including  whales  and  dolphins  and  their 
numerous  relatives,  are  aquatic  mammals  of  fisriTlike  form. 

The  spindle-shaped  body  has 
no  distinct  neck  between  the 
relatively  large  head  and  the 
trunk,  and  tapers  to  a  notched 
tail,  the  horizontal  expansions  of 
which  form  the  flukes.  The  fore 
limbs  are  paddle-like,  and  there 
are  no  external  hints  of  hind 
limbs.  Most  forms  have  a 
median  dorsal  fin.  Hairs  are 
generally  absent,  though  a  few 
bristles  may  persist  near  the 
mouth.  The  thick  layer  of  fat  or 
blubber  beneath  the  skin  serves 
to  retain  the  warmth  of  the  body, 
and  thus  compensates  for  the 
absence  of  hair.  In  one  of  the 
dolphins  dermal  ossicles  occur,  a 
fact  which  has  suggested  the  idea 
that  the  toothed  whales  may  have 
had  mailed  ancestors.  Traces  of 
dermal  armour  have  also  been 
found  in  the  extinct  Zeuglodonts. 
The  general  shape,  the  absence 
of  external  ears,  the  absence  of  an 
eye-cleansing  nictitating  membrane,  the  dorsal  position  and 
valvular  aperture  of  the  single  or  double  nostril,  the  sponginess 
of  the  bones,  the  retia  mirabilta  in  different  parts  of  the  body, 
may  be  associated  with  the  aquatic  life  of  these  mammals. 
45 


FIG.  258.— Left  fore-limb 
of  Balsenoptera. 

Sc. ,  Scapula  with  spine  (sp)  ; 
//".,  humerus  ;  fi.,  radius;  £/., 
ulna;  C.,  carpals  embedded  in 
matrix;  Me.,  metacarpals ;  Pk., 
phalanges. 


7o6 


MAMMALIA. 


The  cervical  vertebrae  are  thin  and  more  or  less  fused. 
There  is  no  union  of  vertebrae  to  form  a  sacrum,  for  the 
hind-limbs  are  at  most  very  rudimentary.  Under  the  caudal 
vertebrae  there  are  wedge-shaped  chevron  bones. 

The  brain-case  is  almost  spherical ;  the  supra-occipital 
meets  the  frontals  and  shuts  out  the  parietals  from  the  roof 
of  the  skull ;  the  frontals  arch  over  the  orbit ;  the  snout  or 
rostrum  of  the  skull  is  composed  of  premaxillae,  maxillae, 
and  vomer,  and  of  the  mesethmoid 
cartilage. 

There  are  at  least  rudiments  of  two 
sets  of  teeth,  as  in  other  Mammals, 
but  in  baleen  whales  only  the  teeth  of 
the  milk  set  are  calcified,  and  they 
come  withal  to  nothing,  being  to  some 
extent  replaced  by  the  horny  baleen 
plates  developed  on  the  palate.  In 
toothed  whales  the  two  sets  are  said 
by  Kiikenthal  to  fuse,  but  the  usual 
interpretation  is  that  the  functional 
teeth  belong  to  the  milk  set.  It  is 
possible  that  the  simple,  homodont, 
conical  teeth  of  Odontoceti  have  re- 
sulted from  a  splitting  of  more  com- 
plex cusped  teeth.  No  clavicles  are"  de- 
veloped. Excepting  the  humerus,  the 
bones  of  the  fore-limb  are  stiffly  jointed 
and  flattened.  The  carpals  are  fixed 
in  a  fibrous  matrix,  tend  to  be  rudi- 
mentary, and  are  often  unossified. 
There  are  four  or  five  digits,  of  which 
the  second  and  third  have  more  than  the 
usual  number  of  phalanges,  a  peculiarity 
possibly  due  to  a  duplication  and  sepa- 
ration of  epiphyses.  The  pelvis  may 
exhibit  two  rudimentary  ischia,  with 
small  vestiges  of  femur  and  tibia. 

The  rounded  brain  is  relatively  large,  with  well-convoluted 
cerebral  hemispheres. 

As  to  the  alimentary  system,  salivary  glands  are  rudi- 
mentary or  absent,  the  stomach  is  chambered,  the  intestine 


FIG.     259.  —  Fore- 
limb  of  Whale  (Me- 

gaptera     longiviand]. 
(After  STRUTHERS.) 


CRT  ACE  A. 


707 


has  rarely  a  caecum,  the  liver  is  but  slightly  lobed,  there  is 
no  gall  bladder. 

The  heart  is  often  cleft  between  the  ventricles.  Both 
arteries  and  veins  tend  to  form  retia  mirabilia. 

The  larynx  is  elongated  so  that  it  meets  fthe  posterior 
nares,  and  forms  a  continuous  canal  down  which  air  passes 
from  nostrils  to  lungs.  Cetaceans  must,  of  course,  rise  to 
the  surface  to  inspire,  but  the  expiration  occurs  at  longer 
intervals  than  in  terrestrial  mammals.  The  water  vapour 
expelled  along  with  the  air  from  the  lungs,  condenses  into 

a  cloud,  which  is  some- 
times increased  by  an 
accidental  puff  of  spray. 

The  kidneys  are  lobu- 
lated.  The  testes  are  ab- 
dominal. There  are  no 
seminal  vesicles.  The 
uterus  is  bicornuate. 
The  placenta  is  non- 
deciduate  and  diffuse. 
The  two  mammae  lie  in 
depressions  beside  the 
genital  aperture,  and  the 
milk  is  squeezed  from 
special  reservoirs  into  the 
mouth  of  the  young. 
Usually  a  single  young 
one  is  born  at  a  time, 
and  there  are  never  more 
than  two. 

All  are  carnivorous,  but 
while  many  feed  on  small 
pelagic  animals,  others 
swallow  cuttles  and  fish, 
and  Orca  attacks  other 
Most  are  gregarious  and  live  in 


FIG.   260. — Pelvis   and   hind-limb 
of  Greenland  Whale  (Babena). 
(After  STRUTHERS.) 

P.,  Pelvis;  F.,  femur  ;  T.,  tibia. 


Cetaceans   and   seals, 
schools  or  herds. 

The  living  Cetaceans  are  ranked  in  two  sub-orders — the 
Mystacoceti  or  Balaenoidea  without  functional  teeth  but  with 
whalebone  or  baleen  plates  on  the  palate,  and  the  Odontoceti 
or  Delphinoidea,  with  functional  teeth  and  without  baleen. 


708  MAMMALIA. 

Some  Eocene  fossils,  known  as  Zeuglodonts,  are  regarded 
by  some  (Lydekker,  Dames)  as  primitive  Cetaceans — 
Archaeoceti — less  specialised  than  modern  forms,  but  Prof. 
D'Arcy  Thompson  has  advanced  strong  arguments  in  favour 
of  their  affinities  with  Pinniped  Carnivores. 

In  regard  to  the  possible  affinities  of  the  Cetacea,  Sir 
Wm.  Flower  maintains  (i)  that  the  hypothesis  of  their 
descent  from  Ichthyopterygian  reptiles  is  untenable,  (2) 
that  they  are  separated  from  an  alliance  with  Carnivora  by 


FIG.   261. — Vertebra,  rib,  and  sternum  of  Babenoptera. 
(From  specimen  in  Anatomical  Museum,  Edinburgh.) 

C.,  Centrum  ;  n.a.,  neural  arch  ;  n.sp.,  neural  spine  ;  t.p.,  transverse 
process  ;  R.,  rib  ;  St.,  sternum. 

many  essential  characters,    (3)    that   they   exhibit   several, 
though  by  no  means  close,  affinities  with  Ungulata. 

The  same  authority  refers  to  several  facts  which  suggest 
that,  in  their  transition  from  terrestrial  to  marine  life,  the 
Cetaceans  may  have  passed  through  a  stage  in  which  they 
lived  in  fresh  water. 


[TABLE. 


CETACEA. 


709 


The  two  Sub-Orders  of  living  Cetaceans  may  be  contrasted 
as  follows  (after  Flower) : — 


MYSTACOCETI  or  BAL^ENOIDEA, 
baleen  Cetaceans. 


ODONTOCETI  or  DELPHINOIDEA. 
toothed  Cetaceans. 


The  teeth  are  absorbed  before  birth. 


Whalebone  or  baleen  plates  develop  as 
processes  from  the  palate. 

The  skull  is  symmetrical. 


The  nasals  roof  the  anterior  nasal  pas- 
sages, which  are  directed  upwards  and 
forwards. 

The  maxilla  does  not  overlap  the  orbital 
process  of  the  frontal. 

The  lachrymal  is  small,  and  distinct  from 
the  jugal. 

The  tympanic  is  ankylosed  to  the  periotic. 


The  rami  of  the  mandible  are  arched  out- 
wards, and  have  no  true  symphysis. 

All  the  ribs  articulate  only  with  the  trans- 
verse processes  of  the  vertebrae. 

The  sternum  is  a  single  piece,  and  articu- 
lates with  a  single  pair  of  ribs. 


The  external  nostrils  are  separate. 


The    olfactory    organ    is    distinctly    de- 
veloped. 

There  is  a  short  caecum. 

Examples : — 

The  right-whale  (Bala>na),  the 
hump-back  (Megafitera),  the 
rorqual  (Balcenoptera). 


The    teeth   persist  after  birth,   and   are 
generally  numerous  and  functional. 

There  is  no  baleen. 


The  skull  on  its  upper  surface  is  more  or 
less  asymmetrical. 

The  nasals,  often  small,  do  not  roof  the 
anterior  nasal  passages,  which  are  di- 
rected upwards  and  backwards. 

The  maxilla  covers  most  of  the  orbital 
process  of  the  frontal. 

The  lachrymal  is  fused  to  the  jugal,  or  is 
large,  and  helps  to  roof  the  orbit. 

The  tympanic  is  not  ankylosed  to  the 
periotic. 

The  rami  of  the  mandible  are  straight, 
and  form  a  symphysis. 

Several  anterior  ribs  articulate  by  capi- 
tula  with  the  centra  of  vertebrae. 

The  sternum  has  usually  several  segments 
with  which  several  sternal  ribs  articu- 
late. 

The  nostrils  unite  in  a  single  blow-hole  on 
the  top  of  the  head. 

The  olfactory  organ  is  rudimentary  or 
absent. 

There  is  no  caecum,  except  in  Platanista. 

Examples : — 

The  Sperm  whale  (Physeter),  the 
dolphin  (Delphinus),  the  por- 
poise (Phoc&na),  the  "Gram- 
pus "  (Oreo),  the  Ca'ing  whale 
(Globicephalus),  Grampus,  the 
Narwhal  {Monodon),  with  a 
horn-like  tusk  in  the  male  only, 
the  Beluga  {Delphinapterus), 
the  blind  Platanista  of  the 
Ganges. 


7io  MAMMALIA. 

Order  5.     RODENTIA. 

Rodents  are  represented  in  all  parts  of  the  world,  and  by 
more  species  than  any  other  order  of  mammals.  Most  of 
them  are  small,  and  most  are  terrestrial,  but  there  are  some 
arboreal  and  aquatic  forms.  All  are  herbivorous,  and  gnaw 
their  food  in  a  characteristic  way. 

The  dentition  is  characteristic.  The  incisors  have  chisel- 
like  edges,  for  as  the  enamel  is  either  entirely  restricted  to 
the  front,  or  is  at  most  thin  posteriorly,  the  back  part  wears 
away  more  rapidly.  The  incisors  are  always  rootless,  grow- 
ing from  persistent  pulps  as  they  are  worn  away,  and  the 
same  is  sometimes  true  also  of  the  back  teeth.  On  the 
lower  jaw  there  is  never  more  than  a  pair  of  incisors,  and 
in  most  cases  the  upper  jaw  also  has  only  a  pair.  There 
are  no  canines,  and  the  skin  projects  as  a  hairy  pad  into  the 
mouth  through  the  large  gap  between  incisors  and  pre- 
molars. 

The  feet  are  plantigrade  or  semi-plantigrade,  generally 
with  five  clawed  or  slightly  hoofed  digits.  Clavicles,  though 
often  rudimentary,  are  generally  present.  The  scapula  has 
usually  a  long  acromion  process. 

The  condyle  of  the  mandible  is  elongated  from  before 
backwards,  and  in  gnawing  the  jaw  moves  backward  and 
forward  (unimpeded  by  any  postglenoid  process  of  the 
squamosal).  The  mandible  has  an  abruptly  narrowed  and 
rounded  symphysis,  and  a  very  large  angular  portion.  The 
orbits  are  confluent  with  the  temporal  fossae.  The  zygomatic 
arch  is  complete.  There  is  generally  a  distinct  interparietal 
bone.  The  tympanic  bullae  are  always  developed,  and  are 
often  large. 

The  cerebral  hemispheres  are  smooth,  and  leave  the  cere- 
bellum uncovered. 

The  skin  is  generally  thin,  and  the  panniculus  carnosus 
but  slightly  developed. 

The  intestine  has  a  large  caecum,  except  in  Myoxidae. 
Special  anal  or  perineal  or  other  glands  secreting  odoriferous 
substances  are  usually  developed. 

The  testes  are  inguinal  or  abdominal  in  position ;  only  in 
the  hares  and  rabbits  do  they  completely  descend  into 
scrotal  sacs. 


RODENTIA.  711 

The  mammae  are  on  the  abdomen,  or  on  the  abdomen 
and  thorax. 

The  uterus  is  double  or  very  markedly  bicornuate.  There 
is  a  provisional  yolk  sac  placenta ;  the  allantoic  placenta  is 
discoidal  and  deciduate. 

Sub-order  SiMPLlciDENTATA. — Rodents  with  only  one  pair  of  upper 
incisors,  with  the  enamel  restricted  to  the  front. 

Squirrel-like  (Sciuromorpha),  including  the  following  and  some 
other  families  : — 

Anomaluridse — the  Ethiopian  arboreal  genus  Anomalurus, 
with  a  lateral  parachute  of  skin. 

Sciuridse — the  squirrels  (Sciurus),  the  flying  squirrels — 
Pteromys  and  Sciuropterus — with  a  parachute  of  skin 
connecting  the  fore  and  hind  limbs,  the  marmots  (Arc- 
tomys],  the  prairie  dogs  (Cynomys],  the  pouched  marmots 
or  sousliks  (Spermophilus}. 

Castoridse — the  beaver  ( Castor]. 

Mouse-like  (Myomorpha),  including  the  following  and  some  other 
families  : — 

Myoxidse — the  dormice  (Myoxtts,  <2rY. ) 

Muridse — e.g.,  the  brown  rat  (Mus  decumamts],  the  black 
rat  (M.  rattus],  the  house  mouse  ( M.  musculus),  the  wood- 
mouse  (M.  sylvaticus],  the  harvest  mouse  (M.  minutus\ 
the  water-voles  (Arvicold],  the  American  musk  rat  (Fiber 
zibethicus},  the  lemming  (Myodes],  the  hamsters  (Cricetus). 

Geomyidoe — e.g.,  the  American  pouched  rat  (Geomys  bur- 
sarius}. 

Dipodidae — e.g.,  the  Jerboas  (Dipiis,  &*c.) 

Porcupine-like  (Hystricomorpha),  including  the  following  and 
some  other  families  : — 

Octodontidae — e.g.,  the  aquatic  Coypu  (Myopotamus  coypu}. 

Hystricidse — e.g.,  the  porcupine  (Hystrix,  &c. ) 

Chinchillidse—  e.g.,  the  squirrel-like  Chinchilla. 

Dasyproctidae — e.g.,  the  Agoutis  (Dasyprocta),  and  the  paca 
( Ccelogenys. ) 

Caviidse — e.g.,  the  guinea  pig  (Cavid),  and  the  S.  American 
Capybara  (Hydrochcerus],  the  largest  living  Rodent,  meas- 
uring about  four  feet  in  length. 

Sub-order  DUPLICIDENTATA. — Rodents  with  two  pairs  of  incisors  in 
the  upper  jaw,  the  second  pair  small  and  behind  the  first  pair ;  the 
enamel  extends  to  the  posterior  surfaces,  but  is  thinner  there. 

Lagomyidae — The  Picas  or  tailless  hares  (Lagomys),  guinea- 
pig-like  animals  found  on  the  mountains  of  N.  Asia,  in 
S.-E.  Europe,  and  on  the  Rocky  Mountains. 
Leporidae — e.g.,    the   common    hare    (Lepus  timidus],    the 
mountain  hare  (L.  variabilis],  the  rabbit  (L.  cuniculus}. 


712 


MAMMALIA. 


Order  6.  CARNIVORA. 

This  order  includes  (a)  the  true  Carnivores,  such  as  lions 
and  tigers,  foxes  and  dogs,  bears  and  otters  ;  (^)  the  aquatic 
Pinnipedia,  such  as  seals  and  walruses ;  and  (c)  the  extinct 
Creodonta  with  several  generalised  types. 

Most  of  the  Carnivora  feed  on  animal  food,  and  the  most 


Pal. 


mx. 


Pmx. 


FIG.  262. — Lower  surface  of  Dog's  skull. 

o.c.,  Occipital  condyle  ;  B.O.,  basi-occipital ;  T.,  tympanic  bulla 
m.c.,  post-glenoid  process  behind  fossa  for  condyle  of  mandible 
B.S.,  basi-sphenoid  ;  P.S.,  base  of  presphenoid  ;  V.,  vomer 
M.-2,  second  molar  ;  M.\,  first  molar  ;  Pm.\-^t  premolars  ;  c.,  canine 
/.i-3,  incisors:  Pmx.,  premaxilla  ;  nix.,  maxilla;  Pal.,  palatine 
/.,  jugal  ;  AS.,  alisphenoid;  Pt.,  pterygoid  ;  Sg.,  squamosal. 


CARNIVORA.  713 

typical  forms  prey  upon  other  animals  and  devour  their 
warm  flesh.  Most  are  bold  and  fierce  animals,  with  keen 
senses  and  quick  intelligence. 

Almost  all  have  well-developed  claws ;  there  are  never 
fewer  than  four  toes.  The  teeth  are  always  rooted  except 
in  the  case  of  the  tusks  of  the  walrus  ;  the  canines  are  strong 
and  sharp  ;  some  of  the  back  teeth  are  generally  sharp  and 
specially  adapted  for  cutting. 

"  The  condyle  of  the  lower  jaw  is  a  transversely  placed 
half-cylinder,  working  in  a  deep  glenoid  fossa  of  corre- 
sponding form."  Thus  the  jaw  moves  only  up  and  down. 
The  zygomatic  arch  within  which  lie  the  powerful  jaw 
muscles  is  generally  prominent,  and  the  widening  of  this 
has,  as  it  were,  broken  the  bridge  behind  the  orbit,  so  that 
the  orbit  is  confluent  with  the  temporal  fossa.  There  are 
generally  strong  occipital  and  sagittal  crests  for  the  insertion 
of  muscles.  The  tympanic  bullae  are  in  most  cases  large. 

The  clavicles  are  incomplete  or  absent ;  the  radius  and 
ulna  are  always  distinct ;  the  fibula  is  slender  but  distinct. 

The  cerebrum  is  well  convoluted,  and  the  cerebellum  is 
more  or  less  covered  by  the  cerebrum. 

The  stomach  is  always  simple ;  the  caecum  is  absent,  or 
short,  or  simple ;  the  colon  is  not  sacculated. 

There  are  no  vesiculae  seminales.  The  uterus  is  bicor- 
nuate.  The  mammae  are  abdominal.  The  placenta  is 
deciduate  and  zonary. 

Representatives  of  Carnivora  are  found  in  all  parts  of 
the  world. 

Sub-Order  Carnivora  Vera  or  Fissipedia. 

The   true   Carnivores  are   for  the  most   part  terrestrial.      The 

incisors  are  almost  always  -»  the  canines  are  usually  large,  one 

of  the  back  teeth  is  modified  as  a  trenchant  carnassial  or 
sectorial.  The  digits  generally  have  sharp  claws,  which  may 
be  retractile.  Within  the  sub-order  there  are  three  sections — 
^Eluroidea,  Cynoidea,  and  Arctoidea — represented  respectively 
by  cat,  dog,  and  bear,  but  these  types  are  connected  by  extinct 
forms. 


[TABLE. 


714 


MAMMALIA. 


^LUROIDEA. 

e.g.,  cat,  civet,  hyaena. 

CYNOIDEA. 

e.g.,  dog,  fox,  wolf,  jackal. 

ARCTOIDEA. 

e.g.,  bear,  otter. 

Digitigrade. 

Digitigrade. 

Plantigrade  or  sub-planti- 
grade. 

Typical  dentition,  — 

Typical  dentition,  * 
'SHS 

Typical  dentition,  —  —  * 

The  tympanic  bulla  is  much 
dilated,  rounded  and  thin- 
walled,  and  is  divided  in- 
to two  chambers  by  an 
internal  septum  (except 
in  Hyaenidae). 

The  tympanic  bulla  is  dila- 
ted, but  the  internal  sep- 
tum is  rudimentary. 

The  tympanic  bulla  is  often 
depressed,  and  there  is 
no  hint  of  an  internal 
septum. 

The  paroccipital  process  of 
the  ex-occipital  is  applied 
to  the  hinder  part  of  the 
tympanic  bulla. 

The  paroccipital  process  is 
in  contact  with  the  bulla, 
but  it  is  prominent. 

The  paroccipital  process 
is  quite  apart  from  the 
bulla. 

The  caecum  is  small,  rarely 
absent. 

The   caecum   is  sometimes 
short  and   simple,   some- 
times long  and  peculiarly 
folded. 

The  caecum  is  absent. 

In  retractile  claws,  the  last  phalanx  of  the  digit  with  its  attached  claw 
is  drawn  back  into  a  sheath  on  the  outer  side  of  the  middle  phalanx  in 
the  fore-foot,  on  the  upper  side  in  the  hind-foot.  When  the  animal  is 
at  rest  or  is  walking,  the  claw  is  retained  in  this  bent  position  by  an  elas- 
tic ligament,  and  is  in  this  way  protected.  When  the  animal  straightens 
the  phalanges,  the  claws  are  protruded. 

Digitigrade  animals  walk  on  their  toes  only,  plantigrade  forms  plant 
the  whole  sole  of  the  foot  on  the  ground,  but  between  these  conditions 
there  are  all  possible  gradations.  Most  Carnivores  are  sub-plantigrade, 
often  when  at  rest  applying  the  whole  of  the  sole  to  the  ground,  but 
keeping  the  heel  raised  to  a  greater  or  less  extent  when  walking. 

/ELUROIDEA — Cat-like  Carnivores. 

Family  FelidcS,  including  the  most  specialised  forms.     The  canines 

T 
are  large,  the  ,molars  are  reduced  to  -,  the  carnassials    are   the 

i 

last  premolars  above  (with  a  three-lobed  blade),  and  the  molars 
beneath  (with  a  two-lobed  blade).  The  skull  is  generally 
rounded,  the  zygomatic  arches  are  wide  and  strong,  the  tym- 
panic bullse  are  large  and  smooth.  The  limbs  are  digitigrade, 
the  claws  retractile.  There  is  no  alisphenoid  canal.  The 

3I3I 

dentition  of  the  typical  genus  Felis  is • 

3121 

Examples: — The  lion  (Felis  led]  in  Africa,  Mesopotamia,  Persia, 
N.-W.  India  ;  the  tiger  (F.  Tigris],  widely  distributed  in 
Asia  ;  the  leopard  (F.  Pardus}  in  Africa,  India,  Ceylon, 
Sumatra,  Borneo,  &c.  ;  the  wild  cat  (F.  catus] ;  the  Caffre 


CARNIVORA.  715 

cat  (F.  caffra)  of  Africa  and  S.  Asia,  venerated  and  mummi- 
fied by  the  Egyptians,  perhaps  ancestral  to  the  domestic  cat  ; 
the  puma  or  couguar  (F.  concolar]  from  Canada  to  Pata- 
gonia ;  the  jaguar  (F.  onca\  also  American. 

A  high  degree  of  specialisation  for  carnivorous  habit  is  well  illus- 
trated by  the  sabre-toothed  tigers  (Machcerodus]  of  Tertiary 
ages, whose  serrated  upper  canines  were  sometimes  seven  inches 

long- 
Family  Viverridse — Old  World  forms,  such  as  civets  (  Viverra},  of 

Africa  and   India,  genets  (Genetta),  of  S.  Europe,  Africa,  and 

S.-W.  Asia,  ichneumons  or  mongooses  (fferpestes),  from  Spain, 

Africa,  India,  Indo-Malaya. 
Family  Proteleidae — represented  by  Proteles  cristatus,  the  hyaena-like 

Aard-wolf  of  Cape  Colony. 
Family  Hysenidae — represented  by  the  genus  Hycena,  found  in  Africa 

and  S.  Asia.     The  tympanic  bulla  is  not  divided  by  a  septum. 

CYNOIDEA — Dog-like  Carnivores. 

Family  Canidse — including  forms  intermediate  between  the  cats  and 
the  bears.  The  dentition  is  more  generalised  than  in  the  Felidse, 

3T42 

its  usual  formula  is .     Within  the  tympanic  bulla  there  is 

3J43 

only  a  rudimentary  septum.     The  paroccipital  process  in  contact 
with  the  bulla  is  prominent.     The  caecum  is  either  short  and 
simple,  or  long  and  peculiarly  folded  upon  itself. 
Examples : — The  genus  Canis  has  representatives  in  all  parts  of 
the  world,  the  wolves  (C.  lupus,  &c.),  the  jackals  (C.  aureus), 
inesomelas,  &c.),  the  domestic  dogs  ( C.  familiaris],  the  foxes 
(C.  vulpes,  &c.),  the  Cape  hunting  dog  (Lycaori],  the  bush- 
dog  (Icticyon]  of  Guiana  and  Brazil,  and  the  primitive  Otocyon 
megalotis  from  S.  Africa,  with  the  maximum  number  of  back 

3,  i,  4,  3-4  _      3142 

teeth .     In  the  dog  the  dental  formula  is ;  the 

3,  ii  4>  4  3H3 

upper  carnassial  or  fourth  premolar  has  a  stout  bilobed  blade, 
the  lower  carnassial  or  first  molar  has  a  compressed  bilobed 
blade.  The  skull  is  more  elongated  than  in  the  cats ;  the 
orbits  are  very  widely  open  posteriorly  ;  the  clavicles  are  very 
small ;  the  limbs  are  digitigrade ;  there  are  five  toes  on  the 
fore-feet,  but  the  short  thumb  does  not  reach  the  ground  ;  there 
are  only  four  toes  on  the  hind-feet,  but  in  domestic  dogs  the 
rudiment  of  the  hallux  is  sometimes  enlarged  as  the  "dew- 
claw  ; "  the  claws  are  non-retractile  and  blunt. 

ARCTOIDEA — Bear-like  Carnivores. 

The  tympanic  bulla  shows  no  trace  of  an  internal  septum  ;  the  paroc- 
cipital process  of  the  ex-occipital  is  quite  apart  from  the  bulla, 
and  widely  separated  from  the  mastoid  process  of  the  periotic. 
The  limbs  are  plantigrade  or  sub-plantigrade,  and  always  bear 
five  toes.  There  is  no  csecum. 


;i6  MAMMALIA. 

Family  Ursicbe — Bears.     The  molars  have  broad  tuberculated  crowns. 
The    three   anterior   premolars   are    usually  rudimentary.      The 

3T42 

auditory  bulla  is  depressed.      Ursus, ,  absent  from  Ethiopian 

3M3 

and  Australian  regions,  represented  in  the  Neotropical  region  by 

only  one  species,  elsewhere  widespread. 
Family  Procyonidae — The  Himalayan  Panda  (AZlurzis  fulgens],  the 

American  raccoon  (Procyori}. 
Family  Mustelidae — The  otter  (Lutra\  the  sea-otter  (Latax  lutris], 

the  skunk  (Mephitis},  the  badger  (Meles],  the  ratel  (Mellivora), 

the  marten,  sable,  polecat,  stoat,  weasel  (Mustela}. 

Sub-Order  PINNIPEDIA.     Seals,  Eared  Seals,  and  Walruses. 

These  are  marine  Carnivores,  unable  to  move  readily  on  land,  but 
coming  ashore  for  breeding  purposes.  They  feed  for  the  most  part  on 
fish,  molluscs,  and  crustaceans.  Absent  from  the  Tropics,  they  are 
represented  on  most  of  the  coasts  in  Temperate  and  Arctic  zones.  Many 
are  markedly  gregarious. 

The  upper  parts  of  the  limbs  are  included  within  the  skin  and  general 
contour  of  the  body.  There  are  five  well-developed  digits  connected 
by  a  web  of  skin.  In  the  hind-foot  the  first  and  fifth  toes  are  generally 
stouter  and  longer  than  the  rest.  There  are  no  clavicles.  The  tail  is 
very  short. 

The   small    milk-teeth    are   absorbed    before    or    immediately  after 

birth.     The  incisors  are  always  fewer  than  -  ;   there  are  no  carnassials  ; 

the  back  teeth  have  pointed  cusps  often  sloping  slightly  backwards. 

The  brain  is  large  and  well-convoluted.  The  eyes  are  large  and  pro- 
minent, with  a  flat  cornea.  The  external  ear  is  small  or  absent. 

The  caecum  is  very  short.     The  kidneys  are  divided  into   lobules. 
The  mammae  are  two  or  four  in  number,  and  lie  on  the  abdomen. 
Family  Otariidae — Eared  or  fur-seals,  connecting  the  Pinnipeds  with 
the  Fissipeds.     The  hind-feet  can  be  turned  forward  and  used  on 
land   in   the  usual   fashion.     The   palms  and  soles  are  naked. 
There  is  a  small  external  ear.     The  testes  lie  in  an  external 
scrotum. 

3,   i,  4,   1-2 

The  sea-lion  Otaria, >  supplies  the  seal  skin  of  commerce. 

2,    I,    4,    I 

Family  Trichechidae — Walruses,  intermediate  between  the  Otariidae 

and  the  seals.     The  hind  feet  can  be  turned  forwards  and  used 

on  land.     The  upper  canines  form  large  tusks ;  the  other  teeth 

are  small,  single  rooted,  and  apt  to  fall  out  ;  those  generally  in 

1130  _    3132 

use  are >  but  the  dentition  of  the  fcetus  is • 

0130  3131 

The  jaw  seems  relatively  short,  an  adaptation  perhaps  to  mussel- 
crushing  instead  of  fish-catching. 
There  are  no  external  ears. 
The  walrus  or  morse,  Trichechus. 
Family  Phocidae — Seals,  the  most  specialised  Pinnipeds.     The  hind- 


INSECTIVORA.  717 

limbs  are  stretched  out  behind,  and  the  strange  jumping  move- 
ments on  land  are  effected  by  the  trunk,  sometimes  helped  by 
the  fore-limbs.  The  palms  and  soles  are  hairy.  There  are  well- 
developed  canines,  the  upper  incisors  have  pointed  crowns,  there 

are  -  back  teeth.      There  is  no  external    ear.      The   testes  are 

5 
abdominal. 

3I4I 

The  common  seal  (Phoca).  ;  the  grey  seal  (Halich<xrus\ 

2141 

the  monk  seal  (Monachus),  the  large  elephant  seal  (Macrorhinus 
leoninus}. 

Sub-Order  CREODONTA  (extinct). 

In  Eocene  and  early  Miocene  strata,  in  Europe  and  America,  there 
are  remains  of  what  seem  to  be  generalised  Carnivora,  ancestral  to  the 
modern  types,  and  apparently  related  to  Insectivora  as  well.  Those 
included  in  the  sub-order  Creodonta  have  strong  canines  but  no  single 
carnassials,  while  the  molars  are  often  like  those  of  Marsupials.  The 
brain  seems  to  have  been  small. 

Examples  : — Hycenodon,  Pterodon^  Proviverra^  Arctocyon. 

Order  7.     INSECTIVORA. 

This  order  includes  hedgehog,  mole,  shrews,  and  related 
mammals.  There  is  much  diversity  of  type,  so  that  a  state- 
ment of  general  characters  is  very  difficult. 

Most  Insectivores  run  about  on  the  earth;  the  mole 
( Talpa),  and  others  like  it,  are  burrowers ;  Potamogale, 
Myogale,  and  others  are  aquatic :  Tupaia  and  its  relatives 
live  like  squirrels  among  the  branches;  and  the  aberrant 
" flying  Lemur" — Galeopithecus  takes  swoops  from  tree  to 
tree. 

Most  feed  on  Insects,  but  Galeopithecus  and  some  other 
arboreal  forms  eat  leaves  as  well,  the  moles  eat  worms, 
Potamogale  is  said  to  feed  on  fish. 

The  body  is  usually  covered  with  soft  fur,  but  the  hedge- 
hog (Erinaceus)  is  spiny,  and  so  to  a  less  extent  is  Centetes, 
the  groundhog  of  Madagascar.  The  digits,  usually  five  in 
number,  are  clawed,  and  the  animals  walk  in  plantigrade 
or  semi-plantigrade  fashion.  In  most,  the  mammae  are 
thoracic  or  abdominal ;  in  Galeopithecus ',  there  are  two  pairs 
in  the  axillary  region. 

The  cranial  cavity  is  small ;  the  skull  is  never  high ;  the 
facial  region  is  long;  the  zygomatic  arch  is  slender  or 
absent.  Except  in  Potamogale,  there  are  clavicles. 


7i8  MAMMALIA. 

There  are  more  than  two  incisors  in  the  mandible.  The 
enamelled  molars  have  tuberculated  crowns  and  well-de- 
veloped roots.  In  many  cases  it  is  not  easy  to  distinguish 
the  usual  division  of  the  teeth  into  incisors,  canines,  pre- 
molars,  and  molars,  but  in  many  the  dentition  is  typical — 
3,  i,  4,  3=44- 

In  the  hedgehog,  according  to  Leche,  i.  3,  pm.  2,  m.  1-3, 
of  the  upper  jaw,  and  i.  3,  c,  pm.  3,  m.  1-3,  of  the  lower  jaw, 
are  persistent  milk  teeth, — a  mixed  and  primitive  condition. 

The  cerebral  hemispheres  are  smooth  and  leave  the 
cerebellum  uncovered ;  the  olfactory  lobes  are  large ;  the 
corpus  callosum  is  short  and  thin.  Thus,  as  regards  the 
brain,  the  Insectivora  represent  a  low  grade  of  organisation. 

Except  in  Galeopithecus,  the  stomach  is  a  simple  sac ;  the 
intestine  is  long  and  simple,  but  the  vegetarian  forms  have  a 
caecum.  In  most,  there  are  odoriferous  glands,  axillary  in 
shrews,  but  usually  near  the  anus. 

The  testes  are  inguinal  or  in  the  groin,  or  near  the 
kidneys,  not  in  a  scrotum.  The  penis  may  be  pendent 
from  the  wall  of  the  abdomen,  but  is  usually  retractile. 
There  is  a  bicornuate  or  two-horned  uterus.  Except  in 
GaleopitkecuS)  several  offspring  and  usually  many  are  born 
at  once. 

The  allantoic  placenta  is  discoidal  and  deciduate.  There 
is  a  provisional  yolk  sac  placenta. 

Insectivora  are  represented  in  the  temperate  and  tropical 
zones  of  both  hemispheres,  but  not  in  S.  America  nor 
Australia. 

Sub-Order  Insectivora  Vera  : — Insectivores  with  free  limbs  suited  for 
movement  on  land,  climbing,  burrowing,  or  swimming.  "  The  upper 
and  lower  incisors  are  conical,  unicuspidate  or  with  basal  cusps  only, 
the  lower  not  pectinated." 

Examples  : — the  hedgehogs  (Erinaceus],  throughout  Europe,  Africa, 
3133 

and  most  of  Asia,  dentition  ;  the  shrews  (Sorex\  in  Europe, 

2123 

4I23 

Asia,    and    N.    America,    dentition    —  ;    the    moles    (Taipei), 

2013 

throughout  the  Palaearctic  region  ;  the  tailless  tenrec  ( Centetes) 
of  Madagascar;  the  S.  African  golden  moles  (Chrysochloris}'; 
the  African  jumping  shrews  (Macroscelides)  ;  the  Oriental  tree 
shrews  ( Tupaia], 

Sub- Order  Dermoptera  : — represented  by  the  very  divergent  Galeopi- 
thecus,  which  almost  requires  an  order  for  itself.  The  fore  and  hind 


CHIROPTERA.  7^9 

limbs  are  connected  by  a  parachute,  and  the  animals  can  glide  from  tree 
to  tree,  ' '  sometimes  traversing  a  space  of  seventy  yards  with  a  descent 
of  only  about  one  in  five."  The  upper  and  lower  incisors  are  compressed, 
multicuspidate,  the  lower  deeply  pectinated.  Two  species  of  this  genus 
live  in  the  forests  of  the  Malayan  region.  They  are  nocturnal,  and  feed 

2123 
on  leaves  and  fruit.     The  dentition  is .     There  are  numerous  skeletal 

3123 
peculiarities. 

Order  8.     CHIROPTERA — Bats. 

Bats  are  specialised  Mammals  related  to  Insectivores. 
They  have  the  power  of  flight,  the  fore  limbs  being  modified 
as  wings.  The  wing  is  mainly  due  to  an  extension  of  the 
skin  stretched  between  the  very  long  fingers.  The  fold  of 
skin  usually  begins  from  the  shoulder,  extends  along  the 
upper  margin  of  the  arm  to  the  base  of  the  thumb,  thence 
between  the  fingers,  and  along  the  sides  of  the  body  to  the 
hind  legs  or  even  to  the  tail.  Contrasted  with  the  wing  of  a 
bird,  that  of  a  bat  has  a  rudimentary  ulna  beside  a  long 
curved  radius,  a  wrist  with  six  bones,  five  free  digits  with 
long  metacarpals  on  the  four  fingers.  The  shoulder  girdle 
is  very  strong,  there  is  a  long  curved  clavicle,  a  large  tri- 
angular scapula,  a  long  coracoid  process ;  the  presternum 
bears  a  slight  keel  on  which  are  inserted  some  of  the  muscles 
used  in  flight.  The  thumb  is  always  clawed ;  the  other 
digits  are  unclawed,  except  in  most  frugivorous  bats,  where 
the  second  digit  bears  a  claw. 

The  hind  limb  is  relatively  short  and  weak,  the  pelvic 
girdle  is  also  weak,  and  in  most  cases  the  pubic  symphysis 
is  loose  in  the  males,  unformed  in  the  females.  The  knee  is 
turned  backwards  like  the  elbow,  the  ankle  has  a  cartilaginous 
prolongation  or  calcar  which  supports  the  fold  of  skin 
between  limb  and  tail,  the  five  toes  are  clawed. 

The  vertebral  column  is  short,  there  is  little  mobility 
between  the  vertebrae,  neural  spines  are  absent  behind  the 
third  cervical  except  in  Pteropidae,  the  caudal  vertebrae  are 
very  simple.  The  ribs  are  usually  flat.  The  maximum 

2133 
dentition  is  —  :  the  milk  teeth  are  very  different  from  the 

3133 
permanent  set.    All  the  bones  are  slender,  and  have  relatively 

large  medullary  canals. 

The  cerebral  hemispheres  are  smooth  and  leave  the 
cerebellum  uncovered.  The  spinal  cord  is  at  first  very 


720 


MAMMALIA. 


broad,  but  narrows  rapidly  behind  the  neck.  The  sense  of 
touch  is  remarkably  developed  in  the  hot  skin  of  the  wing, 
the  large  mobile  external  ears,  the  whisker  hairs  of  the 
snout,  and  in  the  strange  plaited  "  nose  leaves  "  around  the 
nostrils.  Even  when  deprived  of  sight,  hearing,  and  smell, 


FIG.  263. — Skeleton  of  Fox  Bat, — Pteropus. 

C7,  Clavicle;  H,  humerus  ;  /?,  radius;  U,  incomplete  ulna;  Th, 
thumb ;  Co.,  carpals ;  M,  metacarpals  3  and  4 ;  P/t,  phalanges ;  F, 
femur ;  T,  tibia  ;  F,  fibula ;  To.,  tarsals  ;  mt>  metatarsals. 

bats  will  fly  about  in  a  room  without  striking  numerous 
wires  stretched  across  it. 

The  temperature  of  the  body  is  high.  The  testes  are 
abdominal  or  inguinal ;  the  penis  is  pendent.  The  uterus 
is  simple,  with  cornua  generally  short.  There  is  usually  but 


LEMUROIDEA. 


721 


one  offspring  at  a  time.  The  mammae  are  thoracic,  gener- 
ally post-axillary.  As  in  Insectivora  and  Rodents,  the  yolk 
sac  forms  a  provisional  placenta,  and  the  allantoic  placenta 
is  discoidal  and  deciduate. 

Fossil  Chiroptera  occur  in  Upper  Eocene  strata,  but  are 
quite  like  the  modern  forms. 

The  two  sub-orders  of  bats  may  be  contrasted  as  follows : — 


MEGACHIROPTERA. 


Frugivorous  bats,  usually  large. 

The  molars  have  smooth  crowns,  with  a 
longitudinal  groove. 

The  thumb  is  clawed,  and  generally  also 
the  second  digit. 

The  tail,  if  present,  is  below,  not  bound  up 
with  the  interfemoral  membrane. 


The  pylortc  part  of  the  stomach  is  in  most 
cases  much  elongated. 

Found  in  warm  and  tropical  parts  of  the 
Eastern  hemisphere. 

Examples  : — 

The  "  flying-foxes  "  or  fox-bats  (Ptero- 
pus),  large,  tailless  bats,  distributed  from 
Madagascar  to  India,  Ceylon,  Malaya, 
S.  Japan,  Australia,  Polynesia.  The  lar- 
gest species  (P.  edulis)  measures  five  feet 
across  its  spread  wings.  Dentition,  f  yff . 

In  India,  Cynopterus  marginatus  is 
very  common.  Xantharpyia  tegyptiaca 
inhabits  the  Pyramids. 


MlCROCHIROPTERA. 


Usually  insectivorous  bats,  small  in  size. 

The   molars    have  cusped   crowns,   with 
transverse  grooves. 

In  the  hand  the  thumb  only  is  clawed. 


The  tail,  if  present,  is  bound  up  with  the 
interfemoral  membrane,  or  lies  along  its 
upper  surface. 

Except  in  one  family  the  stomach  is 
simple. 

Found  in  the  tropical  and  temperate 
regions  of  both  hemispheres. 

Examples  : — 

The  horse-shoe  bats  (Rhinolophus),  the 
common  pipistrelle  {Vesptrugo  pipistrel- 
tus),  the  genus  Vespertilio  with  four 
British  species,  Vampyrus  spectrum,  a 
large  Brazilian  form,  which  seems  to  have 
been  erroneously  credited  with  blood- 
sucking habits,  the  common  vampire 
(Desmodus  rufus)  an  American  bat — a 
formidable  blood-sucker. 


Order  9.    LEMUROIDEA.     Lemurs. 

Opinions  differ  as  to  whether  the  monkey-like  animals 
known  as  Lemurs  should  be  ranked  with  monkeys  as  a  sub- 
order of  Primates  or  referred  to  a  separate  order.  They 
differ  from  monkeys  and  men  (Anthropoidea)  in  the  follow- 
ing points  : — The  orbit  opens  freely  into  the  temporal  fossa 
(except  in  Tarsius) ;  the  lachrymal  foramen  lies  outside  the 
orbit ;  the  first  pair  of  upper  incisors  is  separated  in  the 
middle  line  (except  in  Chiromys]  •  the  cerebral  hemispheres 
are  but  slightly  convoluted  and  do  not  completely  overlap 

46 


722  MAMMALIA. 

the  cerebellum;  "the  middle  or  transverse  portion  of  the 
colon  is  almost  always  folded  or  convoluted  on  itself;"  there 
may  be  abdominal  mammae ;  the  uterus  is  bicornuate  ;  the 
placenta  is  diffuse.  The  dentition  of  Lemurs  varies  greatly; 

in  some  it  is  ^|. 

The  Lemurs  are  small,  furry,  monkey-like  quadrupeds. 
Many  are  nocturnal,  all  arboreal.  They  feed  on  fruits 
and  leaves,  on  eggs  and  small  animals.  Seven  genera 
live  in  Madagascar,  three  genera  occur  in  the  African 
continent,  and  other  three  genera  are  represented  here 
and  there  in  Oriental  forests  as  far  east  as  the  Philippines 
and  Celebes. 

As  remains  of  extinct  Lemurs  are  found  in  Europe  and  N.  America, 
the  distribution  of  the  order  is  now  greatly  restricted,  and  no  less 
than  thirty  out  of  the  total  of  fifty  species  are  confined  to  Madagascar. 
Wallace  concludes  from  the  distribution  of  Lemurs  that  there  must  have 
been  "a  large  tract  of  land  in  what  is  now  the  Indian  Ocean,  connect- 
ing Madagascar  on  the  one  hand  with  Ceylon,  and  with  the  Malay 
countries  on  the  other.  About  the  same  time  (but  perhaps  not  contem- 
poraneously) Madagascar  must  have  been  connected  w7ith  some  portion 
of  Southern  Africa  ;  and  the  whole  of  the  country  would  possess  no 
other  Primates  but  Lemuroidea."  Whether  this  be  altogether  true  or 
not,  it  is  certain  that  the  Lemurs  are  absent  from  regions  where  once 
they  lived,  that  most  of  the  modern  forms  are  found  (like  the  Marsupials) 
on  an  island,  that  this  insulated  race  has  evolved  in  several  specialised 
directions,  that  outside  of  Madagascar  the  Lemurs  maintain  their  exist- 
ence on  a  few  other  islands,  or  by  hiding  in  the  forests. 

There  are  three  chief  types  : — 

(a)  That  of  the  Lemuridoe,  e.g.,   in   Madagascar  Lemur,  and  the 

large    Indris   (2   feet   long),   in  Africa    Galago,    in    Malay 
Nycticebus,  in  India  and  Ceylon  Loris. 

(b)  Tarsius,  a  specialised  Indo-Malayan  type  with  many  peculiari- 

ties, e.g.,  the  calcaneum  and  navicular  are  elongated  like  the 
calcaneum  and  astragalus  in  the  frog. 

(c)  Chiromys,  the  Aye- Aye,  a  specialised  Madagascar  type,  with 

many  peculiarities,  e.g. ,  with  incisors  like  those  of  Rodents, 
and  with  a  very  much  attenuated  middle  finger. 

Order  10.     ANTHROPOIDEA. 

This  order  includes  five  families. 
Family  5.  Hominidse.     Man. 

4.  Simiidae.     Anthropoid  Apes.   \  Old  World 

3.  Cercopithecidse.     Baboons.     /  Catarrhini. 

2.  Cebidae.    American  Monkeys.  \  New  World 

i.   Hapalidae.     Marmosets.  /  Platyrrhini. 


ANTHROPOIDEA. 


723 


The  following  characteristics  are  generally  true. 

The  body  is  hairy,   least   so   in   man ;    the  dentition  is 


FIG.  264.— Skeleton  of  Male  Gorilla.     (From  Edinburgh 
Museum  of  Science  and  Art. ) 

cL,  Clavicle  ;  sc.,  tip  of  scapula  ;  S.,  praesternum  ;  //.,  humerus  ; 
r.,  radius  ;  u.,  ulna  ;  //.,  ilium  ;  C.,  coccyx  ;  P.,  pubis  ;  Is.,  ischium  ; 
F.>  femur  ;  t.,  tibia  ;  f.,  fibula. 


724  MAMMALIA. 

diphyodont  and  heterodont ;  the  incisors  do  not  exceed  -  ; 

the  molars  are  -  except  in  the  marmosets  where  they  are  -  ; 

3  2. 

the  axis  of  the  orbit  is  directed  forward,  and  the  orbit  is 

closed  off  from  the  temporal  fossa;  the  clavicles  are  well 
developed ;  the  radius  and  ulna  are  never  united ;  the 
scaphoid,  the  lunar,  and  usually  the  os  centrale  remain 
distinct  from  one  another  ;  there  are  usually  five  fingers  and 
five  toes,  but  the  thumb  may  be  absent  or  rudimentary ; 
the  big  toe  is  opposable  except  in  man,  and  has  a  flat  nail 
except  in  the  orang ;  the  thumb  is  usually  more  or  less 
opposable;  the  cerebral  hemispheres  have  numerous  con- 
volutions and  overlap  the  cerebellum  ;  the  stomach  is  simple 
except  in  Semnopithecus  and  its  relatives,  in  which  it  is 
sacculated,  and  there  is  a  caecum  which  is  often  large ; 
there  are  two  mammae  on  the  breast ;  the  uterus  is  simple ; 
the  testes  lie  in  a  scrotum  ;  the  placenta  is  meta-discoidal, 
being  developed  by  the  concentration  of  the  villi  from  a 
diffuse  area  into  a  well-defined  disc. 

Some  of  the  characteristics  in  which  the  Anthropoidea 
differ  from  Lemuroidea  may  be  re-emphasised  : — the  orbit  is 
separated  from  the  temporal  fossa  by  a  bony  partition  ;  the 
lachrymal  foramen  is  situated  within  the  margin  of  the 
orbit;  the  median  upper  incisors  are  in  contact;  the  cerebral 
hemispheres  are  richly  convoluted  and  hide  or  almost  cover 
the  cerebellum ;  "  the  transverse  portion  of  the  colon 
extends  uninterruptedly  across  the  abdomen;"  the  mammae 
are  never  abdominal ;  the  uterus  is  not  bicornuate  but 
simple ;  the  placentation  is  meta-discoidal. 

Family  i.  HAPALID^E  ( =  Arctopithecini).     Marmosets. 

The  marmosets  are  the  smallest  monkeys,  being  no  larger 
than  squirrels.  They  live  in  companies  in  the  Neotropical 
forests,  especially  in  Brazil,  and  feed  on  insects  and  fruit. ' 

Their  dentition  ^  is  distinctive,  for  other  Anthropoidea 
have  -  molars.  There  is  a  broad  septum  between  the 

3 

nostrils,  as  in  the  other  New  World  monkeys ;  the  external 
auditory  meatus  is  not  bony.  The  tail  is  long,  hairy,  and 


ANTHROPOIDEA.  725 

non-prehensile.  The  arms  are  not  longer  than  the  legs  ; 
there  are  no  cheek  pouches  nor  ischial  callosities.  The 
thumb  or  pollex  is  long  but  not  opposable ;  all  the  digits 
have  a  pointed  claw  except  the  great  toe  or  hallux  which  is 
very  small.  The  marmosets  often  bear  three  young  ones 
at  a  birth,  whereas  the  other  monkeys  usually  bear  but  one. 
There  are  two  genera,  f[apale  and  Midas. 

Family  2.  CEBID^E  (  =  Platyrrhini).     American  Monkeys. 

In  the  American  monkeys  the  nose  is  flat,  with  a  broad 
internarial  septum.  They  occur  throughout  tropical  America, 
but  are  most  at  home  in  Brazil.  All  are  arboreal,  and  many 
have  prehensile  tails.  The  digits  have  nails,  not  claws  ;  the 
thumb,  though  not  opposable,  is  divergent  from  the  ringers, 
except  in  the  spider  monkey — Ateles — in  which  it  is  rudi- 
mentary. The  skull  is  rounded,  and  the  frontals  form  a 
V-shaped  suture  with  the  parietals.  The  dentition  is  char- 
acteristic, for  there  are  six  back  teeth ;  the  formula  being 

2133- 

Examples : — The  howling  monkeys  (Mycetes),  with  large  vocal 
organs  protected  by  the  expanded  mandibles,  and  with  an 
inflated  hyoid  bone  forming  a  resonating  chamber  ;  the  sakis 
(Pithecia)  with  very  long  tail  ;  Nyctipithecus ;  Chrysothrix ; 
the  spider  monkeys  (Ateles}  with  exceedingly  prehensile  tail ; 
the  capuchins  (Ce&us),  often  imported  into  Europe. 

Family  3.  CERCOPITHECID^:  ( =  Cynomorph  Catarrhini). 
Old  World  dog-like  Apes. 

The  Old  World  apes  of  this  family  are  still  quadrupeds, 
and  the  snout  or  muzzle  often  justifies  the  term  Cynomorph 
or  dog-like.  There  is  a  narrow  internarial  septum,  to  which 
the  term  Catarrhini  refers.  The  dentition  is  like  that  of  the 
anthropoid  apes  and  man,  2123.  The  external  auditory 
meatus  is  bony.  The  thumb  is  opposable,  except  when  it 
is  rudimentary  as  in  Colobus.  The  tail  is  not  prehensile. 
Over  the  rough  surfaces  of  the  everted  ischia  the  skin  forms 
callosities  often  brightly  coloured.  The  breast  bone  is 
narrow.  The  caecum  has  no  vermiform  appendix. 

In  the  sub -family  Cercopithecinse,  there  are  cheek  pouches,  the 
stomach  is  simple,  the  fore  and  hind  limbs  are  almost  equal. 

Examples  : — the  African  baboons  ( Cynocephalus]  e.g.,  the  mandrill 
( C.  maimori)  notable  for  the  bright  colours  of  the  face  and  hips 


726  MAMMALIA. 

in  the  adult  males,  the  macaques  (Macacus]  all  Asiatic  except 
the  Barbary  ape  ( M.  inuus}  of  N.  Africa  and  Gibraltar  ;  the 
African  Cercopithecus. 

In  the  sub-family  Semnopithecinoe,  there  are  no  cheek  pouches,  the 
stomach  is  sacculated  in  a  complex  fashion,  the  hind  limbs  are  longer 
than  the  fore  limbs. 

Examples: — the  sacred  Indian  apes  (Semnopithecus],  the  African 
Colobtis,  and  the  proboscis  monkey  (Nasalis]  of  Borneo. 


FIG.  265. — Skull  of  Orang-Utan.     (From  Edinburgh 
Museum  of  Science  and  Art. ) 

/.,  Parietal ;/.,  frontal ;  sq.,  squamosal ;  j.  jugal ;  ;;;.,  maxilla. 

Family  4.  SIMIID^E  ( =  Anthropomorph  Catarrhini). 
Anthropoid  Apes. 

The  Old  World  apes  of  this  family  are  the  Gibbons 
(Hylobates),  the  Orangs  (Simia\  the  Chimpanzees  (Troglo- 
dytes or  Anthropopithecus\  and  the  Gorillas  (Gorilla).  As 


ANTHROPOIDEA. 


727 


they  are  the  highest  apes  and  likest  man,  they  are  called 
Anthropoid. 

These  apes  are  less  like  quadrupeds  trian  the  others  ;  they 
have  no  distinct  tail  nor  cheek  pouches.  Only  in  the  Gibbon 
are.  there  ischial  callosities,  and  these  are  small.  The  arms 
are  much  longer  than  the  legs.  The  sternum  is  broad. 
The  caecum  has  a  vermiform  appendix.  As  in  the  lower 
Old  World  apes,  the  dentition  is  like  that  of  man — 2123. 


FIG.  266. — Skull  of  Gorilla.     (From  Edinburgh  Museum 
of  Science  and  Art.) 

The  Gibbons  (Hylobates]  live  in  S.-E.  Asia,  especially  in  the  Malayan 
region.  The  largest  attains  a  height  of  three  feet.  They  walk  erect 
with  the  hands  reaching  the  ground.  The  skull  is  not  prolonged  into 
a  vertical  crest.  There  is  an  os  centrale  in  the  carpus.  The  hallux  is 
well-developed.  They  are  the  highest  apes  with  hints  of  ischial  callosi- 
ties. They  are  mainly  arboreal  in  their  habits.  They  feed  on  fruits, 
leaves,  shoots,  eggs,  young  birds,  spiders,  and  insects.  Their  voice  is 
powerful.  As  regards  teeth,  the  gibbons  are  most  like  man. 

The  Orangs  (Simia]  live  in  swampy  forests  in  Sumatra  and  Borneo. 
The  males  measure  over  four  feet.  They  walk  on  their  knuckles  and 


728  MAMMALIA. 

on  the  outer  edges  of  the  feet.  The  skull  is  prolonged  into  a  vertical 
crest.  There  are  but  slight  supra-orbital  ridges.  The  canines  are  very 
large.  There  are  twelve  ribs  as  in  man,  and  sixteen  dorso-lumbar 
vertebrae.  The  larynx  is  connected  with  two  large  sacs  which  unite 
ventrally.  There  are  no  ischial  callosities.  They  are  arboreal  in  their 
habits,  and  make  nests  in  the  branches.  They  are  exclusively  vege- 
tarian. As  regards  the  structure  of  the  brain,  the  orangs  are  most  like 
man. 

The  Gorillas  (Gorilla]  live  in  Western  Equatorial  Africa.  They  are 
larger  than  all  other  apes,  and  larger  than  man,  though  not 'over  5^  feet 
in  height.  The  arms  reach  to  the  middle  of  the  lower  leg,  and  the 
animals  walk  with  the  backs  of  their  closed  hands  and  the  flat  soles  of 
their  feet  on  the  ground.  The  skull  is  not  prolonged  into  a  vertical 
crest.  There  are  prominent  supra-orbital  ridges.  The  canines  of  the 
males  are  very  large.  The  cervical  vertebrae  bear  very  high  neural 
spines,  on  which  are  inserted  the  muscles  which  support  the  heavy  skull. 
There  are  thirteen  ribs,  and  seventeen  dorso-lumbar  vertebrae.  There 
is  no  os  centrale  in  the  carpus.  There  are  no  ischial  callosities.  They 
live  in  families  in  the  forest,  and  feed  on  fruits.  As  regards  size, 
the  gorillas  are  most  like  man.  The  males  are  much  larger  than  the 
females. 

The  Chimpanzees  (Anthropopithecus]  live  in  Western  and  Central 
Equatorial  Africa.  They  do  not  exceed  a  height  of  5  feet.  The  arms 
reach  a  little  below  the  knee.  They  walk  on  the  backs  of  their  closed 
hands  and  on  their  soles  or  closed  toes.  •  The  skull  has  no  high  crests. 
The  supra-orbital  ridges  are  distinct.  The  canines  are  smaller  than  in 
Gorilla  or  Orang.  There  is  no  centrale  in  the  carpus.  There  are  no 
ischial  callosities.  The  chimpanzees  live  in  families  in  the  forest,  and 
are  chiefly  arboreal,  making  nests  in  trees.  They  seem  to  feed  on  fruits. 
In  the  sigmoid  curvature  of  the  vertebral  column  the  chimpanzees  are 
most  like  man. 

Family  5.  HOMINID^E.     Genus  Homo. 

The  distinctiveness  of  man  from  his  nearest  allies  de- 
pends on  his  power  of  building  up  ideas  and  of  guiding 
his  conduct  by  ideals.  But  there  are  some  structural 
peculiarities  of  interest. 

Man  alone,  after  his  infancy  is  past,  walks  thoroughly 
erect.  Though  his  head  is  weighted  by  a  heavy  brain,  it 
does  not  droop  forwards.  With  his  upright  attitude,  the 
increased  command  of  vocal  mechanism  is  perhaps  in  part 
connected. 

Man  plants  the  soles  of  his  feet  flat  on  the  ground ;  the 
great  toes  are  often  longer,  never  shorter  than  the  others, 
and  lie  in  a  line  with  them ;  he  has  a  better  heel  than 
monkeys  have.  No  emphasis  can  be  laid  on  the  old  dis- 
tinction which  separated  two-handed  men  (Bimana)  from 


ANTHROPOIDEA.  729 

the  "  four-handed  "  monkeys  (Quadrumana),  nor  on  the  fact 
that  men  are  peculiarly  naked.  But  "the  arms  are  shorter 
than  the  legs,  and,  after  birth,  the  latter  grow  faster  than  the 
rest  of  the  body." 

Compared  with  the  anthropoid  apes,  man  has  a  bigger 
forehead,  a  less  protrusive  face,  smaller  cheek  bones  and 
supra-orbital  ridges,  a  true  chin,  and  more  uniform  teeth 
(2,  i,  2,  3),  forming  an  uninterrupted  horse-shoe-shaped 
series  without  conspicuous  canines. 

More  important,  however,  is  the  fact  that  the  weight  of 
the  gorilla's  brain  bears  to  that  of  the  smallest  brain  of 
an  adult  man  the  ratio  of  2  :  3,  and  to  the  largest  human 
brain  the  ratio  of  i  :  3  ;  in  other  words,  a  man  may  have 
a  brain  three  times  as  heavy  as  that  of  a  gorilla.  The  brain 
of  a  healthy  human  adult  never  weighs  less  than  31  or  32 
ounces  ;  the  average  human  brain  weighs  48  or  49  ounces ; 
the  heaviest  gorilla  brain  does  not  exceed  20  ounces.  "The 
cranial  capacity  is  never  less  than  55  cubic  inches  in  any 
normal  human  subject,  while  in  the  Orang  and  Chimpanzee, 
it  is  but  26  and  27^  cubic  inches  respectively." 

But,  as  Owen  allowed  long  since,  there  is  an  "  all-pervad- 
ing similitude  of  structure  "  between  man  and  the  anthropoid 
apes.  As  far  as  structure  is  concerned,  there  is  much  less 
difference  between  man  and  the  gorilla  than  there  is  between 
the  gorilla  and  the  marmoset. 

The  arguments  by  which  Darwin  and  others  have  sought 
to  show  that  man  arose  from  an  ancestral  type  common  to 
him  and  to  the  higher  apes,  are  the  same  as  those  used  to 
substantiate  the  general  doctrine  of  descent.  The  "  Descent 
of  Man "  is  the  expansion  of  a  chapter  in  the  "  Origin 
of  Species."  The  arguments  may  be  briefly  summarised. 

(1)  Physiological.     The  bodily  life  of  man  is  like  that  of 
monkeys  ;  men  and  monkeys  are  subject  to  similar  diseases  ; 
various  human  traits  of  gesture,  expression,  &c.,  are  paralleled 
among  the  "brutes;"  reversions  and  monsters  corroborate 
the  alliance  sadly  enough. 

(2)  Morphological.     The  structure  of  man  is  like  that  of 
the  anthropoid  apes;  none  of  his  distinctions,  except  that 
of  a  heavy  brain,  are  momentous ;  there  are  about  eighty 
vestigial    structures   in    his    muscular,    skeletal,    and   other 
systems. 


730  MAMMALIA. 

(3)  Historical.  Certainties  in  regard  to  remains  of  pri- 
mitive man  are  few,  but  his  individual  development  reads 
like  a  recapitulation  of  ancestral  history. 

To  many,  man  seems  too  marvellous  to  have  been  natur- 
ally evolved,  to  others  the  evidence  seems  insufficient,  but 
if  the  doctrine  of  descent  is  true  for  other  organisms,  it  is 
surely  true  for  man  also. 

As  to  the  antiquity  of  the  human  race,  it  is  certain  that 
men  lived  in  Europe  in  the  later  stages  of  the  Ice  Age,  and 
there  are  indications  of  human  life  in  Pliocene  times.  But 
as  it  is  certain  that  man  could  not  have  arisen  from  any  of 
the  known  anthropoid  apes,  and  likely  that  he  arose  from 
an  ancestral  stock  common  to  them  and  to  him,  it  seems 
justifiable  to  date  the  antiquity  of  the  race  not  later  than  the 
time  when  the  anthropoid  apes  are  known  to  have  existed 
as  a  distinct  race.  This  takes  us  back  to  Miocene  ages. 

If  man  was  naturally  evolved,  the  factors  in  the  process 
require  elucidation,  but  in  regard  to  these  we  can  only 
speculate.  From  what  we  know  of  men  and  monkeys,  it 
seems  likely  that  in  the  struggles  of  primitive  man  wits 
were  of  more  use  than  strength.  When  the  habits  of  using 
sticks  and  stone,  of  building  shelters,  of  living  in  families 
began — and  they  have  begun  among  monkeys — it  is  likely 
that  wits  would  grow  rapidly.  The  prolonged  infancy, 
characteristic  of  human  offspring,  would  help  to  evolve 
gentleness.  But  even  more  important  is  the  fact  that  among 
monkeys  there  are  distinct  societies.  Families  combine  for 
protection,  the  combination  favours  the  development  of 
emotional  and  intellectual  strength.  u  Man  did  not  make 
society ;  society  made  man." 

Finally,  it  is  plain  that  all  repugnance  to  the  doctrine  of 
descent  as  applied  to  man  should  disappear  when  we 
clearly  realise  the  great  axiom  of  evolution,  that  "there 
is  nothing  in  the  end  which  was  not  also  in  the  beginning." 


CHAPTER    XXVII. 

COMPARATIVE    PHYSIOLOGY. 

THE  comparative  study  of  the  Physiology  of  the  Inverte- 
brates has  not  as  yet  been  carried  very  far,  though  there  are 
several  careful  investigations  of  particular  problems.  This 
chapter  is  an  attempt  to  gather  up  some  of  the  most  im- 
portant facts,  in  order  especially  to  show  what  is  sometimes 
forgotten,  that  physiology  has  much  to  say  upon  the  general 
problem  of  the  origin  and  maintenance  of  particular  charac- 
ters. A  short  note  on  abnormal  physiological  conditions 
and  their  bearing  upon  evolution  has  also  been  added. 

The  Physiology  of  the  Nervous  System  has  been  very 
fully  investigated  in  several  cases  among  the  Invertebrates, 
and  we  will  therefore  begin  our  survey  with  it. 

We  may  say,  in  the  most  general  way,  that  the  function 
of  the  nervous  system  is  to  bring  the  organism  into  relation 
with  the  external  world.  The  mechanism  by  which  this  is 
effected  consists  typically  of  three  parts: — (i)  the  peripheral 
nerve  endings,  which  receive  the  stimuli;  (2)  the  nerves,  or 
paths  by  which  the  stimuli  are  conveyed  to  or  from — (3) 
the  central  nerve  cells.  The  peripheral  end-organs  with 
which  we  are  most  familiar  are  those  of  eye,  ear,  and  the 
other  special  senses ;  but  we  must  not  forget  that  the 
termination  of  nerve  in  muscle — the  so-called  end-plate — is 
equally  a  peripheral  nerve  ending.  All  nerves  are  in  com- 
munication on  the  one  hand  with  a  peripheral  organ,  and 
on  the  other  with  central  cells. 

It  is  obvious,  from  the  above  definition,  that  neither 
Protozoa  nor  Sponges  possess  a  nervous  system.  For  in  a 
Protozoon  the  receptive  and  perceptive  mechanism  is  con- 
tained in  the  single  cell, — any  part  of  the  protoplasm  will 


732  COMPARATIVE  PHYSIOLOGY. 

respond  to  external  stimuli.  In  Sponges,  the  transmission  of 
stimuli  is  effected  by  the  general  protoplasm  of  the  cells — 
little  division  of  labour  being  apparent — though  here  and 
there  so-called  nerve  cells  have  been  described. 

Among  the  Ccelentera,  we  find  in  Hydra  special  nerve 
cells,  but,  as  proved  by  the  familiar  regeneration  experi- 
ments, these  are  all  similar  and  equivalent.  On  the  other 
hand,  among  the  "jellyfish,"  we  find  nerve  centres  and 
nerves  quite  distinctly  differentiated.  As  we  should  expect, 
the  nerve  physiology  differs  in  the  Craspedota  and  the 
Acraspeda. 

In  the  Craspedote  forms  the  nervous  system  consists  of  a 
ring  round  the  margin  of  the  bell,  giving  off  nerves  which 
form  a  plexus  among  the  muscles,  and  furnished  with  slight 
thickenings  —  the  marginal  bodies  —  at  the  bases  of  the 
tentacles.  The  ring  controls  the  movements  of  the  swim- 
ming bell ;  if  it  is  totally  destroyed  the  movement  ceases, 
but  the  retention  of  a  very  small  part  is  sufficient  to  main- 
tain the  movement.  The  parts  of  the  ring  are  apparently 
equivalent  to  each  other,  any  part  being  capable  of  trans- 
mitting motor  impulses  to  the  whole  of  the  muscles  effecting 
movement.  The  thickened  areas  of  the  ring  seem  to  have 
a  slightly  more  powerful  effect  than  the  undifferentiated 
parts,  but  the  difference  is  not  very  marked ;  the  marginal 
bodies  are,  however,  distinctly  sensitive  to  light.  If  a  strong 
beam  of  light  be  thrown  upon  a  swimming  bell,  it  responds 
by  more  active  contractions,  and  as  the  organisms  are  more 
active  in  light  than  in  darkness,  we  may  conclude  that  light 
(along  with  heat)  acts  as  a  constant  stimulus.  If  the  nerve 
ring  is  totally  destroyed,  the  animal  becomes  motionless, 
and  does  not  recover  itself;  if  stimulated  electrically  or 
mechanically,  it  responds  by  a  single  contraction,  or  occa- 
sionally, in  very  vigorous  specimens,  by  several. 

In  the  Acraspeda  the  eight  separate  nerve  centres  preside 
over  the  swimming  movements ;  if  these  are  all  destroyed, 
the  movements  cease.  If  the  specimen  is  vigorous,  how- 
ever, it  not  infrequently,  after  a  period  of  rest,  resumes  its 
movements,  sometimes  only  feebly,  sometimes  with  a  speed 
quite  comparable  to  that  of  an  uninjured  specimen.  If 
stimulated  during  the  latent  period,  the  Medusa  usually 
responds  with  more  than  one  contraction,  thus  being  again 


PHYSIOLOGY  OF  NERVOUS  SYSTEM.  733 

contrasted  with  the  Craspedote  forms.  Sensitiveness  to 
light  is  exhibited  in  the  same  way  as  in  the  latter.  The 
central  nervous  system  is  connected  by  a  nerve  plexus  with 
the  muscles  which  effect  movement.  Although  little  is 
known  histologically  of  the  way  in  which  the  nerves  end  in 
the  muscles,  yet  physiologically,  in  its  relation  to  poisons,  the 
peripheral  termination  shows  a  remarkable  resemblance  to 
the  "end  plate,"  which  characteristically  occurs  in  the 
muscles  of  Vertebrates.  We  find  here,  therefore,  even  at 
this  low  stage,  that  the  three  distinct  parts  of  a  nervous 
system  are  quite  clearly  defined.  It  seems  unlikely  that 
division  of  labour  has  gone  so  far  as  to  definitely  differ- 
entiate sensory  and  motor  nerves,  but  it  is 'important  to 
note  that  muscular  contraction  does  follow  the  application 
of  a  stimulus.  The  difference  as  to  the  effect  of  the  removal 
of  the  nerve  centre  in  the  two  types  is  extremely  interesting, 
but  as  yet  unexplained. 

In  Sea  Anemones  the  nervous  system  has  been  less  fully 
investigated  than  in  the  Medusae.  There  are  no  specialised 
nerve  centres ;  nearly  all  parts  of  the  body  when  separated 
seem  to  be  able  to  respond  to  stimuli,  so  that  the  nerve 
cells  must  be  scattered.  The  relation  of  the  muscles  to 
the  nervous  tissue  has  the  same  physiological  complexity 
as  in  the  Medusae.  An  interesting  point  is  the  absence  of 
the  spontaneous  movement  which  is  so  characteristic  of  the 
Medusae.  We  have  the  same  contrast  often  presented  even 
in  the  life  history  of  the  individual, — compare  the  sessile 
hydroid  and  the  active  swimming  bell,  the  fixed  hydra-tuba 
and  the  pelagic  jellyfish.  Recently  in  the  Great  Barrier 
reef  of  Australia  an  Alcyonarian  has  been  found,  in  which 
the  polypes,  though  sessile,  exhibit  a  constant  rhythmic 
contraction  and  expansion  of  their  tentacles,  so  that  the 
tendency  to  exhibit  continuous  rhythm  is  widely  spread 
in  the  Ccelentera.  The  nervous  system  is  apparently 
tolerably  uniform  in  type  throughout  the  group;  what 
determines  the  physiological  peculiarities  has  yet  to  be 
investigated.  There  are  two  rival  explanations  of  rhythmic 
movements,  such  as  those  of  the  umbrella  of  the  Medusae. 
According  to  the  first,  it  is  caused  by  rhythmic  stimuli, 
passing  out  from  the  nerve  centres  to  the  muscles  con- 
cerned, and  thereby  causing  the  contractions.  The  other 


734  COMPARATIVE  PHYSIOLOGY. 

view  is  that  the  regular  contractions  are  due  to  the  activity 
of  the  muscles  themselves.  On  this  hypothesis,  building-up 
processes  go  on  in  the  muscles  until  extremely  unstable 
substances  are  produced ;  these  explode  and  break  down 
into  simpler  compounds,  the  process  being  accompanied 
by  an  evolution  of  energy  manifested  by  the  contraction 
of  the  muscle.  The  process,  repeated  at  regular  intervals, 
causes  the  regular  contractions.  But  this  view  seems  to 
minimise  unduly  the  function  of  nerve  cells. 

In  Beroe,  representing  the  Ctenophora,  we  can  only 
notice  that  the  sense  organ,  which  is  placed  at  the  aboral 
pole,  has  to  do  with  the  movements.  In  contra-distinction 
to  the  conditions  found  in  the  Medusae,  we  find  that  special 
parts  of  the  central  nervous  system  preside  over  special 
areas  of  the  organism.  This  is  a  distinct  advance  in  the 
direction  of  division  of  labour,  and  recalls  the  state  of 
affairs  in  higher  forms,  where  clusters  of  brain  cells  form 
what  are  called  centres,  which  preside  over  particular  organs. 

Little  is  known  of  the  nerve  physiology  of  the  members 
of  the  very  heterogeneous  group  of  "  Worms."  It  is  said 
that  a  decapitated  earthworm  can  regenerate  the  anterior 
end  with  its  cerebral  ganglia.  This  would  seem  to 
indicate  that  there  is  little  centralisation  of  the  nervous 
system,  and  that  the  ganglia  are  all  of  nearly  equal  physio- 
logical importance.  It  seems  more  likely,  however,  that  in, 
at  any  rate,  most  Annelids,  the  so-called  "  brain "  does 
perform  to  some  extent  the  function  of  a  central  nervous 
system,  although  the  centralisation  is  only  partial.  In 
Lumbricus,  sensory  and  motor  nerve  fibres  are  differentiated. 

The  nerve  physiology  of  the  Echinoderms  has  been  very 
fully  worked  out,  except  in  the  case  of  the  Holothurians. 
In  the  starfish,  the  nervous  system  consists  of  a  ring  round 
the  mouth,  from  which  nerves  pass  out  to  the  rays,  giving 
off  branches  to  the  tube  feet.  The  whole  surface  of  the 
body  is  sensitive  to  stimuli.  The  ring  round  the  mouth 
co-ordinates  the  action  of  the  different  rays  ;  if  it  is  severed, 
the  rays  lose  their  power  of  acting  in  concert. 

In  Echinus  also  the  ring  round  the  mouth  has  a  co- 
ordinating function  ;  only  when  it  is  intact  do  the  segments 
of  the  body  act  in  unison.  The  ambulacral  nerves  branch 
freely  to  form  the  inner  nerve  plexus ;  from  this,  nerves  pass 


PHYSIOLOGY  OF  NERVOUS  SYSTEM.  735 

out  through  the  shell  to  the  outer  nerve  plexus.  If  any 
spot  on  the  outside  of  the  shell  be  lightly  stimulated,  all  the 
spines,  pedicellariae,  and  tube-feet  in  the  neighbourhood 
bend  towards  the  spot ;  if  it  be  more  strongly  irritated,  the 
spines  and  tube  feet  of  the  other  segments  come  into  play, 
and  by  their  co-ordinated  activity  move  the  animal  in  a 
straight  line  away  from  the  point  of  injury.  The  spines  and 
tube  feet  thus  exhibit  two  different  forms  of  activity — one  a 
mere  local  response  to  stimuli,  the  other  a  more  compli- 
cated and  co-ordinated  action.  The  first  is  presided  over 
by  the  external  plexus,'  but  for  its  complete  accomplishment 
the  internal  plexus  must  be  intact ;  a  connection  with  the 
gullet  ring  is  unnecessary,  as  the  action  is  quite  as  efficiently 
performed  when  the  ambulacral  nerves  are  severed.  Over 
the  co-ordinated  action  of  the  spines  and  tube  feet  the 
internal  nerve  plexus  presides,  but  connection  with  the 
gullet  ring  is  absolutely  necessary.  The  gullet  ring  is  thus 
of  great  importance,  but  the  co-ordinating  action  is  not 
entirely  limited  to  it.  Each  ambulacral  nerve  can  co- 
ordinate the  action  of  the  tube  feet  of  its  own  segment, 
when  quite  detached  from  the  ring  and  the  other  ambulacral 
nerves.  This  nervous  system  is  a  considerable  advance  on 
that  of  the  jellyfish,  but  the  centralisation  is  still  small. 

In  the  Arthropods,  as  in  the  Annelids,  the  question  of 
the  value  of  the  supra-cesophageal  ganglia  has  been  much 
debated.  In  Insects,  according  to  Krukenberg,  they  are 
not  of  great  importance  as  a  co-ordinating  centre,  many 
complex  movements  being  performed  without  the  head. 
But  this  argument  is  hardly  conclusive,  for  a  decapitated 
tortoise  may  continue  to  walk  along  for  several  yards.  The 
respiratory  movements  appear  to  be  presided  over  by  the 
ganglia  of  the  abdomen  ;  they  are  still  performed  by 
separated  segments,  though  their  depth  or  frequency  is 
often  disturbed  by  the  separation  from  the  brain.  In  spite, 
however,  of  the  independence  of  the  ganglia  of  the  ventral 
chain,  the  brain  here,  as  in  higher  animals,  directs  the  move- 
ments. In  the  Crayfish,  while  voluntary  movements  and  the 
maintenance  of  equilibrium  depend  on  the  supra-cesophageal 
ganglia,  the  infra-cesophageal  contain  the  centres  for  the 
co-ordination  of  the  movements  of  eating,  being  reflex 
centres,  as  are  all  the  remaining  ganglia.  In  the  Crab  there 


736  COMPARATIVE  PHYSIOLOGY. 

is  both  morphologically  and  physiologically  a  much  greater 
amount  of  concentration. 

Among  the  Mollusca  we  find  that  in  the  Lamellibranchs 
the  three  sets  of  ganglia  are  of  nearly  equal  importance. 
There  is  no  defined  central  nervous  system,  a  fact  which 
we  may  correlate  with  the  sedentary  habit.  The  motor 
nerves  to  the  great  retractor  muscles  pass  out  from  the 
adjacent  ganglia ;  that  is,  the  cerebral  ganglia  innervate  the 
anterior  retractor,  the  visceral  the  posterior.  The  closing  of 
the  shell  is  active,  and  is  caused  by  the  passage  of  impulses 
to  the  muscles  along  the  motor  nerves.  The  opening  is 
more  passive,  as  the  elastic  ligament  causes  the  valves  to 
gape  when  the  muscles  relax.  This  relaxation  is  caused  by 
inhibitory  nerves  which  inhibit  the  action  of  the  motor 
nerves,  and  the  muscles  in  consequence  return  to  their 
former  condition.  The  inhibitory  nerves  to  both  muscles 
pass  out  from  the  cerebral  ganglia,  but  there  is  no  evidence 
to  justify  the  assumption  that  these  have  any  "  brain " 
function.  The  motor  cells  of  the  cerebral  and  visceral 
ganglia  can  be  stimulated  through  many  peripheral  sensory 
nerves.  The  heart  is  innervated  from  the  visceral  ganglia, 
but  some  physiologists  who  minimise  the  importance  of  the 
innervation  maintain  that  the  heart's  activity  is  largely  pro- 
toplasmic, and  that  the  nerves  have  chiefly  or  wholly  an 
inhibitory  or  trophic  function. 

Among  the  Gasteropods  there  is  not  much  of  special 
interest  in  regard  to  the  nervous  system. 

In  the  Cephalopoda  the  supra-cesophageal  mass  is  un- 
doubtedly a  true  "  brain."  When  it  is  destroyed  the 
ordinary  vital  functions,  such  as  respiration,  circulation, 
&c.,  are  unaltered ;  the  animal  continues  to  respond  to 
external  stimuli,  but  the  power  of  "  volition  "  is  gone  ;  if 
left  to  itself,  it  remains  in  one  position  until  death  ensues. 
From  this  fact  we  see  that  the  centres,  or  presiding  nerve 
cells,  for  all  the  automatic  functions  are  placed  elsewhere 
than  in  the  brain,  but  that  this  originates  all  the  "  voluntary" 
muscular  movements.  Of  the  various  centres,  the  respiratory 
is  located  in  the  pleural  ganglia ;  from  it  nerves  pass  out 
which  end  in  the  stellate  ganglia,  and  are  both  motor  and 
sensory  for  the  mantle.  This  centre  is  not  self-acting,  that 
is,  not  automatic,  as  are  the  corresponding  centres  in  higher 


THE  PHYSIOLOGY  OF  NUTRITION.  737 

Vertebrates,  but  is  only  reflexly  stimulated  into  activity  by 
impulses  borne  by  afferent  nerves  from  some  part  of  the 
body.  It  seems  most  reasonable  to  suppose  that  this  con- 
dition is  primitive,  and  that  the  automatic  form  of  activity 
is  derived.  The  centre  for  the  movement  of  the  chromato- 
phores  is  in  the  sub-cesophageal  mass.  The  activity  of  the 
heart  is  said  by  some  to  be  purely  "  protoplasmic,"  but 
co-ordination  of  the  parts  of  the  heart,  the  branchial  hearts, 
&c.,  is  effected  by  means  of  the  ganglia  placed  in  the 
course  of  the  visceral  nerves  and  their  branches.  The 
arms  are  very  well  innervated,  containing  a  central  nervous 
axis  ;  even  a  severed  arm  is  said  to  exhibit  powerful  reflex 
movements.  This  property  is  probably  of  some  use  in  the 
free  hectocotylised  arm  of  the  male. 

THE  PHYSIOLOGY  OF  NUTRITION. 

We  have  seen  that  by  means  of  the  nervous  system  the 
animal  is  brought  into  relations  with  the  external  world. 
It  is  in  consequence  constantly  evolving  energy  in  the  form 
of  movement,  heat,  electrical  energy  (Gymnotus,  &c.),  or  light 
(phosphorescent  animals).  We  now  proceed  to  consider 
the  manner  by  which  this  loss  of  energy  is  made  good,  that 
is  the  Nutrition  of  the  Tissues.  Inasmuch,  however,  as  the 
food  of  animals  typically  consists  of  very  complex  organic 
substances,  the  process  of  Digestion  must  first  be  considered. 
Digestion  is  the  process  by  which  the  organic  substances  of 
the  food  are  broken  down  into  simpler  substances,  which 
are  soluble  and  diffusible,  and  capable  of  being  assimilated 
and  built  up  into  the  substance  of  the  tissues. 

Digestion. 

In  the  familiar  case  of  the  Amoeba,  solid  food  particles  are 
ingested,  they  are  surrounded  by  fluid,  and  eventually  the 
fluid  is  absorbed  with  the  products  of  digestion,  while  the 
useless  and  indigestible  residue  is  rejected.  Primarily,  this 
process  differs  from  that  found  in  Vertebrates  in  that  it  is 
zVzMs-cellular,  instead  of  being  the  result  of  the  action  of 
extra-cellular  ferments.  There  is  some  doubt  as  to  whether 
the  Protozoan  type  of  digestion  is  also  due  to  ferments,  or 
whether  the  living  protoplasm  has  the  power  of  directly 

47 


738  COMPARATIVE  PHYSIOLOGY. 

inducing  changes  in  substances  brought  into  contact  with  it. 
Krukenberg  succeeded  in  extracting  a  peptic  ferment  from 
the  plasmodium  of  "  Flowers  of  Tan,"  but  did  not  believe 
that  it  could  have  a  digestive  function  on  account  of  the 
alkalinity  of  normal  protoplasm.  Metchnikoff,  however, 
has  demonstrated  in  some  cases  that  the  fluid  of  "  food 
vacuoles  "  is  acid,  and  seems  to  hold  that  all  digestion  is  due 
to  ferment  action.  Miss  Greenwood  has  also  demonstrated 
an  acid  in  the  vacuoles  of  several  Protozoa,  and  described 
the  process  of  digestion.  In  any  case  we  must  note  that  the 
formation  of  ferments  appears  to  be  a  characteristic  of  pro- 
toplasm ;  but  that  as  we  ascend  in  the  scale  of  being  these 
ferments  are  more  and  more  utilised  in  the  digestive  pro- 
cesses, and  tend  to  be  limited  to  the  walls  and  outgrowths 
of  the  alimentary  canal.  We  may  note  here  (as  is  more  fully 
explained  in  the  section  on  Comparative  Pathology)  that  in 
most  animals  certain  cells  retain  the  primitive  Protozoan 
capacity  for  taking  up  and  digesting  solid  particles,  while  the 
general  body  cells  have  lost  it. 

It  is  a  fact  of  common  observation  that  in  parasites  the 
alimentary  canal  tends  to  be  absent  or  degenerate  ;  nutrition 
is  usually  affected  by  simple  absorption  of  the  juices  of  the 
host.  The  exact  physiological  reason  for  the  disappearance 
of  the  gut  is  not  obvious.  Further,  the  method  by  which 
such  parasites  are  protected  from  the  action  of  the  ferments 
of  their  hosts  is  not  clear.  The  reason  is  perhaps  in  part 
the  thickness  of  the  cuticle,  which  is  composed  of  substances 
not  amenable  to  ferment  action.  Again,  Frenzel  claims  to 
have  found  an  anti-enzyme  in  Gregarines,  which  neutralises 
the  action  of  the  host's  intestinal  juices.  The  problem  is 
analogous  to  that  suggested  by  the  fact  that  the  cells  of 
the  gut  escape  during  life  the  action  of  its  juices,  by  which 
they  are  often  attacked  after  death.  Frenzel,  indeed,  com- 
pares a  Gregarine  to  an  absorbing  intestinal  cell. 

Digestion  in  the  Invertebrata. 

In  the  Ccelentera,  ferments  have  been  extracted  from  the 
bodies  of  jellyfish  and  sea  anemones.  In  some  cases  a 
tryptic  ferment  was  extracted  from  the  reproductive  organs, 
a  peptic  from  the  tentacles  and  mesenteries.  The  secretion 


THE  PHYSIOLOGY  OF  NUTRITION.  739 

of  ferment  is  thus  not  confined  to  the  digestive  region,  and 
according  to  Krukenberg  the  ferments  are  not  employed  for 
the  digestion  of  food  outside  the  formative  cells.  In  his 
experiments  he  found  that  solution  and  absorption  of  food 
particles  only  took  place  when  the  particles  were  in  actual 
contact  with  the  digestive  region.  In  Sponges,  digestion  is 
purely  intra-cellular  ;  in  Hydra,  both  intra-  and  extra- 
cellular digestion  seem  to  occur. 

Among  the  higher  worms,  Hirudo  is  distinguished  by  the 
absence  of  an  enzyme  containing  secretion.  The  blood  con- 
tained in  its  pouched  gut  is  simply  absorbed  by  the  walls. 
The  similarity  of  this  method  of  nutrition  to  the  purely 
parasitic  one  found  in  Cestodes  and  Trematodes,  has  been 
advanced  as  an  additional  reason  for  associating  the  leeches 
with  flat  worms  rather  than  with  the  Chaetopoda.  The  habit 
of  feeding  on  the  blood  of  other  animals  may,  however, 
have  led  to  some  of  the  leech's  peculiarities. 

In  most  of  the  other  Annelida — Aphrodite,  Arenicola, 
Lumbricus,  &c. — a  ferment  capable  of  acting  upon  proteids 
has  been  found.  It  is  closely  allied  to  the  tryptic  ferment 
of  Vertebrates,  but  is  not  identical  in  all  its  chemical  reac- 
tions. It  has  been  termed  iso-trypsin,  and  like  trypsin  it  is 
only  active  in  neutral  or  alkaline  solution.  It  appears  to  be 
confined  to  the  Annelida.  The  intestinal  "  caeca  "  found  in 
Aphrodite  and  others  are  not  absorptive  areas,  but  merely 
reservoirs  of  secretion.  They  are  rendered  necessary  by  the 
fact  that  the  gland  cells  are  constantly  active,  and  not  merely, 
as  in  Vertebrates,  stimulated  to  action  by  the  presence  of 
food  in  the  intestine.  The  process  is  therefore  closely 
analogous  to  the  secretion  of  bile  by  the  vertebrate  liver, 
where  the  liver  cells  are  constantly  active,  and  the  gall 
bladder,  like  the  caeca  of  worms,  serves  as  a  store  chamber. 
But  as  the  bile  is  probably  not  to  any  extent  a  digestive 
fluid,  and  as  the  true  digestive  glands  of  Vertebrates  are 
not  constantly  active,  the  conclusion  is  suggested  that  the 
constant  activity  of  the  cells  in  the  worm  is  a  primitive 
condition.  In  most  Annelida  a  diastatic  ferment  also 
occurs,  which  possesses  as  usual  the  power  of  converting 
starch  into  sugar. 

Turning  to  the  Echinoderms,  we  find  that  in  star  fishes 
tryptic,  peptic,  and  diastatic  ferments  are  all  found.  The 


740  COMPARATIVE  PHYSIOLOGY. 

voluminous  caeca  are  not  areas  where  digestion  goes  on,  but, 
as  in  Aphrodite ,  merely  reservoirs  for  the  secretion.  In  the 
Holothurians  no  digestive  glands  have  been,  as  yet,  found  in 
connection  with  the  gut,  nor  can  any  ferment  be  extracted 
from  its  walls.  The  contents  of  the  gut  are,  however,  mixed 
with  a  peptic  ferment ;  this  can  also  be  extracted  from 
extra-intestinal  parts  of  the  body,  so  that  ferment  secreting 
glands  must  exist.  A  similar  diffuseness  in  the  occurrence 
of  ferments  is  very  common  among  the  Echinoderma.  It  is 
therefore  asserted  that  digestion  must  go  on  in  various  parts 
of  the  body,  and  that  it  is  not  limited  to  the  alimentary 
tract.  Diastatic  ferments  are  very  frequently  present. 

In  Arthropods,  peptic,  tryptic,  and  diastatic  ferments  are 
common.  The  peptic  ferment  is  uniform  throughout  the 
group,  and  has  been  termed  "homaropepsin,"  to  indicate 
that  it  differs  considerably  from  the  pepsin  of  Vertebrates. 
On  the  other  hand,  the  tryptic  ferment  is  not  distinguishable 
from  that  of  Vertebrates.  Both  peptic  and  tryptic  ferments 
are  often  secreted  by  the  same  gland.  The  reason  for  this 
and  its  physiological  consequences  are  unknown. 

In  the  Mollusca,  cesophageal  glands,  usually  called 
"  salivary,"  are  very  common,  and  often  large.  In  some 
cases,  as  in  Dolium  and  others,  these  glands  secrete  only 
mineral  acids  (sulphuric  in  Doliuni).  According  to  Bunge, 
these  acids,  like  the  hydrochloric  of  the  Vertebrate  stomach, 
have  chiefly  an  antiseptic  action,  destroying  Bacteria  intro- 
duced with  the  food.  If  this  be  correct,  the  advantage  of 
the  cesophageal  position  is  very  obvious.  The  true  digestive 
gland  of  Molluscs  is  the  "liver,"  which  is  usually  very  large, 
and  often  secretes  diastatic,  peptic,  and  tryptic  ferments. 
Its  secretion,  like  the  perivisceral  fluid,  is  always  neutral  or 
slightly  alkaline.  Peptic  digestion  may  be  rendered  possible 
(i)  by  the  presence  of  acid  derived  from  the  cesophageal 
glands,  or  (2)  by  the  acid  nature  of  the  food ;  but  nothing  is 
known  with  certainty.  In  the  Eolidae,  the  gut  gives  off 
prolongations  which  pass  upwards  into  the  dermal  papillae. 
Into  these  the  contents  of  the  alimentary  canal  pass,  and 
here  both  digestion  and  absorption  take  place.  They  thus 
become  filled  with  chyle,  which  is  directly  absorbed  by  the 
tissues. 

In  Ascidians,  Krukenberg  was  in  many  cases  quite  unable 


THE  PHYSIOLOGY  OF  NUTRITION.  741 

to  find  clear  indications  of  the  presence  of  ferments  in  the 
gut.  He  inclined  to  the  opinion  that  in  many  members  of 
the  class,  the  digestive  processes  are  as  simple  as  those  of 
the  Ccelentera. 

The  Nutrition   of  the  Tissues. 

After  the  complex  food  substances  have  been  broken 
down  into  simpler  ones,  they  must  be  carried  to  the  tissues, 
there  to  be  employed  in  repairing  waste,  or  in  growth.  In 
a  simple  Protozoon  there  is  no  difficulty ;  like  a  primitive 
community,  the  single  cell  supplies  its  own  wants,  and  the 
question  of  transport  is  never  raised.  In  a  Metazoon,  on 
the  other  hand,  as  in  a  civilised  state,  there  is  much 
division  of  labour,  and  the  question  of  the  transport  of 
manufactured  material  becomes  very  important. 

In  a  Vertebrate  the  blood  is  the  great  transporting 
agent ;  into  it  the  products  of  digestion  are  ultimately 
poured ;  from  it  waste  products  are  filtered.  It  is  itself, 
however,  confined  to  closed  vessels,  and  does  not  come 
into  close  connecton  with  the  tissues  ;  these  are,  strictly 
speaking,  nourished  by  the  lymph,  which  bathes  the  tissues 
throughout,  and  also  communicates  freely  with  the  blood 
stream.  Thus  the  lymph  is  the  "  middleman  "  between 
blood  and  tissues.  In  Vertebrates  the  lymph  has  not  the 
respiratory  significance  which  the  blood  has  in  virtue  of  its 
red  corpuscles. 

In  most  of  the  lower  aquatic  forms  of  life,  the  fluid 
within  the  body  differs  little  from  that  which  surrounds  it. 
Thus,  as  we  should  expect,  the  fluid  which  bathes  the  cavity 
of  a  sea  anemone  or  a  jellyfish,  filling  the  hollow  tentacles 
of  the  one  and  the  canal  system  of  the  other,  is  little  more 
than  sea  water.  It  contains  no  formed  elements,  no  dis- 
solved albumens,  no  organic  substances  capable  of  forming 
a  loose  combination  with  oxygen — that  is,  no  respiratory 
pigment.  It  is  thus  certainly  not  a  nutritive  fluid ;  the  tissues 
must  be  nourished  by  the  products  of  digestion  passing 
from  cell  to  cell.  It  is,  however,  of  use  in  respiration.  Like 
other  sea  water,  it  contains  dissolved  oxygen  ;  and  we  must 
suppose  that  the  endoderm  cells  take  up  the  oxygen  they 
require  directly  from  it,  as  the  ectoderm  cells  do  from  the 
surrounding  water.  The  fluid  has  also  an  excretory  signi- 


742  COMPARATIVE   PHYSIOLOGY. 

ficance;  it  carries  away  waste  products,  both  solid  and 
gaseous,  and  removes  these  from  the  body. 

The  fluids  of  Ascidians,  Lamellibranchs,  and  of  a  few 
Gasteropods,  are  all  classed  by  Krukenberg  as  hydro- 
lymph.  They  consist  largely  of  water,  but  contain  in 
addition  formed  elements,  or  dissolved  proteids.  In 
Ascidians,  the  body  fluid  contains  a  small  amount  of  dis- 
solved proteids,  and  some  pigmented  corpuscles.  Its  real 
function  has  not  been  fully  investigated ;  the  presence  of 
dissolved  proteids  seems  to  suggest  digestion  by  ferments 
in  spite  of  Kruken  berg's  negative  results. 

In  Echinoderms  we  find  that  both  a  perivisceral  fluid 
and  blood  enclosed  in  special  blood  vessels  are  present. 
Of  the  blood  little  or  nothing  is  known,  the  technical 
difficulties  in  the  way  of  isolation  being  very  great.  The 
perivisceral  fluid  contains  numerous  formed  elements,  and 
a  small  amount  of  dissolved  proteids.  It  probably  performs 
the  functions  of  the  lymph  of  Vertebrates,  but  is  said  to 
have  a  respiratory  function  in  addition. 

In  Insects  the  blood  is  of  the  nature  of  Vertebrate  lymph. 
It  is  very  rich  in  dissolved  proteids,  and  undoubtedly  serves 
for  the  nutrition  of  the  tissues.  It  has  no  respiratory 
function,  in  spite  of  the  frequent  occurrence  of  various 
pigments  in  it — a  point  of  some  theoretical  interest.  The 
tracheal  tubes  carry  air,  and  so  oxygen,  to  every  part  of  the 
body  ;  an  oxygen  carrying  fluid  formed  by  the  organism 
itself  thus  becomes  quite  unnecessary.  We  may,  physio- 
logically, compare  the  tracheal  system  of  the  Insect  with 
the  canal  system  of  the  Medusa.  In  both  cases  the  ex- 
ternal medium  is  carried  by  special  channels  to  the  tissues 
themselves ;  in  both  cases  the  body  fluids  have  in  conse- 
quence no  respiratory  significance. 

In  "  Worms,"  Crustaceans,  most  Gasteropods,  and 
Cephalopods,  the  blood  is  both  respiratory  and  nutritive. 
It  is  "  haemolymph,"  combining  the  functions  of  the  blood 
and  the  lymph  in  Vertebrates. 

In  Annelid  worms  the  blood  contains  small  formed 
elements,  and  a  number  of  respiratory  pigments,  some  of 
which  will  be  discussed  later. 

In  Cephalopods  the  blood  contains  formed  elements 
similar  to  leucocytes,  while  in  the  plasma  a  respiratory 


PRODUCTS   OF  METABOLISM.  743 

pigment  known  as  haemocyanin  is  dissolved.  This  con- 
sists of  a  proteid  substance  united  to  copper,  and  is  the  only 
albuminoid  present  in  the  plasma.  It  is  very  widely  spread 
among  Gasteropods,  Crustaceans,  &c.,  but  is  not  universal. 
Its  absence  in  some  Crabs,  which  have  apparently  no  com- 
pensating metal  containing  pigment,  perhaps  indicates  that 
too  much  stress  should  not  be  laid  upon  its.  respiratory 
significance.  Lipochrome  pigments  are  very  frequently 
present  in  the  blood  of  Crustaceans  and  Cephalopods ;  their 
use  is  unknown. 

If  we  compare  the  condition  seen  in  Cephalopods  with 
that  found  in  Vertebrates,  we  find  that  in  the  latter  it  is  the 
red  blood  corpuscles  which  are  the  oxygen  carriers,  while  in 
the  former  the  plasma  alone  subserves  respiration.  Even  in 
Vertebrates,  however,  the  waste  carbonic  acid  is  carried  in 
the  plasma  in  combination  with  its  soda,  so  that  the  plasma  is 
not  entirely  unconcerned  with  respiration.  In  both  Verte- 
brates and  Cephalopods  the  plasma  has  a  nutritive  function. 

PRODUCTS  OF  METABOLISM. 

In  the  course  of  those  processes  of  breaking  down  and 
building  up  of  protoplasm  which  constitute  what  is  called 
the  metabolism  of  the  animal,  we  constantly  find  that 
certain  by-products  are  formed.  These  may  be  simply 
waste  matters,  capable  of  subserving  no  useful  purpose  in 
the  animal  economy,  or  they  may  have  important  functions. 
As  we  ascend  in  the  scale  we  find  that  these  by-products  are 
more  and  more  utilized  for  different  purposes.  Thus  many 
pigments  which  are  widely  distributed  seem  to  be  practi- 
cally functionless,  but  in  particular  cases  they  come  to  be  of 
importance  in  producing  protective  coloration,  and  so  on. 
Among  the  products  of  metabolism,  we  will  discuss  here  only 
two  groups,  the  skeletal  tissues  and  the  colouring  matters. 

The  Skeletal  Tissues  of  Animals. 

Even  in  the  very  simplest  forms  of  life  we  find  that  the 
soft  protoplasm  is  frequently  provided  with  protective  struc- 
tures. In  many  cases  the  organism  merely  takes  up  inor- 
ganic particles  from  the  surrounding  medium,  and  with  these 
fashions  a  shell  for  itself,  as  we  find  in  some  of  the  Fora- 
minifera.  In  most  of  the  Foraminifera,  however,  a  true 


744  COMPARATIVE  PHYSIOLOGY. 

shell  of  lime  is  "  secreted  "  by  the  protoplasm.  This  taking 
up  of  inorganic  particles  is  not  the  only  way  in  which  the 
tendency  to  form  a  protective  covering  is  manifested  in  the 
Protozoa.  The  Corticata  are  encased  in  a  firm  sheath  which 
shows  many  of  the  characters  of  true  skeletal  substances ; 
while  familiar  organic  compounds  such  as  cellulose,  gelatine, 
and  horny  substances,  are  not  unknown.  Even  in  the 
Protozoa,  therefore,  we  see  in  germ  the  power,  so  charac- 
teristic of  higher  animals,  of  producing  by  modifications  of 
their  protoplasm,  specific  substances  capable  of  affording 
both  support  and  protection. 

Skeletal  tissues  are  usually  characterised  by  the  physical 
property  of  being  firm  and  often  hard  to  the  touch,  while 
generally  retaining  some  elasticity,  and  the  chemical  one  of 
offering  great  resistance  to  ordinary  chemical  agencies. 
They  are  naturally  passive  and  inert,  and,  so  far  as  the 
internal  skeleton  is  concerned,  are  formed  in  the  connective 
tissues,  and  not  in  relation  to  important  organs,  except  in 
pathological  conditions.  Lime  salts  are  frequently  associated 
with  some  of  the  common  skeletal  substances,  but  this  is  by 
no  means  universal  even  for  the  same  substance.  Thus  the 
collagen  of  the  bones  of  Vertebrates  is  associated  with 
abundant  lime  salts,  while  that  of  the  cartilages  contains  an 
inconsiderable  quantity.  Again,  chitin  in  the  Crustacea  is 
strongly  impregnated  with  lime,  while  in  Insecta  lime  salts 
are  practically  absent.  Within  the  limits  of  the  Cephalo- 
poda, the  conchiolin  of  the  "  shell "  may  be  associated  with 
lime  in  one  genus  and  quite  devoid  of  it  in  another. 
Within  the  Mollusca,  indeed,  we  find  every  stage  in  shell 
development  represented  ;  from  the  papery  "  shell "  of 
Aplysia  to  the  enormous  edifices  seen  in  some  of  the 
tropical  forms.  It  seems  difficult  in  these  cases  to  avoid 
the  conclusion  that  the  disproportionate  bulk  is  due  to 
necessities  of  growth,  and  has  no  relation  to  the  needs  of 
the  animal. 

The  following  is  a  brief  account  of  some  of  the  more 
important  skeletal  substances  : — 

Tunicin. 

Tunicin,  or  animal  cellulose,  is  a  carbohydrate  very 
similar  to,  if  not  identical  with,  the  cellulose  of  plants.  It 


SKELETAL   SUBSTANCES.  745 

occurs  in  the  test  of  Tunicates  as  a  cuticular  product  of  the 
epidermal  cells,  and  is  said  to  have  been  also  found  in  some 
cases  in  the  body  of  the  animal.  Dr.  Ambronn  asserts  that 
he  has  found  a  body  giving  similar  chemical  reactions  in 
connection  with  the  chitin  of  Arthropods,  and  also  in  some 
Molluscs. 

Chitin  and  Conchiolin. 

Chitin  and  Conchiolin  may  serve  as  examples  of  skeletal 
substances  containing  nitrogen,  but  giving  only  one  of  the 
proteid  reactions.  Several  other  well-known  substances  are 
included  in  this  group,  such  as  spongin,  byssus-substance, 
&c.  All  are  characterised  by  their  great  resistance  to 
chemical  agents. 

Chitin  is  characteristic  of  Arthropods,  but  also  occurs  in 
the  shell  of  Lingula  and  in  "cuttle-bone."  It  yields  on 
decomposition  reducing  substances  of  the  nature  of  sugar, 
and  is  a  derivative  of  a  carbohydrate.  It  is  a  product  of 
ectodermal  cells,  and  is  the  only  organic  skeletal  substance 
in  Arthropods.  In  the  Crustacea  it  is  usually  associated 
with  lime  salts  and  with  various  pigments. 

Conchiolin  is  found  in  Bivalves,  Gasteropods,  and  some 
Cephalopods.  It  strongly  resists  the  action  of  mineral 
acids,  and,  like  Chitin,  is  unaffected  by  ferments.  It  varies 
greatly  in  composition,  even  within  the  limits  of  a  species, 
and  is  probably  a  mixture  of  nearly  related  substances. 
The  substance  which  forms  the  horny  axis  in  Gorgonida 
and  Antipatharia  is  closely  allied  to  conchiolin. 

Collagen  and  Keratin. 

Collagen  and  Keratin  are  well-known  examples  of  skeletal 
substances  which  contain  sulphur  as  well  as  nitrogen,  and 
give  some,  though  not  all,  of  the  chemical  reactions  of 
proteids.  Collagen  is  found  in  the  bones  and  cartilages  of 
Vertebrates ;  it  is  characterised  by  yielding  gelatin  when 
boiled  with  water.  Unlike  the  substances  previously  men- 
tioned, it  is  readily  digested  by  pepsin,  but  is  not  affected 
by  tryptic  ferments.  In  Vertebrates  it  is  found  as  an  intra- 
cellular  matrix,  secreted  by  little  patches  of  formative  cells. 
In  Cephalopods  in  the  head  region  there  is  a  modified  form 
of  collagen  which  is  readily  acted  on  by  trypsin.  Collagen 


746  COMPARATIVE  PHYSIOLOGY. 

is  said  to  have  been  found  in  Sipunculus,  in  Holothurians, 
and  in  Brachiopods. 

The  dead  epidermal  cells  of  many  Vertebrates  form  a 
cuticle  of  keratin  over  the  living  cells  below.  The  process 
is  said  to  be  one  of  dehydration ;  but  it  is  not  a  simple 
drying  up  as  it  occurs  quite  as  markedly  in  aquatic  animals. 
In  the  hairs  and  nails  of  mammals,  the  feathers  of  birds, 
the  scales  of  fishes,  keratin  forms  a  protective  covering; 
in  some  mammals  it  further  furnishes  powerful  offen- 
sive "horns."  Keratin  is  also  found  in  the  egg  shells  of 
Birds,  Reptiles,  and  Selachians  ;  in  the  first  group  it  is 
associated  with  lime  salts.  It  also  occurs  in  the  sheath  of 
nerve  fibres,  which  is  explicable  enough  when  we  remember 
that  in  development  the  nerves  arise  from  the  ectoderm. 
Keratin  has  also  been  found  among  worms.  It  is  extremely 
resistant  to  the  action  of  ferments. 

The  Colouring  Matters  of  Animals. 

Colour  in  animals  is  either  due  directly  to  pigments,  or, 
as  in  the  case  of  structural  colours,  is  simply  a  light  effect. 
To  the  latter  division  belong  the  often  brilliant  colours  of 
some  Annelids,  and  the  gorgeous  metallic  tints  of  the 
plumage  of  some  birds.  In  this  section  we  confine  our- 
selves to  the  pigments. 

Physiologically,  we  may  classify  pigments  in  various  ways: 
there  are  the  respiratory  pigments,  of  which  Haemoglobin  is 
perhaps  the  best  example  ;  the  waste  products,  such  as  the 
pigments  of  some  butterflies'  wings  (which  are  allied  to  uric 
acid),  and  probably  the  pigments  of  bile ;  finally,  there  are 
numerous  pigments  of  whose  primary  physiological  meaning 
we  can  say  nothing,  but  which  may  be  secondarily  of  use 
in  producing  protective,  warning,  or  sexual  colouring.  Such 
are  the  pigments  of  the  skin  in  Crustacea,  caterpillars, 
Amphibians,  and  so  on. 

The  most  important  respiratory  pigments  are  Haemo- 
globin, Haemocyanin,  and  Haemerythrin ;  some  others  have 
been  named  by  different  authors,  but  their  respiratory 
significance  seems  uncertain. 

Hemoglobin  occurs  in  all  the  Craniate  Vertebrates,  and 
also  not  infrequently  among  the  different  Invertebrate 


PIGMENTS.  747 

classes,  usually  in  isolated  members  of  groups.  It  consists 
of  a  pigment,  Haematin,  united  to  a  proteid ;  the  pigment 
contains  iron  in  its  molecule.  In  the  higher  Vertebrates, 
Haemoglobin  is  during  life  continually  undergoing  decom- 
position. The  iron  is  mostly  retained  within  the  body, 
and  is  probably  re-utilised  in  metabolism  ;  the  proteid  is 
probably  also  utilised,  while  the  iron-free  Haematin  under- 
goes chemical  changes,  and  is  excreted  as  the  pigments  of 
bile  and  urine.  In  pathological  conditions  Haematin 
may  be  deposited  in  the  tissues  in  different  forms.  This 
deposition  of  pigments  derived  from  Haematin,  which  only 
occurs  in  disease  in  Vertebrates,  seems  to  occur  normally 
in  certain  Invertebrates,  in  the  shells  of  some  Gasteropods, 
the  skin  of  star  fishes,  &c.,  apparently  in  some  cases  in  forms 
in  which  Haematin  itself  does  not  occur.  With  regard  to 
the  distribution  of  Haemoglobin,  we  must  note  that  the 
occurrence  of  the  same  pigment  in  widely  separated  forms 
indicates  similar  physiological  processes,  but  not  necessarily 
a  similar  function.  Thus,  Haemoglobin  is  said  to  occur  in 
considerable  quantity  in  the  perivisceral  fluid  of  Holothurians, 
where  we  can  hardly  suppose  that  its  respiratory  importance 
is  very  well  marked.  In  fact,  the  wide  and  irregular  dis- 
tribution of  Haemoglobin  among  Invertebrata  forbids  the 
supposition  that  it  can  there  possess  the  supreme  importance 
which  it  has  in  higher  Vertebrates. 

The  efficiency  of  Haemoglobin  is  of  course  due  to 
its  power  of  forming  a  loose  combination  with  oxygen ; 
it  is,  however,  also  capable  of  uniting  with  other  gases, 
as  CO  and  CO2. 

Hamocyamn  is  found  in  many  Crustacea,  and  in  Molluscs. 
In  the  oxidised  state  it  is  a  colourless  substance,  but  turns 
blue  when  reduced.  It  is  absent  in  the  few  Crustaceans 
(Daphnia,  &c.),  which  contain  Haemoglobin,  and  is  a  true 
respiratory  pigment.  It  consists  of  a  proteid  united  to 
copper,  but  in  a  few  cases  it  is  said  that  the  copper  is  replaced 
by  iron.  There  is  said  to  be  more  difficulty  in  reducing 
Haemocyanin  than  there  is  with  Haemoglobin. 

Hcemerythrin  occurs  in  the  blood  of  Gephyreans ;  it 
undergoes  a  colour  change  dependent  on  processes  of 
oxidation  and  reduction. 

The  number  of  pigments  which  we  can  definitely  classify 


74§  COMPARATIVE  PHYSIOLOGY. 

as  respiratory,  or  as  waste  products  resulting  from  the 
decomposition  of  such,  is  very  small ;  in  the  great  majority 
of  cases  we  can  say  nothing  as  to  function.  In  some  cases, 
however,  we  can  point  to  the  physical  or  chemical  conditions 
which  favour  the  development  of  pigments.  Thus  in  some 
animals  the  pigments  indicate  the  normal  reaction  of  the 
tissues.  For  example,  those  sea  anemones  which  contain 
peptic  ferments  are  red,  those  which  contain  tryptic,  yellow 
or  brown.  Again,  light  and  absence  of  oxygen  are  neces- 
sary for  the  development  of  certain  of  the  black  pigments ; 
the  black  pigment  in  a  frog's  skin  disappears  in  an 
atmosphere  of  pure  oxygen.  It  is  a  fact  of  common  observa- 
tion that  portions  of  animal's  bodies  which  are  shaded  from 
the  light  tend  to  be  pale  in  colour.  Another  interesting 
point  about  colouring  matters  is  that  they  are  not  always 
produced  by  the  animal  in  which  they  occur.  Thus  green 
oysters  owe  their  colour  to  an  insoluble  pigment  taken  from 
the  diatoms  of  the  food,  and  deposited  in  the  tissues ; 
the  colour  of  "  red  mullet "  is  perhaps  due  to  the  crustaceans 
of  the  food.  It  has  been  suggested  that  the  uniformity  of 
tint  observed  in  many  animals  living  in  the  same  environ- 
ment, as,  for  example,  in  the  Sargasso  Sea,  may  be  due  to 
a  similar  cause. 

Most  of  the  pigments  fall  into  chemical  groups ;  of  these 
the  best  defined  and  perhaps  most  widely  spread  is  the 
Lipochrome  group.  The  Lipochromes  are  characterised  (i) 
by  their  colour,  which  varies  from  yellow  through  orange  to 
red ;  (2)  by  giving  in  the  dry  state  a  blue  coloration  with 
strong  H2SO4;  (3)  by  their  ready  decomposition  when 
exposed  to  light,  when  they  lose  their  colour  and  yield 
cholesterin ;  (4)  by  the  fact  that  they  consist  only  of 
carbon,  hydrogen,  and  oxygen.  They  occur  in  both  plants 
and  animals,  especially  in  the  "liver"  in  Arthropods  and 
Molluscs,  in  the  skin  and  body  fluids  of  Crustacea,  in  the 
skin  of  many  Vertebrates,  &c.  According  to  Krukenberg, 
the  lipochromes  probably  often  take  origin  from  fatty  bodies; 
if  this  is  correct,  their  association  with  the  "  liver "  in 
Invertebrates  is  interesting. 

The  above  is  a  very  brief  account  of  some  of  the  chief 
known  facts  of  animal  coloration.  It  may  serve  to  show 
that  much  must  be  discovered  as  to  the  primary  physiological 


COMPARATIVE  PATHOLOGY.  749 

import    of    pigments,    before   the   vexed   subject   of   their 
secondary  import  can  be  definitely  dealt  with. 

COMPARATIVE  PATHOLOGY. 

Within  recent  years  pathologists  have  begun  to  study 
diseased  conditions  comparatively — an  obviously  rational 
method  which  promises  to  lead  to  very  important  results, 
both  practical  and  theoretical.  For  man  has  no  monopoly 
of  disease,  and  some  of  the  processes  by  which  unhealthy 
conditions  are  dealt  with  by  the  organism  are  more  readily 
studied  in  lower  animals  than  in  him.  Of  this  we  shall  give 
one  illustration.  In  1862,  Haeckel  observed  that  grains  of 
indigo  injected  into  the  mollusc  Thetys  were  surrounded  by 
the  amoeboid  blood  corpuscles.  Other  observers  followed 
the  hint  which  this  suggestive  fact  supplied,  and  MetchnikorT, 
above  all  others,  has  shown  the  important  role  which  these 
amoeboid  cells  fill  in  waging  war  against  intruding  germs 
and  parasites,  in  surrounding  irritant  particles,  in  repairing 
injuries,  and  the  like.  In  fact,  MetchnikorT  has  worked  out 
the  evolution  of  the  phagocyte,  as  he  terms  the  amoeboid  cell 
whose  function  it  is  to  discharge  the  role  above  indicated. 
It  is  this  evolution,  as  stated  in  Metchnikoffs  lectures  on 
the  comparative  pathology  of  inflammation  (Trans.,  London, 
1893),  which  we  shall  take  in  illustration  of  comparative 
pathology. 

The  simplest  conditions  are  of  course  illustrated  by  the 
Protozoa.  These  enjoy  comparative  immunity  from  the 
injurious  effects  of  wounds  and  from  infectious  disease. 
For  injuries  are  very  rapidly  repaired ;  a  fragment,  if  nucle- 
ated, can  usually  regrow  the  whole ;  infecting  organisms  are 
in  most  cases  digested,  and  irritant  particles  are  got  rid  off. 
This  is  particularly  true  of  the  amoeboid  Protozoa,  the 
Rhizopods.  Sometimes,  moreover,  the  Bacteria  or  other 
micro-organisms  which  produce  disease  are  actually  avoided, 
for  some  of  the  Protozoa  exhibit  that  sensitiveness  (or 
chemiotaxis)  which  distinguishes  the  wandering  amoeboid 
cells  or  phagocytes  of  higher  animals.  Thus,  a  Myxomycete 
will  creep  towards  a  decoction  of  dead  leaves  and  away  from 
a  salt  solution,  and  will  "  prefer  "  a  nutritive  fluid  which  is 
not  swarming  with  Bacteria  to  one  that  is. 


750  COMPARATIVE  PHYSIOLOGY. 

In  Sponges,  infection  is  often  avoided  and  parasites  are 
excluded  by  the  closure  of  the  inhalent  pores.  But  if 
entrance  be  effected,  the  microbe  or  irritant  is  dealt  with  by 
the  amoeboid  cells  of  the  middle  stratum,  which  have  also 
to  do  with  ordinary  digestion.  Thus  disease  in  Sponges  is 
very  rare.  In  Hydra,  where  there  is  virtually  no  mesoglcea, 
the  flagellate  or  amoeboid  cells  lining  the  gut  act  as  so  many 
"stationary  phagocytes."  Thus,  in  these  two  cases,  the 
functions  of  intra-cellular  digestion  and  of  "  phagocytosis  " 
are  combined. 

In  other  Coelentera,  as  in  Hydra,  the  ordinary  digestive 
functions  are  restricted  to  the  endoderm  cells  lining  the  gut, 
but  most  of  them  have,  what  Hydra  has  not,  wandering 
amoeboid  cells  in  the  mesoglcea,  and  these  deal  with 
microbes,  parasites,  and  irritants.  The  same  is  true  of 
simple  worms,  such  as  Turbellarians. 

In  higher  wrorms  and  in  Echinoderms  the  phagocytic  cells 
are  usually  situated  on  the  peritoneal  epithelium,  or  float  in 
the  perivisceral  fluid.  They  may  have  many  functions, 
respiratory  and  excretory,  for  instance,  but  the  phagocytic 
function  is  of  great  importance,  all  the  more  so  that  the 
gut  has  now  lost  its  power  of  intra-cellular  digestion. 

Crustaceans,  insects,  molluscs  have  a  more  or  less  well- 
developed  blood  vascular  system,  and  there  are  often 
amoeboid  cells  in  the  blood  like  the  white  blood  corpuscles 
of  most  Vertebrates.  But  the  phagocytic  function  still 
depends,  largely  at  least,  on  wandering  phagocytes  in  the 
body  cavity  or  in  the  mesodermic  tissues.  But  as  the 
vascular  system  in  these  forms  is  usually  lacunar,  no  rigid 
distinction  can  be  drawn  between  phagocytes  in  the  blood 
and  phagocytes  in  the  body  cavity.  No  case  is  known, 
however,  in  which  the  leucocytes  or  white  blood  corpuscles 
of  an  Invertebrate  exhibit  the  power  of  migrating  through 
the  walls  of  the  blood  vessels  to  the  seat  of  irritation  or  in- 
jury, as  is  common  among  Vertebrates. 

Among  Vertebrates,  as  the  circulatory  system  becomes 
gradually  more  highly  developed  from  Tunicates  onwards, 
the  number  of  extra-vascular  phagocytes  is  reduced,  and 
more  and  more  depends  on  those  of  the  blood.  In  the  fin 
of  a  young  newt  an  injury  or  an  infection  may  be  dealt  with 
solely  by  the  migratory  phagocytes  of  the  connective  tissue ; 


COMPARATIVE  PATHOLOGY.  751 

in  the  most  frequently  observed  case — the  tail  of  a  tadpole 
in  which  the  blood  vessels  are  formed — the  extra-vascular 
phagocytes  are  greatly  aided  by  leucocytes,  which  work  their 
way  through  the  walls  of  the  vessels  or  are  liberated  by  a 
lesion ;  in  other  cases  all  may  depend  on  these  leucocytes. 
It  is  important  also  to  notice  that  the  endothelial  cells  of 
the  blood  vessels  seem  by  their  contractility  to  assist  the 
passage  (or  diapedesis)  of  the  leucocytes ;  sometimes,  more- 
over, they  may  themselves  leave  the  wall  of  the  vessel  to 
deal  with  Bacteria  introduced  into  the  blood. 

We  are  not  here  concerned  with  MetchnikofP s  thesis  that 
"  inflammation  generally  must  be  regarded  as  a  phagocytic 
reaction  on  the  part  of  the  organism  against  irritants — a 
reaction  carried  out  by  the  mobile  phagocytes  sometimes 
alone,  sometimes  with  the  aid  of  the  vascular  phagocytes  or 
of  the  nervous  system."  We  are  immediately  interested 
only  in  noticing  how  these  mobile  cells,  retaining  many  of 
the  qualities  of  the  ancestral  Amoebae,  perform  in  the  animal 
body  numerous  functions,  struggling  with  invading  Bacteria, 
surrounding  and  engulfing  irritant  particles,  and  repairing 
wounds.  And  from  the  most  general  point  of  view  it  is 
evident  that  one  of  the  numerous  factors  determining  the 
fate  of  an  organism  in  the  struggle  for  existence  is  its  power 
of  resisting  Bacteria.  If  phagocytes  be  not  present,  there 
must  be  some  other  means  of  defence ;  thus  the  Nematodes 
have  found  this  in  their  firm  resistent  cuticles. 

The  processes  of  disease  in  higher  animals  have  been  very 
carefully  investigated  from  the  evolutionist's  point  of  view 
by  Sutton.  He  points  out  that  some  of  the  causes  which 
pathologists  recognise  as  operating  to  produce  disease, 
(viz.,  hypertrophy  or  atrophy  of  organs  or  structures,  and 
coalescence  of  parts  originally  distinct),  are  also  "  factors  in 
evolution,"  which  biologists  recognise  in  their  theories  of 
the  progress  of  life.  Thus,  descending  to  particular  cases,  we 
find  that  the  long  claws  of  the  sloth  and  bat,  the  great 
curved  teeth  of  the  Babimssa,  are  paralleled  in  pathological 
conditions  by  the  elongated  nails  and  hoofs  of  Birds  and 
Ungulates  kept  in  unnatural  conditions,  by  the  curved 
incisors  of  Rodents  which  have  lost  the  corresponding  teeth 
of  the  other  jaw.  It  is  unnecessary  here  to  multiply  examples 
of  greatly  hypertrophied  organs,  normally  present  in  certain 


752  COMPARATIVE  PHYSIOLOGY. 

animals,  but  occurring  in  disease  in  others  ;  many  will  occur 
on  reflection.  In  considering  many  of  these  cases,  we 
must  recognise  the  law  of  correlation,  and  realise  that  the 
structures  of  a  particular  animal  are  not  commonly  the  best 
conceivable,  but  the  best  that  can  be  attained  under  the 
given  conditions. 

Pathological  new  formations  may  arise  in  response  to 
mechanical  stimulation,  as  in  the  case  of  corns  and  warts, 
or  may  be  due  to  aberrant  physiological  processes.  Thus, 
cancer  is  regarded  by  many  as  in  origin  an  aberrant  gland- 
formation,  and  only  occurs  in  regions  of  the  body  where 
glands  are  normally  found.  It  is  a  senile  modification  of 
an  ordinary  developmental  process.  Pathological  bony 
growths  seem  to  have  their  origin  in  patches  of  cartilage 
remaining  from  the  primitive  cartilage  of  limb  or  brain  case, 
and  so  are  continuations  of  the  ordinary  process  by  which 
cartilage  is  replaced  by  bone. 

Rudimentary  or  imperfectly  formed  organs  are  in  general 
specially  liable  to  disease.  Such  rudimentary  organs  are 
usually  relics  of  past  history.  We  must  thus  recognise  that, 
just  as  in  the  history  of  civilisation  some  of  the  most  cruel 
wrongs  are  only  good  institutions  belated,  so  in  the  history  of 
disease  the  most  dire  pathological  conditions  may  be  histori- 
cally only  the  result  of  belated  physiological  processes.  In 
some  cases  we  may  perhaps  say  more  hopefully  that  patho- 
logical processes  may  be  the  starting  point  for  new  physio- 
logical evolutions. 

Brief  as  the  above  comparative  survey  of  Physiology  and 
Pathology  is,  it  may  serve  to  give  the  student  some  impression 
of  the  intricacy  of  life,  and  act  as  a  relief  from  mechanical 
theories  of  Variation,  Selection,  and  Heredity.  It  is  an 
attempt  to  look  from  the  inner  side  upon  the  great  problem 
which  is  constantly  being  worked  out  before  us : — given 
the  potentialities  of  protoplasm  and  certain  chemical  and 
physical  conditions,  to  find  the  best  adaptation  to  a  given 
environment. 


CHAPTER    XXVIII. 

GEOGRAPHICAL     DISTRIBUTION     OF     ANIMALS. 

As  similar  animals  tend  to  occur  where  the  conditions  of 
life  are  similar,  we  are  warranted  in  speaking  of  a  pelagic 
fauna,  an  abyssal  fauna,  a  littoral  fauna,  and  so  on.  Let  us 
briefly  consider  this  grouping  of  animals  according  to  their 
haunts. 

The  Pelagic  Fauna. 

The  pelagic  fauna  includes  all  the  animals  of  the  open 
sea,  both  drifters  (Plankton)  and  swimmers  (Nekton).  The 
physical  conditions  in  which  they  live  are  very  favourable: — 
there  is  room  for  all,  sunshine  without  risk  of  drought,  and 
an  evener  life  throughout  the  day  and  throughout  the  year 
than  is  to  be  found  elsewhere  except  in  the  abysses  of  the 
deep  sea.  Moreover,  the  minute  pelagic  Algae  afford  an 
inexhaustible  food  supply  to  the  animals.  It  is  not  sur- 
prising, therefore,  to  find  that  the  open  sea  has  been  peopled 
from  the  earliest  times  of  which  the  rocks  give  us  any  life 
record. 

The  fauna  is  representative,  exhibiting  great  variety  of 
types,  from  the  minute  Noctiluca  which  sets  the  waves  aflame 
in  the  short  summer  darkness,  to  the  giants  of  modern  times 
— the  whales.  It  includes  a  few  genera  of  Foraminifera,  rich 
in  species,  all  the  Radiolarians,  many  Infusorians,  Medusae 
and  Medusoids,  Siphonophora  and  Ctenophora,  many 
"  worm  "  types  and  a  Holothurian,  a  legion  of  Crustaceans 
and  a  few  Insects  (Halobatidae),  such  Molluscs  as  Pteropods, 
Heteropods,  and  many  of  the  Cephalopods,  such  Tunicates 
as  Salpa  and  Pyrosoma,  many  fishes,  a  few  turtles  and 
snakes,  besides  some  well-known  birds  and  mammals. 

48 


754     GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS. 

The  fauna  of  the  open  sea  is  representative,  but  there  are 
few  of  the  types  which  we  can  suppose  to  have  lived  there 
always.  It  may  be  that  forms  like  the  minute  water-fleas 
have  been  there  almost  from  the  first,  but  most  bear  the 
impress  of  lessons  which  the  open  sea  could  never  have 
taught  them. 

Pelagic  animals  tend  to  be  delicate  and  translu- 
cent ;  many  are  phosphorescent.  The  number  of  species, 
differing  from  one  another  within  a  relatively  narrow 
range,  is  often  enormous,  thus  about  5000  species  of 
Radiolarians  are  known.  The  huge  number  of  individuals, 
which  frequently  occur  in  great  swarms,  is  equally  character- 
istic. Perhaps  both  facts  indicate  that  the  conditions  of  life 
are  relatively  easy,  as  is  also  implied  in  the  limitless  food 
supply  afforded  by  the  unicellular  Algae. 

The  Abyssal  Fauna. 

Through  the  researches  of  the  Challenger  and  similar 
expeditions,  we  know  that  there  is  practically  no  depth-limit 
to  the  distribution  of  animal  life,  though  the  population  is 
denser  at  moderate  depths  than  in  the  deepest  abysses,  and 
though  there  is  probably  a  thinly  peopled  intermediate  zone 
between  the  light-limit  and  the  greatest  depths.  We  know, 
too,  that  there  are  representatives  of  most  types  from 
Protozoa  to  Fishes,  though  Sponges  and  Echinoderms 
preponderate,  and  that  the  distribution  tends  to  be 
cosmopolitan,  in  correspondence  with  the  uniformity  of  the 
physical  conditions. 

The  abyssal  fauna  includes  many  flinty  sponges,  some 
corals  and  sea-anemones,  possibly  a  few  medusae,  annelids 
and  other  "  worms  "  on  the  so-called  red  clay,  representatives 
of  the  five  extant  orders  of  Echinoderms,  abundant  Crusta- 
ceans, representatives  of  most  of  the  Mollusc  types,  and 
peculiarly  modified  fishes,  many  more  than  half-blind,  others 
catching  with  darkness-eyes  the  fitful  gleams  of  phos- 
phorescence. 

As  to  the  physical  conditions,  the  deep-sea  world  is  in 
darkness,  for  a  photographic  plate  is  not  influenced  below 
250-500  fathoms;  it  is  extremely  cold,  about  34°  F.,  for 
the  sun's  heat  is  virtually  lost  at  about  150  fathoms;  the 


THE  LITTORAL  FAUNA.  755 

pressure  is  enormous,  thus  at  2500  fathoms  it  is  about  2\ 
tons  per  square  inch ;  the  cold  water  in  sinking  brings  down 
much  oxygen ;  it  is  quite  calm,  for  even  the  greatest  storms 
are  relatively  shallow  in  their  influence ;  there  are  no  plants 
(except  perhaps  the  resting  phases  of  some  Algae),  for 
typical  vegetable  life  depends  upon  light,  and  not  even 
Bacteria,  otherwise  almost  omnipresent,  are  known  to 
flourish  in  the  great  depths.  A  strange,  silent,  cold,  dark, 
plantless  world  !  The  animals  feed  upon  one  another  and 
upon  the  debris  which  sinks  from  above,  including  the  rain 
of  pelagic  Protozoa,  whose  continual  dying  seems  rather  to 
contradict  Weismann's  doctrine  of  their  immortality. 

We  do  not  clearly  know  when  the  colonising  of  the  depths  began, 
but  there  is  much  to  be  said  for  the  view  that  an  abyssal  fauna  was,  at 
most,  scanty  before  Cretaceous  ages.  One  of  the  arguments  is  as 
follows  : — In  ancient  days,  when  warmth-loving  plants  flourished  in  the 
far  north,  when  there  was  no  ice-bound  polar  sea,  the  abyssal  water 
cannot  have  been  so  cold  as  it  is  now,  it  would  therefore  contain  less 
abundant  oxygen,  and  this  scantiness  would  make  life  more  difficult. 
But  whenever  the  peopling  of  the  abysses  occurred,  it  must  have  been 
gradual.  It  is  likely  that  most  of  the  pioneers  migrated  outwards  and 
downwards  from  the  shore  region  (in  a  wide  sense),  following  the  drift 
of  food  ;  it  is  possible  that  others,  e.g.,  some  Crustaceans,  sank  from  the 
surface  of  the  open  sea.  The  boreal  character  of  many  deep-sea  animals 
has  been  often  remarked,  and  it  is  plausible  to  suppose  that  there  was  a 
particularly  abundant  colonisation  in  the  Polar  regions,  and  a  gradual 
spreading  towards  the  Equator  as  the  Poles  became  colder.  Perhaps 
the  richness  of  the  fauna  at  the  Equator  may  be  thought  of  as  in  part 
due  to  the  meeting  of  two  great  waves  of  life  from  the  Poles. 

The  abyssal  conditions  of  life  tend  to  uniformity  over 
vast  areas,  just  as  in  the  open  sea.  But,  on  the  whole,  life 
must  always  have  been  harder  in  the  depths  than  on  the 
surface.  The  absence  of  plants,  for  instance,  involves  a 
keener  struggle  for  existence  among  animals.  Thus, 
although  many  abyssal  forms,  e.g.,  sea  anemones,  live  a 
passive  sedentary  life,  waiting  for  food  to  drop  into  their 
mouths,  the  majority  are  less  easy-going.  The  deep-sea 
has  been  a  sterner  school  of  life  than  the  surface. 


The  Littoral  Fauna. 

At  a  very  early  date  the  shores  were  peopled,  and  the 
fauna  is  very  rich  and  representative.      From  the  strictly 


756     GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS. 

Littoral  zone,  exposed  at  low  tide,  with  its  acorn  shells  and 
periwinkles,  limpets  and  cockles,  to  the  Laminarian  zone 
(to  15  fathoms)  with  its  sea  slugs  and  oysters,  where  the 
great  seaweeds  wave  listlessly  amid  an  extraordinary  keen 
battle,  to  the  Coralline  zone  (15-40  fathoms),  with  its  carni- 
vorous buckies,  what  variety  and  abundance,  what  crowd- 
ing and  struggle  ! 

There  are  Infusorians  and  Foraminifera,  sponges  horny, 
flinty,  and  limy,  zoophytes  and  sea-anemones,  a  mob  of 
worms,  star-fishes  and  sea-urchins,  crabs  and  shrimps, 
acorn  shells  on  the  rocks  and  sandhoppers  among  the 
jetsam,  a  few  insects  about  high-tide  mark,  sea-spiders 
clambering  on  the  seaweeds,  abundant  bivalves  and  gastero- 
pods,  sea-squirts  in  their  degeneracy,  besides  fishes,  a  few 
reptiles,  numerous  shore  birds,  and  an  occasional  mammal. 
The  shore  fauna  is  thus  very  representative,  rivalling  in  its 
range  that  of  the  open  sea,  far  exceeding  that  of  the  abysses. 

The  conditions  of  life  on  the  shore  are  in  some  ways  the 
most  stimulating  in  the  world.  It  is  the  meeting  place  of 
air,  water,  and  land.  Vicissitudes  are  not  exceptional,  but 
normal.  Ebb  and  flow  of  tides,  fresh-water  floods  and  desic- 
cation under  a  hot  sun,  the  alternation  of  day  and  night  felt 
much  more  markedly  than  on  the  open  sea,  the  endless 
variations  between  gently  lapping  waves  and  blasting 
breakers,  the  slow  changes  of  subsidence  or  elevation, — 
these  are  some  of  the  vicissitudes  to  which  shore  animals 
are  exposed.  The  shore  is  rich  in  illustrations  of  keen 
struggle  for  existence  and  of  life-saving  shifts  or  adaptations, 
such  as  masking,  protective  coloration,  surrender  of  parts, 
and  "  death  feigning."  We  may  think  of  it  as  a  great  school 
where  many  of  the  great  lessons  of  life,  such  as  moving  head 
foremost,  were  learned. 

The  Fresh  Water  Fauna. 

Perhaps  the  most  striking  fact  in  regard  to  the  animals 
which  live  in  fresh  water  is  their  uniformity.  The  number 
of  individuals  in  a  lake  is  often  immense,  but  the  number 
of  species  is  relatively  small,  the  number  of  types  still 
smaller.  In  widely  separated  basins  and  in  different 
countries  the  same  forms  occur. 


MINOR  FAUNAS.  757 

We  may  distinguish  a  littoral,  a  surface,  and  a  deep- 
water  lacustrine  fauna.  The  deep-water  forms  are  chiefly 
Rhizopods,  Turbellarians,  Nematodes,  Leeches,  Chaetopods, 
Amphipods,  Isopods,  Entomostraca,  a  few  Arachnids,  some 
insect  larvae,  and  molluscs,  and  the  general  opinion  is  that 
these  are  derivable  from  the  shore-fauna  of  the  lake,  which 
includes  similar  forms,  along  with  a  few  others,  such  as  the 
fresh-water  sponge  and  Hydra.  On  the  other  hand,  the 
surface  lacustrine  fauna,  consisting  of  water-fleas,  Rotifers, 
Infusorians,  &c.,  widely  and  uniformly  distributed,  is  said 
not  to  be  derivable  from  the  shore  forms.  In  trans- 
parency, in  gregariousness,  in  nocturnal  habit,  and  in  other 
ways  they  present  a  marked  analogy  with  the  marine 
Plankton.  How  are  we  to  account  for  their  origin  and 
wide  distribution  ? 

(i.)  To  explain  the  uniformity  Darwin  referred  to  the  birds  which 
carry  organisms  from  watershed  to  watershed,  to  the  carrying  power  of 
the  wind,  and  to  changes  of  land  level  which  bring  different  river  beds 
into  communication.  But  this  is  not  enough. 

(2.)  It  seems  very  likely  that  some  of  the  fresh  water  forms  have 

migrated  from  the  sea  and  seashore  through  brackish  water  to  rivers 

and  lakes.     As  the  possibility  of  making  the  transition  depends  on  the 

constitution  of  the  animal,  it  is  intelligible  that  similar  forms  should 

,  succeed  in  different  areas. 

(3.)  There  seems  much  force  in  what  Credner  and  Sollas  emphasise 
that  many  lakes  are  dwindling  relict-seas  of  ancient  origin.  Granted  a 
fairly  uniform  pelagic  fauna,  e.g.,  before  Cretaceous  times,  we  can 
understand  that  the  conversion  of  land-locked  seas  into  lakes  would 
imply  a  decimating  elimination,  and,  as  the  conditions  of  elimination 
would  be  much  the  same  everywhere,  the  result  would  be  uniformity 
in  the  survivors. 

Minor  Faunas. 

(a. )  Of  Brackish  Water.  —  We  are  warranted  in  speaking  of  a 
brackish-water  fauna,  because  of  its  uniformity  in  widely  separated 
regions.  It  does  not  seem  to  be  a  mere  physiological  assemblage,  vary- 
ing in  each  locality,  but  rather  a  transition  fauna  of  ancient  date,  a  relic 
of  a  littoral  fauna  once  more  uniform.  The  fact  is  that  the  power  to 
live  in  brackish  water  is  not  very  common ;  it  runs  in  families. 

(b. )  Cave  fauna. — In  America,  thanks  very  largely  to  the  labours  of 
Packard,  about  100  cave  animals  are  known  ;  in  Europe  the  number 
is  about  300,  the  increase  being  largely  due  to  the  occurrence  of  about 
100  species  of  two  genera  of  beetles  in  European  caves.  In  the  famous 
Mammoth  Cave  of  Kentucky,  which  has  over  100  miles  of  passages, 
with  streams,  pools,  and  dry  ground,  there  are  over  40  different  species 


758     GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS. 

of  animals.  The  temperature  is  very  equable,  varying  little  more  than 
a  degree  throughout  the  year  ;  it  is,  of  course,  dark  ;  and  there  are  no 
plants  other  than  a  few  Fungi.  Thus  the  conditions  present  some 
analogy  with  those  of  the  deep  sea.  The  fauna  is  of  much  interest  to 
the  evolutionists,  for  we  wonder  how  far  the  peculiarities  of  the  cave- 
animals,  e.g.,  absence  of  coloration  and  frequent  blindness,  are  due  to 
the  cumulative  effect  of  the  environment  and  disuse,  or  how  far  they 
represent  the  survival  of  fortuitous  variations,  and  the  result  of  the 
cessation  of  natural  selection  along  certain  lines.  Have  the  seeing 
animals  found  their  way  out,  leaving  only  the  blind  sports,  which  crop 
up  even  in  daylight  ?  or  is  the  loss  of  eyes  the  result  of  disuse  and 
absence  of  stimulus  ?  Or  again,  if  it  be  granted  that  pigment  is  an 
organic  constitutional  necessity,  e.g.,  a  waste-product,  while  coloration 
is  explicable  as  an  adaptation  wrought  out  in  the  course  of  natural 
elimination,  then  the  question  arises  whether  the  cessation  of  natural 
selection — a  condition  awkwardly  called  "panmixia" — which  might 
account  for  the  disappearance  of  the  coloration  when  there  is  no 
premium  set  upon  it,  can  also  account  for  the  loss  of  pigment,  that  is  of 
a  character  which  was  not  acquired  in  the  course  of  natural  selection  ? 
(See  Beddard's  Animal  Coloration}.  Our  only  answer  at  present  is 
that  there  is  need  for  experiment. 

(c.)  Parasitic  fauna. — It  seems  legitimate  to  rank  together  those 
animals  whose  habitat  is  in  or  on  other  organisms,  from  which  they 
derive  subsistence,  without  in  most  cases  killing  them  quickly,  if  at  all, 
nor  on  the  other  hand,  rendering  them  any  service.  Among  ectopara- 
sites, there  are  such  forms  as  fish  lice  and  many  other  Crustaceans, 
numerous  insects  such  as  lice  and  fleas,  and  Arachnids  such  as  mites. 
Among  endoparasites,  there  are  Gregarines,  some  Mesozoa,  many 
Nematodes,  most  Trematodes,  all  the  Cestodes,  many  Crustaceans, 
insect  larvoe,  and  Arachnids. 

The  parasitic  habit  is  a  common  one,  illustrated  by  many  different 
types.  It  is  associated  with  degeneration,  varying  according  to  the 
degree  of  dependence,  with  great  nutritive  security  and  prolific  repro- 
duction, but  with  enormous  hazards  in  the  fulfilment  of  the  life  history. 

Parasitic  animals  must  be  distinguished  (a)  from  epiphytic  or  epizoic 
animals  which  live  attached  to  plants  or  animals,  but  are  in  no  way 
dependent  upon  them,  e.g.,  barnacle  on  Norway  lobster  ;  (I)}  from  com- 
mensals (p.  1 60),  who  live  in  some  degree  of  partnership,  but  without  in 
any  way  preying  upon  one  another,  e.g.,  crab  and  sea  anemone  ;  and  (c) 
from  symbions,  who  live  in  close  partnership,  or  symbiosis  (p.  114),  e.g., 
Radiolarians  and  Algae.  But  between  these  habits  there  are  many  grada- 
tions, and  from  close  association  there  is  always  an  easy  transition  to 
parasitism. 

Terrestrial  Fauna. 

The  colonising  of  dry  land  has  doubtless  been  a  gradual 
process,  as  different  types  wandered  inland  from  the  shore, 
or  became  able  to  survive  the  drying  up  of  fresh  water  basins. 
The  fauna  includes  some  Protozoa,  e.g..  Amoeba  terricola, 


THE  AERIAL  FAUNA.  759 

which  lives  in  moist  earth,  some  of  the  Planarians,  Nema- 
todes,  Leeches,  Chaetopods,  and  other  "worms,"  a  few 
Crustaceans  like  the  wood  lice  (Oniscus),  many  insects  and 
Arachnids,  a  legion  of  slugs  and  snails,  most  adult 
Amphibians,  most  Reptiles,  many  Birds,  and  most  Mam- 
mals. Among  Vertebrates  certain  fishes  are  of  interest  in 
having  learned  to  gulp  mouthfuls  of  air  at  the  surface  of 
the  water,  to  clamber  on  the  roots  of  the  mangrove  trees, 
or  to  lie  dormant  through  seasons  of  drought.  But  among 
Vertebrates,  Amphibians  were  the  first  successfully  to 
make  the  transition  from  water  to  dry  land. 

It  is  important  to  bear  in  mind  that  many  a  stock  may,  in  the 
course  of  its  evolution,  have  passed  through  a  variety  of  environments. 
Thus  the  thoroughly  aquatic  Cetaceans  were  probably  derived  from  a 
land  stock  common  to  them  and  to  the  Ungulates,  and  may  have  passed 
through  a  fresh  water  stage.  Without  going  further  back,  we  have  here 
an  illustration  of  the  zigzag  course  of  evolution. 

We  cannot  believe  in  any  abrupt  transition  from  the  shore  to  terra 
fir  ma.  It  has  been  a  slow  ascent,  slow  as  the  origin  of  dry  land 
itself.  Thus,  mud-inhabiting  worms,  dwellers  in  damp  humus,  bank- 
frequenting  animals,  those  which  find  a  safe  retreat  in  rottenness  or 
within  bolder  forms,  dot  the  path  from  the  shore  inland.  Many  have 
lingered  by  the  way,  many  have  diverged  into  cul-de-sacs,  many  have 
been  content  to  keep  within  hearing  of  the  sea's  lullaby,  which  soothed 
them  in  their  cradles. 

Simroth,  in  his  work  on  the  origin  of  land  animals,  seeks  to  show 
that  hard  skins,  cross-striped  muscle,  brains  worthy  of  the  name,  red 
blood,  and  so  on,  were  acquired  as  the  transition  to  terrestrial  life 
was  effected.  Let  us  take  the  last  point  by  way  of  illustration.  Iron 
in  some  form  seems  essential  to  the  making  of  haemoglobin,  but  iron 
compounds  are  relatively  scarce  and  not  readily  available  in  the  sea, 
they  are  more  abundant  in  fresh  water,  and  yet  more  so  as  the  land  is 
reached.  Therefore  it  is  suggested  that  it  was  as  littoral  animals 
forsook  the  shore  for  the  land,  via  fresh  water  paths,  that  iron,  in  some 
form,  entered  into  their  composition,  became  part  and  parcel  of  them, 
helped  to  form  haemoglobin  or  some  analogous  pigment,  and  thus  opened 
the  way  to  a  higher  and  more  vigorous  life. 


The  Aerial  Fauna. 

The  last  region  to  be  conquered  was  the  air.  Insects 
were  the  first  to  possess  it,  but  it  was  long  before  they  were 
followed.  The  flying  fishes  flapped  their  fore-fins  above 
the  foam  as  they  leapt ;  the  web-footed  tree  frogs,  Draco 
volans,  with  skin  spread  out  on  elongated  ribs,  and  various 


760     GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS. 

lizards  began  to  swoop  from  branch  to  branch ;  some  of  the 
ancient  Saurians  flopped  their  leathery  skin-wings ;  a  few 
arboreal  mammals  essayed  what  the  bats  perfected ;  and 
the  feverish  birds  flew  aloft  gladly. 

Perhaps  a  keen  struggle  among  insects,  or  such  events  as  floods, 
storms,  and  lava-flows  would  prompt  to  flight,  perhaps  it  was  the 
eager  males  who  led  the  way,  perhaps  the  additional  respiratory 
efficiency,  produced  by  the  outgrowth  of  wings,  gave  these  a  new  use. 
Perhaps  the  high  temperature  of  birds — an  index  to  the  intensity  of 
their  metabolism — may  have  had  to  do  with  the  development  of  those 
most  elaborate  epidermic  growths  which  we  call  feathers.  But  we  must 
still  be  resigned  to  a  more  or  less  ingenious  "  perhaps." 


The  Relation  of  the  different  Faunas  to  one  another. 

As  we  have  already  hinted,  the  problem  of  the  evolution 
of  faunas  is  still  beyond  solution,  and  as  this  is  not  the 
place  for  the  marshalling  of  arguments,  I  shall  content 
myself  with  stating  various  possibilities. 

(a.)  According  to  Moseley,  "The  fauna  of  the  coast  has  not  only 
given  origin  to  the  terrestrial  and  fresh-water  faunas,  it  has  throughout 
all  time,  since  life  originated,  given  additions  to  the  pelagic  fauna  in 
return  for  having  received  from  it  its  starting  point.  It  has  also  received 
some  of  these  pelagic  forms  back  again  to  assume  a  fresh  littoral 
existence.  The  terrestrial  fauna  has  returned  some  forms  to  the  shores, 
such  as  certain  shore-birds,  seals,  and  the  polar  bear;  and  some  of 
these,  such  as  the  whales  and  a  small  oceanic  insect,  Halobates>  have 
returned  thence  to  pelagic  life." 

"  The  deep-sea  has  probably  been  formed  almost  entirely  from  the 
littoral,  not  in  the  most  remote  antiquity,  but  only  after  food,  derived 
from  the  debris  of  the  littoral  and  terrestrial  faunas  and  floras,  became 
abundant  in  deep  water." 

"It  was  in  the  littoral  region  that  all  the  primary  branches  of  the 
zoological  family  tree  were  formed  ;  all  terrestrial  and  deep-sea  forms 
have  passed  through  a  littoral  phase,  and  amongst  the  representatives 
of  the  littoral  fauna  the  recapitulative  history,  in  the  form  of  series  of 
larval  conditions,  is  most  completely  retained." 

(b. )  According  to  Agassiz,  Simroth,  and  others,  if  one  may  venture  to 
compress  their  views  into  a  sentence,  a  littoral  fauna  was  the  original 
one,  whence  have  been  derived,  on  the  one  hand,  the  pelagic  and 
abyssal  faunas ;  on  the  other  hand,  the  fresh-water  and  terrestrial 
faunas. 

(e.)  According  to  Brooks,  a  pelagic  fauna  was  primitive,  whence 
have  been  derived  the  tenants  of  the  shore  and  the  inhabitants  of  the 
deep  sea.  To  the  latter,  however,  a  possibility  of  ascending  again  is  not 
denied. 


PROBLEMS  OF  GEOGRAPHICAL  DISTRIBUTION.       761 

(d. )  Personally,  I  regard  the  most  probable  ancestral  home  of 
animals  as  some  region  not  far  from  the  shore,  and  I  picture  the 
relations  as  follows  : — 


Dry  Land 


Original 
Home? 


The  more  detailed  Problems  of  Geographical  Distribution. 

Leaving  the  general,  and  at  present  very4  obscure,  problem 
of  the  evolution  of  faunas,  let  us  briefly  notice  some  of  the 
more  detailed  questions  of  distribution.  We  shall  content 
ourselves  with  stating  (i)  a  few  of  the  outstanding  facts,  (2) 
the  factors  determining  why  some  animals  are  here  and 
others  there,  and  (3)  the  usually  recognised  zoo-geographical 
regions. 

Some  of  the  Outstanding  Facts  of  Geographical  Distribution. 

(a.)  Widely  separated  countries  may  have  an  essentially 
similar  fauna.  Thus,  there  is  much  in  common  between 
Britain  and  Northern  Japan,  and  there  is  so  much  agree- 
ment between  the  North  European  (Palaearctic)  and  the 
North  American  (Nearctic)  fauna  that  many  unite  the  two 
regions  in  one  (Holarctic). 


762       GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS. 

(b.)  Closely  adjacent  countries  may  have  quite  different 
faunas.  Thus,  the  Bahamas  and  Florida,  Australia  and 
New  Zealand  are  peopled  by  very  different  animals.  But 
the  best  illustration  is  that  of  two  little  islands,  Bali  and 
Lombok,  in  the  Malay  Archipelago,  which  are  separated  by 
"  Wallace's  Line,"  a  strait  only  fifteen  miles  wide  at  its 
narrowest  part.  They  differ  from  each  other  in  their  birds 
and  quadrupeds  far  more  widely  than  Britain  and  Japan. 

(c.)  Regions  with  very  different  faunas  are  in  many  cases 
connected  by  transition  areas.  Thus  a  journey  from  the 
North  of  Canada  to  Brazil  would  show  a  fairly  gradual 
transition  from  an  Arctic  to  a  tropical  fauna. 

(d.)  At  the  same  time  there  are  regions  whose  fauna  is 
exceedingly  distinctive  and  sharply  defined.  Thus  the 
Mammalian  fauna  of  Australia  is  distinctively  Marsupial, 
and  nowadays  there  is  only  one  family  of  Marsupials — the 
American  opossums — found  beyond  the  Australasian  limits. 

(e.)  Another  striking  fact  is  the  "  discontinuous  dis- 
tribution "  of  certain  types,  by  which  we  mean  that  examples 
of  a  type  may  occur  in  widely  separated  regions  without 
there  being  any  representatives  in  the  intermediate  area. 
The  general  explanation  is  that  the  type  in  question  once 
enjoyed  a  wide  distribution,  as  the  rock  record  shows,  and 
that  the  conditions  favourable  to  survival  have  been  found 
in  widely  separated  places.  Thus,  of  the  genus  Tapir, 
there  are  some  four  species  in  South  and  Central  America, 
while  the  only  other  species  occurs  in  Malacca  and  Borneo. 
Similarly  the  Camelidae  are  represented  by  one  genus  in 
the  Old  World  and  another  in  South  America,  and  the 
insectivorous  Centetidae  are  represented  by  five  genera  in 
Madagascar,  and  one  in  Cuba  and  Hayti. 

The  Factors  determining  Distribution. 

There  are  six  factors  which  combine  to  determine  the  particular  distri- 
bution of  an  animal.  These  may  be  conveniently  considered  in  pairs. 

(a)  Distribution   is  in  part  determined   by  the  constitution   of  the 
animal   and    the   physical    conditions   of    the     region.       Thus   snakes 
dimmish  rapidly  in  numbers  towards  the  poles,  their  constitution  being 
in  most  cases  ill-adapted  to  withstand  cold  ;  thus  crayfishes  are  absent 
from  districts  where  the  fresh  water  does  not  contain  sufficient  lime  salts 
for  their  needs. 

(b)  Distribution  is  in  part  determined  by  the  position  of  the  animal's 
original  home  (which  is  often  an  unknown  fact),  and  by  the  available 


ZOO-GEOGRAPHICAL   REGIONS.  763 

means  of  dispersal.  Thus,  so  far  as  we  know,  the  Old  World  has  been 
the  exclusive  home  of  the  anthropoid  apes,  and  there  they  have 
remained  ;  thus  bats,  being  able  to  fly,  have  a  more  cosmopolitan 
distribution  than  most  other  mammals  ;  thus  amphibians,  being  unable 
to  withstand  salt  water,  are  absent  from  almost  all  oceanic  islands. 

(c]  Distribution  is  in  part  determined  by  the  actual  changes  (geological, 
climatic,  &c.)  which  have  affected  different  regions,  and  by  "bionomic" 
factors,  i.e.,  the  relations  between  the  animal  in  question  and  other 
organisms,  whether  animals,  plants,  or  man.  Thus  it  is  plain  that  we 
cannot  understand  the  fauna  of  Australia  without  knowing  the  geological 
fact  that  part  of  this  island  was  once  connected  with  the  Oriental 
continent  by  a  bridge  of  land  across  the  Java  Sea.  The  Australasian 
mammalian  fauna  consists  of  survivals  and  descendants  of  a  Mesozoic 
mammalian  fauna  which  has  been  exterminated  everywhere  else,  except 
in  the  case  of  the  American  opossums.  The  original  Australian  mammals 
were  saved,  not  by  any  virtue  of  their  own,  but  by  the  earth-change 
which  insulated  them.  Similarly,  it  is  the  geologist  who  helps  us  to 
understand  the  faunal  diversity  on  the  two  sides  of  "  Wallace's  line," 
or  the  absence  of  amphibians,  reptiles,  and  mammals  from  the  Canaries. 
That  much  will  also  depend  on  the  animal's  power  of  surviving  the 
struggle  for  existence  in  different  regions  is  too  obvious  to  require 
exposition.  We  need  only  think  of  the  way  in  which  man  has  in  a  few 
years  altered  the  distribution  of  many  birds  and  mammals,  sometimes 
indeed  reducing  it  to  nil,  or  increasing  it  to  desperation. 

To  sum  up,  the  chief  factors  determining  geographical 
distribution  are — (i)  the  constitution  of  the  animal,  (2)  the 
physical  conditions  of  the  region,  (3)  the  position  of  the 
original  home,  (4)  the  means  of  dispersal,  (5)  the  historical 
changes  of  the  earth  and  its  climate,  and  (6)  the  bionomic 
relations. 

Zoo- Geographical  Regions. 

I  shall  simply  quote  a  paragraph  from  Professor  Heil- 
prin's  work — The  Geographical  and  Geological  Distribution 
of  Animals  (Internat.  Sci.  Series.  London,  1887),  a  very 
valuable  book  for  the  student,  especially  as  it  considers  dis- 
tribution in  space  and  time  together. 

"  By  most  naturalists  (Wallace,  Sclater,  and  others)  the 
terrestrial  portion  of  the  earth's  surface  is  recognised  as 
consisting  of  six  primary  zoological  regions,  which  corre- 
spond in  considerable  part  with  the  continental  masses  of 
geographers.  These  six  regions  are  : — 

"  i.  The  Palcearctic,  which  comprises  Europe,  temperate 
Asia  (with  Japan),  and  Africa  north  of  the  Atlas  Moun- 
tains ;  also  Iceland,  and  the  numerous  oceanic  islands  of 
the  North  Atlantic : 


764       GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS. 

"2.  The  Ethiopian,  embracing  all  of  Africa  south  of  the 
Atlas  Mountains,  the  southern  portion  of  the  Arabian  Pen- 
insula, Madagascar,  and  the  Mascarene  Islands,  and  which, 
consequently,  nearly  coincides  with  the  Africa  of  geo- 
graphers : 

"  3.  The  Oriental  or  Indian,  which  embraces  India  south 
of  the  Himalayas,  Farther  India,  Southern  China,  Sumatra, 
Java,  Bali,  Borneo,  and  the  Philippines : 

"4.  The  Australian,  comprising  the  continent  of  Aus- 
tralia, with  Papua  or  New  Guinea,  Celebes,  Lombok,  and 
the  numerous  islands  of  the  Pacific  : 

"5.  The  Nearctic,  which  embraces  Greenland,  and  the 
greater  portion  of  the  continent  of  North  America  (exclud- 
ing Mexico) : 

"  6.  The  Neotropical,  corresponding  to  the  continent  of 
South  America,  with  Central  America,  the  West  Indies,  and 
the  greater  portion  of  Mexico." 

Professor  Heilprin  makes  several  modifications  on  this 
scheme  of  distribution  :  (a)  uniting  Palsearctic  and  Nearctic 
in  one  Holarctic  realm ;  (b)  establishing  a  special  Poly- 
nesian realm  for  the  scattered  island  groups  of  the  Pacific ; 
and  (c)  defining  three  transition  regions,  (i)  around  the 
Mediterranean,  intermediate  between  Palaearctic,  Ethiopian, 
and  Oriental,  (2)  Lower  California  between  Western  Hoi- 
arctic  and  Neotropical,  and  (3)  the  Austro-Malaysian  islands 
lying  to  the  east  of  Bali  and  Borneo,  inclusive  of  the 
Solomon  islands,  a  region  intermediate  between  Oriental, 
Australian,  and  Polynesian.  It  seems  also  convenient  to 
recognise  two  polar  regions, — Arctic  and  Antarctic.  Of 
the  last,  we  have  had  as  yet  only  glimpses. 

It  may  be  useful  to  map  out  the  divisions  as  follows  : — 

(Arctic. ) 
NEARCTIC.          PAL^ARCTIC. 


|  Holarctic.         | 

Transition  Transition 

to  to — ORIENTAL. 

I 

Transition  to  Polynesian 

and  to 

Polynesian— NEOTROPICAL.       ETHIOPIAN.       AUSTRALIAN. 
(Antarctic.) 


CHAPTER    XXIX. 

THEORY    OF    EVOLUTION. 

IN  Chapter  VI.  we  indicated  the  nature  of  the  evidence 
which  has  led  naturalists  all  but  unanimously  to  accept  the 
doctrine  of  descent  as  a  modal  interpretation  of  organic 
nature.  The  data  of  physiology  and  morphology,  combined 
with  what  is  known  of  the  history  of  the  race  and  the 
development  of  the  individual,  have  led  us  to  believe  that 
the  forms  of  life  now  around  us  are  descended  from 
simpler  ancestors  (except  in  cases  of  degeneration),  and 
these  from  still  simpler,  and  so  on,  back  to  the  mist  of  life's 
beginnings.  In  other  words,  we  believe  that  the  present  is 
the  child  of  the  past  and  the  parent  of  the  future.  This  is 
the  general  idea  of  evolution. 

But  while  this  general  idea,  which  is  a  very  grand  one,  is 
usually  recognised  as  the  simplest  interpretation  of  the 
facts,  we  remain  in  doubt  as  to  the  factors  of  the  process  by 
which  the  world  of  life  has  come  to  be  what  it  is.  This 
uncertainty  is  in  part  due  to  the  complexity  of  the  problem, 
in  part  to  the  relative  novelty  of  the  inquiry — for  precise 
etiology  is  not  yet  fifty  years  old,  in  part  also  to  the  fact 
that  while  there  has  been  much  theorising,  there  has  been 
comparatively  little  experimenting  or  connected  obser- 
vation as  to  the  modes  and  causes  of  evolution. 

With  the  exception  of  Alfred  Russel  Wallace  and  a  few 
others  who  believe  that  it  is  necessary  to  postulate  spiritual 
influxes  to  account  for  certain  obscure  beginnings,  e.g.,  of 
the  higher  human  qualities,  evolutionists  are  agreed  in 
seeking  to  explain  the  evolution  of  plants  and  animals  as  a 
continuous  "  natural "  process,  the  end  of  which  was 


766  THEORY  OF  EVOLUTION. 

implicit  in  the  beginning.  In  so  doing,  they  follow  the 
method  of  analysis,  endeavouring  to  explain  the  facts  in 
their  lowest  terms.  But  as  the  biologist's  lowest  term  is 
living  matter,  and  as  one  aspect  of  this  is,  in  favourable 
conditions,  known  as  thought,  there  is  no  reason  to  call  the 
evolutionist's  analysis  "  materialistic  " — if  anything  oppro- 
brious be  meant  by  that  adjective.  The  common  denom- 
inator of  the  biologist  is  as  inexpressibly  marvellous  as  the 
philosopher's  greatest  common  measure,  if  indeed,  they  are 
not  practically  the  same. 

The  Two  Great  Problems. 

Our  uncertainty  in  regard  to  the  factors  of  evolution  is  so 
great,  that  I  cannot  venture  here  to  do  more  than  indicate 
(a.)  what  the  great  problems  are,  and  (<£.)  the  general  drift 
of  the  most  important  suggestions  which  have  been  made 
towards  their  solution. 

The  two  great  problems  before  the  evolutionist  are  : — 

(a.)  What  is  the  nature  and  origin  of  variations,  i.e.,  of 
those  organic  changes  which  make  an  organism 
appreciably  different  from  its  parents  or  its  species  ? 

(&)  What  are  the  directive  factors  which  may  operate 
upon  given  variations,  determining  their  elimination 
or  their  persistence,  and  helping  towards  the  familiar 
but  puzzling  result — the  existence  of  distinct  and 
relatively  well-adapted  species  ? 

Secure  answers  to  these  two  questions  must  be  found  in 
reference  to  the  present ;  as  our  data  accumulate  it  will  be 
more  possible  to  argue  back  to  the  past. 

It  may  be  convenient  to  speak  of  the  factors  which  cause 
variation  as  primary  or  originative,  and  of  the  factors  which 
operate  upon  or  direct  the  course  of  variation  as  secondary 
or  directive.  As  far  as  practical  results  are  concerned,  the 
two  sets  of  factors  are  of  equal  importance. 

The  Nature  of  Variations. 

We  mean  by  variations  those  changes  in  organisms  which 
make  them  appreciably  different  from  their  parents  or  from 
their  species. 


PRIMARY  OR   ORIGINATIVE  FACTORS.  767 

The  term  of  course  includes  not  only  material  differences, 
but  also  those  whose  only  demonstrable  expression  is 
psychical.  Thus,  an  increase  in  maternal  affection  is  as 
important  and  real  a  variation  as  the  sharpening  of  a  canine 
tooth. 

It  may  also  be  useful  to  distinguish  variations  in  size, 
symmetry,  number  of  appendages,  and  so  on,  from  more 
qualitative  variations  in  chemical  composition,  such  as 
the  appearance  of  a  new  pigment,  but  this  distinction 
is  only  a  matter  of  convenience,  as  it  is  only  a  matter 
of  degree. 

Again,  variations  occur  which  may  be  called  continuous ', 
being  merely  minute  increments  or  diminutions  of  certain 
parental  or  specific  characters.  These  are  related  to  one 
another  much  in  the  same  way  as  are  the  successive  stages 
in  the  continuous  growth  of  an  individual. 

But  other  variations  occur  which  deserve  to  be  called 
discontinuous.  For,  without  the  appearance  of  transitional 
stages,  marked  variations  crop  up,  reaching  with  apparent 
suddenness  to  what  must  be  called  new  and  may  withal 
exhibit  a  measure  of  perfectness. 

That  both  kinds  of  variations  occur  is  a  fact  of  life  ;  the 
possibility  of  both  is  probably  a  primary  quality  of 
organisms  ;  but  we  are  only  beginning  to  know  the  relative 
frequency  of  the  two  kinds  and  their  respective  limits,  and 
we  know  almost  nothing  as  to  their  causes  (see  Bateson's 
"  Materials  for  the  Study  of  Variation,  1894  " ). 

PRIMARY  OR  ORIGINATIVE  FACTORS. 

What  causes  variation?  This  is  the  fundamental  ques- 
tion, but  it  is  the  least  answerable. 

It  is,  indeed,  an  axiom  or  a  truism,  that  changes  in  any 
animate  system  are  evoked  by  changes  in  the  larger  system 
of  which  the  organism  forms  a  part.  In  other  words,  the 
stimulus  to  organic  change  must  always  be  ultimately 
traceable  to  the  environment,  but  this  is  implied  in  our 
conception  of  living  matter,  and  does  not  help  us  to  under- 
stand the  immediate  conditions  which  lead  to  change. 

In  the  absence  of  sufficiently  precise  data,  we  can  do  little 
more  than  point  out  various  possibilities  : — 


768  THEORY  OF  EVOLUTION. 

(a)  Variations  due  to  Change  in  the  Environment. 
There  is  abundant  proof  that  changes  in  surrounding 
pressure,  in  the  chemical  composition  of  the  medium,  in  food 
supply,  in  heat,  light,  &c.,  may  be  followed  by  changes  in 
the  organism  upon  which  these  influences  play.  Changes 
in  the  body  of  the  organism  follow  changes  in  the  environ- 
ment. But  (i)  it  is  difficult  to  discriminate  between 
variations  which  may  be  spoken  of  as  the  direct  results  of 
environmental  influence,  and  those  to  which  the  organism 
was  already  definitely  predisposed,  and  to  which  the 
environmental  change  supplied  only  the  stimulus.  (2)  We 
have  not  at  present  sufficient  data  to  enable  us  to  state  that 
variations  arising  in  or  acquired  by  the  body  of  an  individual 
organism  as  the  result  of  surrounding  change,  do  as  such  in 
any  degree  specifically  affect  the  reproductive  cells.  In 
other  words,  we  cannot  at  present  say  that  "  environmental 
variations  "  are  transmissible.  And  if  they  are  not,  their 
importance  in  evolution  is  only  indirect. 

(b)  Variations  due  to  Change  in  Function. 
It  is  an  undoubted  fact  that  the  bodily  structure  of  an 
animal  may  be  changed  by  the  increased  use  of  certain 
parts,  or  the  disuse  of  others,  in  short,  by  some  change  of 
function.  ,  This  change  of  or  in  function  may  be  directly 
prompted  by  some  change  in  the  external  conditions  of  life, 
or  it  may  be  the  expression  of  a  deeper  variation  in  the 
animal's  material  constitution  or  mental  character.  But 
important  as  these  functional  variations  and  their  results 
are  to  the  individual,  we  are  uncertain  as  to  their  importance 
for  the  race,  for  we  do  not  know  to  what  extent  (if  any)  the 
results  are  transmissible. 

(c)  Variations  due  to  Changes  in  the  Germ  Cells. 
In  many  cases  of  variation,  particularly  those  which  appear 
in  early  life,  it  is  not  possible  to  suggest  any  environmental 
or  functional  condition  which  may  be  regarded  as  the 
stimulus  or  the  cause.  We  are  led  in  such  cases  to  believe 
that  the  variation  in  bodily  structure  or  habit  is  the  expres- 
sion of  some  novelty  in  the  protoplasmic  constitution  of  the 
germ-cells.  Then,  hiding  our  ignorance,  we  say  that  the  varia- 
tion is  germinal,  constitutional,  congenital,  or  blastogenic. 


SECONDARY  OR  DIRECTIVE  FACTORS.  769 

But  why  should  there  be  changes  in  the  germ  cells  ? 
Perhaps,  because  living  matter  is  very  complex  and  un- 
stable, and  because  it  is  of  its  very  nature  to  differentiate 
and  integrate ;  perhaps  because  the  immediate  environment 
of  the  germ  cells  (blood,  body  cavity  fluid,  sea  water,  &c.) 
is  complex  and  variable.  But  it  may  be  more  important 
to  recognise  that  every  multicellular  organism,  reproduced 
in  the  usual  way,  arises  from  an  egg  cell  fertilised  by  a 
spermatozoon,  and  that  the  changes  involved  in  and  pre- 
paratory to  this  fertilisation,  or  "amphimixis,"  make  new 
permutations  and  combinations  of  living  substances  or  vital 
qualities  not  only  possible  but  necessary. 

SECONDARY  OR  DIRECTIVE  FACTORS. 

(i.)  Natural  Selection.  —  The  distinctive  contribution 
which  Charles  Darwin  and  Alfred  Russel  Wallace  made  to 
etiology  was  their  theory  of  Natural  Selection. 

By  Natural  Selection  is  meant  that  process  whereby,  in 
the  ordinary  course  of  nature,  certain  organisms,  e.g.,  certain 
members  of  a  species,  are  more  or  less  rapidly  eliminated, 
while  others  are  allowed  to  survive  longer. 

That  some  forms,  e.g.,  in  one  family,  should  succeed  less 
well  than  others,  depends  obviously  on  the  fact  that  all  are  not 
born  alike,  depends,  in  other  words,  on  the  fact  of  variation. 

That  there  should  be  elimination  is  necessary  (a)  because 
a  pair  of  animals  usually  produce  many  more  than  a  pair, 
and  the  population  tends  to  outrun  the  means  of  sub- 
sistence, and  (b}  because  organisms  are  at  the  best  only 
relatively  well-adapted  to  their  conditions  of  life,  which  are 
variable.  These  two  primary  facts  and  their  subsequent 
consequences,  e.g,  that  some  animals  feed  upon  others,  that 
there  may  be  more  males  than  females,  &c.,  render  some 
struggle  for  existence  necessary,  though  this  phrase  must  be 
used,  as  Darwin  said,  "  in  a  wide  and  metaphorical  sense," 
including  all  endeavours  for  the  well-being,  not  only  of  the 
individual,  but  of  its  offspring. 

The  facts  then  are — that  variations  constantly  occur,  that 
some  members  of  a  species  or  family  are  necessarily  less 
fitly  adapted  than  others,  and  that  the  course  of  nature  is 
such  that  these  relatively  less  fit  forms  will  tend  to  be 

49 


770  THEORY  OF  EVOLUTION. 

eliminated,  while  the  relatively  more  fit  will  tend  to 
survive.  As  many  variations  re-appear  generation  after 
generation,  and  may  become  gradually  increased  in  amount, 
the  continuance  of  the  selective  or  eliminating  process  will 
work  towards  the  origin  of  new  adaptations  and  new  species. 

The  importance  of  Natural  Selection  as  a  secondary  factor 
in  evolution  will  vary  according  to  stringency  of  the  elimin- 
ating process,  and  it  must  be  noted  that  the  "struggle  for 
existence "  varies  in  intensity  within  wide  limits,  that  it 
requires  to  be  investigated  for  each  case,  and  cannot  be 
postulated  as  a  force  of  nature. 

The  importance  of  the  factor  will  also  depend  on  the 
number,  nature,  and  limits  of  the  variations  which  occur. 
Thus  a  new  species  might  arise,  either  by  the  occurrence  of 
a  discontinuous  variation  of  considerable  magnitude,  or  by 
the  eliminating  process  acting  for  many  generations  on  a 
series  of  minute  continuous  variations. 

Darwin  also  believed  in  the  importance  of  sexual  selection, 
in  which  the  females  choose  the  more  attractive  males, 
which,  succeeding  in  reproduction  better  than  their  neigh- 
bours, tend  to  transmit  their  qualities  to  their  numerous 
male  heirs.  But  this  and  other  forms  of  reproductive 
selection  may  be  regarded  as  special  cases  of  natural 
selection,  and  require  no  particular  emphasis.  Nor  is  the 
importance  of  sexual  selection  or  preferential  mating 
admitted  by  so  great  an  authority  as  Wallace. 

(2.)  "Isolation" — Under  this  title  Romanes,  Gulick,  and 
others  include  the  various  ways  in  which  free  intercrossing 
is  prevented  between  members  of  a  species,  e.g.,  by  geo- 
graphical separation,  or  by  a  reproductive  variation  causing 
mutual  sterility  between  two  sections  of  a  species  living  on  a 
common  area.  Without  some  "  isolation  "  tending  to  limit 
the  range  of  mutual  fertility  within  a  species,  or  bringing 
similar  variations  to  breed  together,  a  new  variation  is  liable, 
they  say,  to  be  "  swamped  "  by  intercrossing.  But  definite 
facts  as  to  this  "  swamping,"  and  in  many  cases  as  to  the 
alleged  "  isolation,"  are  hard  to  find,  nor  can  we  say  that 
a  strong  variation  will  not  persist  unless  it  be  "  isolated." 
Romanes's  view,  however,  was  that  "  without  isolation,  or 
the  prevention  of  free  intercrossing,  organic  evolution  is  in  no 
case  possible.  Isolation  has  been  the  universal  condition 


SUMMARY  OF  EVOLUTION  THEORIES. 


771 


of  modification.  Heredity  and  variability  being  given,  the 
whole  theory  of  organic  evolution  becomes  a  theory  of  the 
causes  and  conditions  which  lead  to  isolation." 

SUMMARY  OF  EVOLUTION  THEORIES. 


(Axiom  or  Truism). 

Variations  are  all  ultimately  due  to  the  External  Influences 
and  the  Nature  of  the  Organism,  i.e.,  of  Protoplasm. 

(  Environment}. 

(Organism}. 

(Function}. 

Changes  in  the 

Constitutional, 

Use  and  disuse 

environment 

congenital, 

of  parts, 

are  followed  by 

or  germinal 

or  change  of 

responsive  variations 

variations 

function  (due  to 

in  the  organism, 

arising  from  the 

change  of 

either  (a)     or   (b}  in 

nature  of  protoplasm, 

environment  or  to 

in  its           its  germ- 

or  from  the 

germinal  change) 

body,              cells. 

changes  necessarily 

are  followed  by 

or  (c)  in  (b} 

associated  with 

variations  in 

through  (a)  (?). 

fertilisation, 

(a)  the  body  of  the 

may  be  continuous 

organism,  or  (b} 

or  discontinuous, 

in  the  germ-cells, 

quantitative  or 

either  directly 

qualitative,  &c. 

or  (?)  through  (a). 

Degree  of 

Degree  of 

transmissibility 

transmissible. 

transmissibility 

unknown. 

unknown. 

Such  variations, 

Such  variations 

Such  variations 

if  transmissible,  and 

probably  supply  the 

if  transmissible,  and 

if  the  originating 

usual  material 

if  the  originating 

conditions  persist 

for  the  origin  of 

conditions  persist 

for  some  time, 

new  species, 

for  some  time, 

may  perhaps 

for  the  establishment 

may  perhaps 

give  rise  to  new 

of  which,  more  or 

give  rise  to  new 

species,  especially 

less  natural  selection 

species,  especially 

if  favoured  by 

(elimination)  and 

if  favoured  by 

natural  selection 

isolation  may  be 

natural  selection 

and  isolation. 

necessary,  according 

and  isolation. 

to  the  nature 

of  the 

variation. 

APPENDIX. 

SOME    ZOOLOGICAL    BOOKS. 


INTRODUCTORY  : — 

F.  Jeffrey  Bell,  Comparative  Anatomy  and  Physiology  (Lond. 
1887). 

C.  Lloyd  Morgan,  Animal  Biology  (Lond.  1889). 

T.  H.  Huxley  and  H.  N.  Martin,  A  Course  of  Elementary  In- 
struction in  Practical  Biology  (Lond.  1888),  revised  edition  by 
G.  B.  Howes  and  D.  H.  Scott. 

A.  Milnes  Marshall  and  C.  H.  Hurst,  A  Course  of  Practical  Zoology 
(3rd  Ed.,  Lond.  1892). 

T.  Jeffrey  Parker,  Elementary  Biology  (2nd  Ed.,  Lond.  1893). 

J.  Arthur  Thomson,  The  Study  of  Animal  Life  (Lond.  1892). 

TEXT-BOOKS  OF  ZOOLOGY  : — 

T.   H.   Huxley,  Anatomy  of  Invertebrates  (1877),  and  Anatomy  of 

Vertebrated  Animals  (1871). 
C.   Glaus,    Grundziige  der  Zoologie   (4th   Ed.,    1880-82),   and   his 

smaller  Text-Book  of  Zoology  (translated  by  Sedgwick,  1884-5.) 
Hatchett  Jackson's  edition  of  Rolleston's  Forms  of  Animal  Life 

(Oxford,  1888). 
Text-Books  (mostly  with   similar   titles)    by  Boas,    R.    Hertwig, 

Kennel,  H.  A.  Nicholson,  A.  S.  Packard,  R.  Perrier,  Shipley, 

and  many  others. 
J.  Ritzema  Bos,  Agricultural  Zoology  (translated  by  Davis,  Lond. 

1894.) 

BOOKS  AS  GUIDES  TO  PRACTICAL  WORK  : — 

C.  Vogtand  E.Yung,  Traite  d'Anatomie  Comparee  pratique  (Paris, 

1885-95)  ;  also  in  German. 
T.  J.  Parker,  Zootomy  (Lond.  1884). 
Huxley  and  Martin,  op.  cit. 
A.  Milnes  Marshall  and  C.  H.  Hurst,  op.  cit. 
W.  K.  Brooks,  Handbook  of  Invertebrate  Zoology  for  Laboratories 

and  Sea- side  Work  (Boston,  1882). 
P.  Girod,  Manipulations  de  Zoologie  (1879-81). 


774  APPENDIX. 

BOOKS  AS  GUIDES  TO  PRACTICAL  WORK — continued. 

A.  Bolles  Lee,  Microtomisf  s  Vade-Mecum  (3rd  Ed.,  1893). 
An  Atlas  will  help  the  student  greatly,  if  he  does  not  use  it  too  . 
much,  e.g.  : — 

G.  B.  Howes,  Atlas  of  Practical  Elementary  Biology  (Lond.  1885). 
W.  R.  Smith  and  J.  S.  Norwell,  Illustrations  of  Zoology  (Edin.  1889). 

A.  de  Vayssiero,  Atlas  d'Anatomie  Comparee  des  Invertebres  (Paris, 

1889). 
C.  B.  Briihl,  Zootomie  aller  Thierklassen  (Wien). 

GENERAL  MORPHOLOGY  : — 

Ernst  Hseckel,  Generelle  Morphologie  (Berlin,  1866). 

Herbert  Spencer,  Principles  of  Biology  (Lond.  1864-66). 

W.  His,  Unsere  Kb'rperform  (1875). 

G.  Jaeger,  Allgemeine  Zoologie  (1878). 

P.  Geddes,  Art.  "Morphology"  (Encyclop.  Brit.} 

CLASSIFICATION,  see : — 

E.  Ray  Lankester,  Art.  "Zoology"  (Encyclop.  Brit.} 
W.  A.  Herdman,  Phylogenetic  Classification  of  Animals. 

WORKS  ON  COMPARATIVE  ANATOMY  : — 

Besides  the  classic  works  of  Cuvier,  Meckel,  Milne  Edwards,  &c. — 

Richard  Owen,  Comparative  Anatomy  of  Vertebrate  Animals  (4th 
Ed.,  1871). 

T.  H.  Huxley,  op.  cit. 

C.  Gegenbaur,  Elements  of  Comparative  Anatomy  (trans,  by  F. 
Jeffrey  Bell,  Lond.  1878). 

A  Lang,  Text-Book  of  Comparative  Anatomy  (trans,  by  H.  M.  and 
M.  Bernard,  Lond.,  in  progress). 

R.  Wiedersheim,  Comparative  Anatomy  of  Vertebrata  (trans,  by 
W.  N.  Parker,  Lond.  1886).  And  a  larger  work  untrans- 
lated. 

B.  Hatschek,  Lehrbttch  der  Zoologie  (1888). 

F.  Leydig,  Lehrbuch  der  Histologie  (Comparative)  (1857). 

WORKS  ON  COMPARATIVE  PHYSIOLOGY  : — 

Claude  Bernard,  Phenomenes  de  la  Vie  Commune  aux  Animaux  et 

aux  Vegetaux  (1878). 
Paul  Bert,   Lecons  sur  la  Physiologie  comparee  de  la  Respiration 

(1870). 

C.  F.  W.    Krukenberg,    Vergleichend-Phystologische   Studien   and 

Vortrdge  (1881-89). 
F.    Jeffrey   Bell,    Comparative   Anatomy  and  Physiology    (Lond. 

1887). 

A.  B.  Griffiths,  Comparative  Physiology  (1891). 
Halliburton,  Physiological  Chemistry  (1891). 
Bunge,  Physiological  and  Pathological  Chemistry  (trans.  1890). 


SOME   ZOOLOGICAL   BOOKS.  775 

EMBRYOLOGY  : — 

F.  M.  Balfour,  Comparative  Embryology  (2  vols.,  Lond.  1 880-81). 
M.   Foster  and  F.  M.    Balfour,  revised  by  A.   Sedgwick  and  W. 

Heape,  Elements  of  Embryology  (Lond.  1883). 
A.  C.  Haddon,  Introduction  to  the  Study  of  Embryology  (Lond. 

1887). 
O.   Hertwig,  Lehrbuch  der  Entwicklungsgeschichte  des  Menschen 

und der  Wirbelthiere  (trans,  by  E.  L.  Mark,  3rd  Ed.,  1893). 
E.     Korschelt     and     K.     Heider,    Lehrbuch    der     Vergleichenden 

Entwicklungsgeschichte  der  Wirbellosen  Thiere  (Jena,  1890-93). 
L.  Roule,  Embryologie  Generate  (Paris,  1892). 
A.  Milnes  Marshall,  Vertebrate  Embryology  (1893). 
C.  S.  Minot,  Human  Embryology  (1892). 

PALEONTOLOGY  : — 

II .  A.  Nicholson  and  R.   Lydekker,  Manual  of  Paleontology  (2 

vols.,  Lond.  and  Edin.  1889). 

K.  A.  von  Zittel,  Handbuch  der  Palaontologie  (completed  1893). 
M.  Neumayr,  Die  Stdmme  des  Thierreichs  (vol.  I.,  Wien  and  Prag, 

1889). 

Gaudry,  Les  Enchainements  du  Monde  Animal  (1889-90). 
Cams  Sterne  (Ernst   Krause),    Werden  und   Vergehen    (3rd   Ed., 

Berlin,  1886). 
Also  text-books  by  Bernard  (1893),  Koken  (1893). 


GEOGRAPHICAL  DISTRIBUTION  : — 

A.  R.  Wallace,  Geographical  Distribiition  (2  vols.,  Lond.  1876). 
A.    Heilprin,    The    Geographical  and    Geological   Distribution   oj 

Animals  (Lond.  1887). 

Trouessart,  La  Geographie  Zoologique  (Paris,  1890). 
W.  Marshall,  in  Berghaus'  Physikal  Atlas  (Leipzig,  1887). 


BOOKS  OF  NATURALIST  TRY\VELLERS,  e.g.  :— 

Charles  Darwin,    Voyage  of  the  Beagle  (Lond.    1844,   New  Ed., 

1890). 

H.  W,  Bates,  Naturalist  on  the  Amazons  (New  Ed.,  Lond.  1892). 
T.  Belt,  Naturalist  in  Nicaragua  (2nd  Ed.,  1888). 
A.  R.  Wallace,  Malay  Archipelago  (1869),  Tropical  Nature  (1878), 

Island  Life  (\%&Q\ 
Wyville  Thomson,    The  Depths  of  the  Sea  (1873),    Voyage  of  the 

Challenger  (\^\ 
H.   N.   Moseley,  Naturalist  on  the   Challenger  (1879,  New  Ed., 

1892). 

W.  H.  Hudson,  Naturalist  in  La  Plata. 
A.   E.   Brehm,  From  North  Pole  to  Equator  (translation  Ed.   by 

J.  Arthur  Thomson,  with  bibliography,  1895). 


776  APPENDIX. 

COMPARATIVE  PSYCHOLOGY  : — 

G.   J.    Romanes,   Animal  Intelligence  and   Mental  Evolution   of 

Animals. 
C.  Lloyd  Morgan,  Animal  Life  and  Intelligence  and  Introduction 

to  Comparative  Psychology  (Lond.  1894). 

GENERAL  NATURAL  HISTORY  : — 

Brehm's    Thierleben   (3rd    Ed.,    by    Pechuel  -  Loesche,    10   vols., 

Leipzig  and  Wien,  1890-1893). 
Cassell's  Natttral  History  (Ed.   by  P.    Martin   Duncan,   6  vols., 

1882). 
Standard  or  Riverside  Natural  History  (Ed.   by  J.  S.   Kingsley,  6 

vols.,  1888). 
Royal  Natural  History  (Ed.  by  R.  Lydekker,  in  progress). 

BOOKS  ON  EVOLUTION  : — 

Charles  Darwin,  Origin  of  Species  (1859,  &c. ). 
Alfred  Russel  Wallace,  Darwinism  (1889). 
Herbert  Spencer,  Principles  of  Biology  (1866,  &c.). 
Ernst  Hseckel,  Generelle  Morphologie  (1866,  &c.). 

For  more  recent  books,  see  my  "  Study  of  Animal  Life."  To  the 
list  there  given  must  be  added  two  books  in  particular,  Weismajin's 
Germ- Plasm  (1893)  and  Bateson's  Materials  for  the  Study  of  Variation 
(1894). 

GENERAL  WORKS  OF  REFERENCE  : — 

W.  Hatchett  Jackson's  edition  of   Rolleston's  Forms  of  Animal 

Life  (Oxford,    1888).      A   very  valuable   work,    with    special 

bibliographies. 
Leunis,  Synopsis  des   Thierreichs,  re-edited  by  Ludwig  (Hanover, 

1886). 

Bronn,  Klassen  und  Ordmingen  des  Thierreichs  (1859-1895). 
E.  Ray  Lankester  and  others,  Zoological  Articles  reprinted  from 

Encyclop.  Brit.  (Lond.  1891). 

MONOGRAPHIC  SERIES,  e.g. — 

Reports  of  the  Voyage  of  H.M.S.  "Challenger." 
Fauna  und  Flora  des  Golfes  von  Neapel. 
Catalogues  of  the  British  Museum — Natural  History. 

RECORDS  OF  RESEARCH  : — 

Journal  of  Royal  Microscopical  Society ',  edited  by  F.  Jeffrey  Bell 

(6  parts  in  the  year). 

Zoological  Record^  edited  by  D.  Sharp  (yearly). 
Zoologisches  Jahresbericht  (Naples)  (yearly). 
Anatomische  Ergebnisse  (Merkel  &  Bonnet)  (yearly). 
Science  Progress,  edited  by  F.  Bretland  Farmer  (monthly). 
Natural  Science  (monthly). 


SOME  ZOOLOGICAL   BOOKS.  777 

HISTORY  OF  ZOOLOGY  :— 

W.  Whewell,  History  of  Inductive  Sciences  (1840). 

J.  V.  Carus,  Geschichte  der  Zoologie  (1872). 

E.  Perrier,  La  Philosophic  Zoologique  avant  Darwin  (1884). 

E.  Hoeckel,  Natural  History  of  Creation  (trans.  Lond.  1870). 

E.  Ray  Lankester,  Article  "Zoology"  (Encyclop.  Brit.}. 

H.    A.    Nicholson,   Natural  History:    its  Rise   and  Progress   in 

Britain  (1888). 

E.  Krause,  Die  Allgemeine  Weltanschauungen  (1889). 
H.  F.  Osborn,  From  the  Greeks  to  Darwin  (1894). 

BIBLIOGRAPHY  : — 

Hatchett  Jackson  and  Rolleston,  op.  cit. 

L.  von  Graff,  Bibliothek  des  Professors  der  Zoologie  und  vergleichen- 

den  Anatomie  (Leipzig,  1891). 

Bibliotheca  Historico-naturalis,  Engelmann,  (1700-1846). 
Bibliotheca  Zoologica,  Carus  and  Engelmann,  (1846-61). 
Bibliotheca  Zoologica,  continued  by  Taschenberg,  (1861-1880). 
Bibliotheca  Zoologies  et  Geologies,  L.  Agassiz,  edited  by  Strickland 

and  Jardine  (Lond.  1848-1854). 

Royal  Society's  Catalogue  of  Scientific  Papers  (fr.  1800). 
Annual  Bibliographies  in  Zoological  Record,  &c.,  cited  above. 
C.  Davies  Sherborn,  Books  of  Reference  in  the  Natural  Sciences, 

Natural  Science,  V.  (August  1894). 


INDEX. 


INDEX. 

PAGE 

PAGE 

Aard-vark, 

.    690 

Actinotrocha  (larva), 

.     225 

Aard-wolf, 

•  715 

Actinozoa, 

129,  156 

Abdominal  pores,    . 

472,  477 

Adambulacrals, 

.     230 

,,          ribs,      . 

568,  584 

Adamsia, 

160 

Abducens  nerve,     . 

.     441 

Adder,    .... 

-     583 

Abomasum,     . 

.     697 

Adhesive  cells, 

•     157 

Absorption,     . 

•       23 

Adrenal  body  of  Pigeon, 

.     612 

Abyssal  Fauna, 

•     754 

,,             ,,       Rabbit, 

.     676 

Acanthias, 

.     506 

^Eginopsis, 

.     156 

Acanthin, 

IO2 

yEgithognathse, 

.     620 

Acanthocephala, 
Acanthodrilini, 

.    185 

.       209 

^Eluroidea, 
^Elurus, 

.     714 
.     716 

Acanthometra, 

•       103 

yEpyornis, 

.     619 

Acanthopteri, 

.       510 

Aerial  Fauna, 

•     759 

Acarina, 

336 

^Ethalium, 

99 

Acephala, 

.       362 

^Etiology, 

.     765 

Acetabulum,   . 

605,  666 

Agama,  .... 

•    577 

Achromatin,    . 

•       43 

Agamidae, 

•     577 

Acineta, 

.     105 

Agouti,  .... 

.     711 

Acinetaria, 

.     105 

Air  bladder  of  Fishes, 

.    476 

Acipenser, 

.     508 

Air  sacs  of  Birds,    . 

.    622 

Accela, 

.     162 

„       „    of  Lizards, 

•     576 

Acontia, 

.     151 

Albumen  gland  of  Snail, 

•     354 

Acontias, 

,     578 

of  bird's  egg,    . 

.     613 

Acornshell,     . 

•     274 

,,        of  frog's  egg,     . 

•     549 

Acquired  Characters, 

71,  768 

Alces,     .... 

.     698 

Acrania  =  Cephalochorda, 

.     410 

Alciope, 

209 

Acraspeda,      .         .      142, 

148,  154 

Alcippe, 

•     275 

Acrodont  teeth, 

-     570 

Alcyonaria,     . 

•     157 

Acromion, 

.     664 

,,          and  Zoantharia 

•     153 

Actinia   .... 

.     156 

Alcyonidium, 

•     225 

Actiniaria, 

153,  156 

Alcyonium, 

153,  156 

Actinomma,    . 

.     103 

Alecithal, 

6c 

Actinophrys,  . 

.       IOO 

Alimentary  System  of  — 

Actinosphserium,     . 

.     100  !          ,,           Amphibians, 

•    540 

782 


INDEX. 


Alimentary  System  of  — 

PAGE 

PAGE 

Alternation  in  Spongilla,         .     122 

Amphioxus, 

.     414 

,,         in  Tunicates,          .     407 

Anodonta,    . 

•     367 

Altrices,           ....     628 

Arenicola,     . 

.     203 

Alytes,    556 

Ascidian, 

.     404 

Ambiens  muscle,     .         .         .601 

Aurelia, 

•     143 

Amblypoda,    ....     7°4 

Balanoglossus, 

•     393 

Amblyrhynchus,      .         .         .     577 

Bee,      . 

.     301 

Amblystoma,  .         .         .         .     556 

Cockroach,  . 

.     296 

Ambulacral  areas,  .         .         .     236 

Crayfish, 

.     261 

,,            ossicles,         .         .     230 

Crinoidea,     . 

•     245 

Ameiva,           ....     577 

Crocodilia,    . 

.     587 

Ametabolic  Insects,         .         .     305 

Crustacea,    . 

.     280 

Amia,     .....     508 

Distomum,   . 

.     167 

Amitosis,         ....       45 

Earthworm, 

.     192 

Ammocoetes,  ....     473 

Frog,    . 

•     540 

Ammodiscus,           .         .         .     102 

Haddock, 

.     500 

Ammonites,    ....     385 

Helix,  . 

•     35i 

Ammothea,     ....     342 

Herring, 

•     504 

Amnion,          .         .      451,  529,  590 

Hirudo, 

.     217 

Amniota,         .         .         .         -53° 

Holothurian, 

.     240 

Amoeba,          ...         86,  99 

Insects, 

•     312 

,,        Functions  of,    .         .106 

Limulus, 

•     339 

,,         Physiology  of,  .         .       15 

Lizards, 

•     573 

,  ,        type  of  Rhizopoda,   .       86 

Mollusca, 

•     35i 

Amphibia,       ...          3,  529 

Myxine, 

.    467 

„         Classification  of,      .     555 

Nematoda,    . 

.     181 

„         Fishes  compared  with,  530 

Nemerteans, 

.     178 

,,         General  characters  of,  529 

Peripatus, 

.     287 

,,         History  of,      .         .     559 

Petromyzon, 

.     470 

,,         Mammals  and,         .     559 

Pigeon, 

.     608 

Life  of,  .        .        .     557 

Rabbit, 

.     669 

„          Orders  of,       .         .     529 

Rotifera, 

•     223 

Amphiblastula,        .         .         .124 

Scorpion, 

•     329 

Amphiccelous  vertebra,   .         .     479 

Sea-urchin,  . 

.     238 

Amphidiscs,    ....     122 

Sepia,  . 

•     379 

Amphilina,      .         .         .               175 

Skate   . 

•     487 

Amphimixis,   ....       63 

Spider, 

•     332 

Amphineura,  ....     345 

Starfish, 

.     232 

,,        General  characters  of,  345 

Vertebrates. 

.    448 

Amphioxus,    .         .         .         .410 

Alisphenoid  canal, 

•     693 

Amphipoda,    ....    -&6g 

Allantois,        .         34,  451, 

529>  590 

Amphiporus,  .         .         .         .180 

Alligators, 

.     588 

Amphisbsena,           .         .         -578 

Alloiocoela,     . 

.     162 

Amphisbsenidre,      .         .         .     578 

Alpaca,  .... 

.     696 

Amphistylic,   ....     534 

Alternation  of  Generations, 

•      55 

Amphitherium,        .         .         .     640 

„         in  Aurelia,     . 

.     147 

Amphiuma,     ....     5.56 

„         in  Ccelentera, 

130,  132 

Amphiura,       ....     236 

„         in  Distomum, 

.     167 

Ampullae  of  skate,  .         .         .     478 

„         in  Nematodes, 

.     183 

,,        of  starfish,        .         .     233 

INDEX. 


783 


PAGE    l                                                                                            PAGE 

Anableps,  "  Placenta  "  of, 

591     Anthropoid  Apes,  . 

722 

Anabolism,      .... 

28     Anthropoidea, 

722 

Anacanthini,  .... 

510    Anthropomorph,     . 

726 

Anaconda,       .... 

583     Anthropopithecus,  . 

726 

Anal  cerci  of  Cockroach, 

295     Antilocapra,    .... 

698 

Analogous  organs, 

33     Antilope,        . 

698 

Anamniota,     .... 

530     Antipatharia,            .         .      153, 

157 

Anapophyses, 

66  1     Antiquity  of  man,  . 

730 

Anchinia,         .... 

409 

Antlers,  ..... 

698 

Anchitherium, 

701 

Ant-lion,         .... 

304 

Ancylus,          .... 

358 

Ants,      

304 

Androctonus, 

331 

ere 

Anemonia,      .... 

jj 
156 

Anus  of  Vertebrates, 

DJD 

452 

Angiostomum, 

182 

Apes,     ..... 

726 

Anguillulidse, 

185 

Aphaniptera, 

304 

Anguis,  ..... 

577 

Aphides,         .... 

305 

Angular  bone.          .      431,  586, 

605 

Aphrodite,      .... 

209 

Animalculists, 

49 

Apis,       ..... 

298 

Animal     Kingdom  —  General 

Aplacophora,  .... 

347 

survey  of,     . 

I-I2 

Aplysia,  ..... 

357 

Animals  and  plants  contrasted,  17-19 

Apneustic  Insects,  . 

3H 

Anisopleura,    .... 

388 

Apoda  (Echinoderma)     . 

243 

Anisopoda,      .... 

276 

Apoda  =  Gymnophiona  . 

556 

Annelids.         .... 

1  86 

Apodemata  (Crayfish)     . 

257 

Development  of, 

196 

Appendages  of  Arachnoidea,  . 

329 

,         and  Vertebrates,     . 

424 

Arenicola, 

202 

Anoconta,        .         .         .     363, 

372 

Cockroach, 

295 

External  appearance  of, 

363 

Crayfish,       255, 

257 

Internal            „ 

364 

Crustacea, 

255 

Life  history  of,  . 

370 

Eurypterina,  . 

340 

Mode  of  life  of, 

363 

Haddock, 

496 

Shell  of,    . 

364 

Insects,  . 

307 

Structure  of, 

363 

Limulus, 

339 

Anolis,    ..... 

577 

Myriopods, 

292 

Anomalurus,  .... 

711 

Peripatus, 

287 

Anomia,          .... 

372 

Polychaeta, 

209 

Anomodontia, 

592 

Spider,  . 

332 

Anomura,        .... 

279 

Trilobita, 

34i 

Anoplodium, 

162 

Appendicularia,       .          .     401, 

408 

Ant-eaters,      .... 

68  1 

Appendicular    skeleton    of 

Antedon,         .... 

243 

Vertebrates, 

•433 

,,          Structure  of,     . 

244 

Appendix  vermiformis,    . 

670 

Antennae  of  Cockroach, 

295 

Apseudes,       .... 

277 

,,          of  Crayfish, 

256 

Apterygota,    .... 

3°4 

,,          of  Myriopods,  . 

292 

Apteryx,         .... 

619 

,,          of  Peripatus, 

287 

Aptornis,         .... 

618 

,,          of  Trilobites,     . 

34i 

Apus,     ..... 

269 

Antennules  of  Crayfish,  . 

256 

Aquatic  Mammals, 

638 

Anthomedusce, 

155 

Aqueduct  of  Sylvius, 

438 

Anthozoa,       .... 

156 

Aqueductus  Vestibuli,     . 

444 

784 


INDEX. 


Aqueous  humour  of  Eye, 
Arachnoidea,  .         . 

Arachnoid  membrane,  . 
,,  fluid,  .  . 
Araneidse,  .  .  . 
Area,  .... 
Arcella,  ... 
Archseoceti,  ... 
Archseopteryx,  .  . 
Archegosaurus,  .  . 


PAGE 

.  446 
.  327 
.  438 
-438 
331 

.  372 
.  99 
.  708 
.617 

-557 
67,  448 


Archenteron, 
Archerina,  . 
Archi-  Annelida,  .  .  .211 
Archi-cerebrum,  .  .  .271 
Archi-Chsetopoda,  .  .211 
Archiccele,  .  .  .  .176 
Archigetes,  .  .  .  .171 
Archinephric  duct  —  Segmental 

duct,  ....  461 
Archipterygium  of  Fishes,  .  521 
Archoplasm,  .  .  .  •  45 
Arctocyon,  .  .  .  .  717 
Arctoidea,  .  .  .  .  715 
Arctomys,  .  .  .  .  711 
Arctopithecini,  .  .  .  724 
Arenicola,  .  .  .  .201 
„  Structure  of,  .  202-6 
Argonauta,  ....  386 
Argulus,  ....  269 
Argyroneta,  ....  336 
Arion,  .....  358 
Aristotle's  lantern,  .  .  237 

Arius,     .....     526 
Armadillo  (Crustacean),          .     277 
,,         (Mammal),     Exo- 

skeleton  of,  .          .     690 

Armadillos,  ....  690 
Arrow-worms,  .  .  .  222 
Artemia,  ....  269 
Arterial  arches  of  Vertebrates,  455 
,,  system  of  embryo  of 

higher  Vertebrates,     455 

„     of  Fish,        .     455 

,,     of  Frog,        .     542 

,,  ,,     of  Pigeon,     .     610 

,,  ,,     of  Rabbit,     .     673 

Arthrobranchs  of  Crayfish,      .     264 

Arthropoda,  Classes  of,    .         7,  252 

,,     General  characters  of,    253 

Arthrostraca,  ....     269 


Articular,         .         .      431,  586,  605 
Articulata,       ....     246 
Artiodactyla,  ....     693 
, ,  and     Perisso- 

dactyla  contrasted,  .  693 
Arvicola,  .  .  .  .  711 
Asaphus,  .  .  .  .341 
Ascaridse,  .  .  .  .184 
Ascaris,  .  .  .  .  .183 
Ascetta,  .  .  .  117 

Ascidia,  .         .         .      401, 409 

Ascidiacea,  ....  409 
Ascon  type  (of  Sponge),  .  118 
Asellus,  ....  277 

Asexual  reproduction,  .  .  51 
Aspergillum,  ....  372 
Aspredo,  . .  .  .  .526 
Astacus,  .  .  .  253-69 
Asteracanthion,  .  .  .  235 
Asterias,  .  .  .  229, 235 
Asteroidea,  ....  229 
Astraea,  .  .  .  154,  157 

Astropecten,  ....  235 
Astrophyton,  .  .  .  236 

Astrorhizidae,  .         .         .106 

Atavisms  or  Reversions,          .       83 

Atax, 336 

Ateles, 725 

Atheoe,  ....     566 

Atlanta,  .         .         .         -357 

Atlantosaurus,  .  .  .  593 
Atlas  vertebra,  .  .  .  660 
Atrial  cavity,  .  .  401, 416 

Atriopore  of  Amphioxus,         .     416 

Attidse, 336 

Atypus, 336 

Auchenia,        ....     696 

Auditory  capsules  of  skull,      .     427 

,,         nerve,       .         .         .     441 

,,        organs  of  Insects,     .     311 

Aulastoma,     .         .         .         .214 

Aulosphaera,   .         .         .         .103 

Aurelia,  .         -133,  142,  156 

,,        Life  history  of,        146,  147 

,,        Relatives  of,      .         .147 

Auricles  of  Vertebrate  heart,  .     454 

Auricularia  (of  Holothuria),    .     243 

Australian  region,  .         .         .     764 

Autostylic,      ....     534 

Aves,  see  Birds,       .         .         2,  595 


INDEX. 

785 

PAGE 

PAGE 

Avicula,           .... 

372 

Bionomics  — 

Axial  Skeleton,  see  Backbone, 

432 

Birds,  

621 

Axis  vertebra, 

660 

Coelentera, 

160 

Axolotl,           .... 

556 

Crustacea,  .... 

283 

Aye-Aye,        .... 

722 

Fishes,         .... 

^17 

Azygobranchs, 

"3^7 

Insects,       .... 

j  / 

324 

jji 

Mollusca,    .... 

361 

Babirusa,        .... 

694 

Spiders,       .... 

334 

Baboons,         .... 

725 

Sponges,     .... 

125 

Backbone,      .... 

432 

Bipalium,        »         .         .         . 

163 

Bactrites,        .         .         ... 

-2gq 

Bipinnaria,      .         .         .     234, 

j 

24.0 

Baculites,        .... 

385 

Bird  lice,         .... 

•***T.7 
305 

Badger,            .... 

716 

Birds,     

"59? 

Bakena,           .... 

709 

and  Reptiles, 

jyj 
562 

Balsenoidea,    .... 

709 

Classes  of,     . 

y— 

3 

Balaenoptera, 

709 

Classification  of,    . 

617 

Balanoglossus  — 

Courtship  of, 

626 

Description  of, 

39i 

Diet  of, 

626 

General  characters  of, 

39i 

Eggs  of,         ... 

627 

Habits  of,      . 

392 

Feathers  of,  . 

699 

Invertebrate  characters  of,  398 

Flight  of,      .         . 

621 

Species  of,     . 

392 

General  characters  of,    . 

596 

Vertebrate  characters  of, 

397 

,,       survey  of, 

596 

Balantidium,  .... 

i°5 

Intelligence  of, 

626 

Balanus,          .... 

269 

Life  of, 

621 

Baleen,            .... 

706 

Migration  of, 

628 

Bandicoot,      .... 

686 

Moulting  of, 

628 

Barbary  ape,  .... 

726 

Nests  of,        ... 

626 

Barbule,           .... 

495 

Pedigree  of,  . 

630 

Barnacle,         .... 

272 

Song  of,         ... 

625 

Basiliscus,       .... 

577 

Birgus,  

279 

Basitemporal, 

596 

Bitterling,       .... 

527 

Basipodite  of  Crayfish,    . 

255 

Bivalves,         .... 

362 

Batoidei,         .... 

506 

,          Classification  of. 

372 

Bats,       

719 

,          General  characters  of, 

363 

Bdellostoma,  .... 

469 

,                 ,,        notes  on, 

372 

Bears,     ..... 

716 

,           Habits  of, 

373 

Beaver,  ..... 

711 

,           Life  history  of 

374 

Bees,       

298 

,           Past  history  of 

374 

Beetles,  

3°4 

Bivium  of  Sea  urchin, 

237 

Belemnites,     .... 

386 

,,       of  Starfish, 

230 

Belinurus,        .... 

^dO 

Bladder  of  Frog, 

C4.O 

Belodon,          .... 

o<+w 

589 

,  ,       of  Mammals, 

JT** 

676 

Beluga,  

709 

,,       -worms, 

173 

Bernhardus,    .... 

160 

Blastocyst, 

650 

Beroe,    

*57 

Blastoderm,    . 

65 

Bilateral  symmetry,         .        31, 

161 

Blastoidea, 

246 

Bile,        

22 

Blastomere,     . 

420 

I7O 

Blastopore,     . 

67 

728 

Blastosohere.  . 

65 

50 

;86 


INDEX. 


I 
Blastula,          .... 
Blatta,   
Blind  spot  of  Eye,  . 
Blood,    .         .         .         .25,  26, 
„       of  Vertebrates, 
Boa,       

'AGE 

65 
293 
447 
74i 
453 
tfl 

Branchiostegal  rays, 
Branchipus,     . 
Branchiura,     . 
Breast  bone,   . 
Brisinga, 
Brittle-stars,  .         .         . 

PAGE 

.     498 
.     269 
.     209 

-     433 
•     235 

2^ 

Body  cavity  — 
Characters  of  a  true, 
of  Amphioxus, 
Arenicola,  . 

315 

415 
2Cn 

Brontornis, 
Brontosaurus, 
Bryozoa, 

.       6l9 

-     593 
.     224 
3  5  7 

Balanoglossus, 
Crayfish,      .         .         . 

394 
->6-> 

Buckie  =  Buccinum. 

^2 

Earthworm, 
Hirudo, 
Insects, 
Lamprey,    . 
Peripatus,    . 

190 
216 

315 

472 
31"? 

Bufo,      .... 
Bugs  
Bulbus  arteriosus,  . 
Bulla,     .... 
Bunodont, 

-    555 
-    305 
•     454 
•     357 
.    693 

Scorpion,     . 
Spider, 
Starfish, 
Teleostean, 
Vertebrates, 
„            Primary,  . 
„           Secondary, 
Bombinator,   .... 
Bone,      .     ,    .         .         .38, 
Bonellia,         .... 

330 
332 
232 
453 
453 
426 
426 
555 
373 

211 

Bursa  Fabricii, 
Buthus,  .... 
Butirinus, 
Butterflies, 
Byssus,   .... 

Caddis  flies,    . 
Coeca,     .... 
Csecilia,  .... 
Csecum  of  rabbi  c,    . 

.     609 

•     33i 
.     510 

•     3°4 

•     37i 

•     304 
•     739 
.     556 
.    670 

Books,  Notes  on,    . 
Book  scorpions, 
Bopyridse,       .... 

778 

331 

277 

Ca'ing  whale, 
Calamoichthys, 

.     709 
.     508 
.     116 

Bos,        
Bothriocephalus,     . 
Botryllus,        .         .         .401, 
Bougainvillea, 
Bovidse,  ..... 
Brachiolaria,   .         .         .     234, 

698 
171 
409 

155 
698 

249 

Calciferous  glands, 
Calcispongiae, 
Caligus, 
Callionymus,  . 
Callorhynchus, 

.     192 
.     116 
.     272 
.     520 
.     507 
•     34i 

Brachionus,     .... 
Brachiopoda,  .         .         .      224, 
Brachyura,      .... 

223 

225 
279 

Calyptoblastic, 
CamelidcS, 

•     155 
.     696 
.     696 

Bradypodidae, 
Bradypus,        .... 

689 
680 

Campanularia, 
Campodea, 

•     155 
.     305 

Brain  of  Skate, 
,,     of  Vertebrates, 
,  ,     Summary  of  parts  of,     . 
Branchellion,  .... 

482 

435 
438 
214 

Campodeiform  larva, 
Camptosaurus, 
Cancer,  .... 

-     323 
•     593 
.     279 
.     715 

Branchioe  =  Gills. 
Branchial  arches,    .         .     428, 
,,         sense  organs,  . 
Branchiobdella, 
Branchiopoda, 

43i 
443 
209 
269 

Canis,     .... 
Cannon-bone, 
Capitellidce,    . 
Capitulum  of  rib,    . 
Capra,     .... 

•     7i5 
.     696 
.     209 
.     66  1 
.     698 

INDEX. 


787 


Caprella,  .  .  .  .277 
Capreolus,  ....  698 
Capuchins,  ....  725 
Capybara,  .  .  .  .  711 
Carapace  of  Chelonia,  .  .  254 
„  of  Crayfish,  .  .  564 
Carcharias.  ....  506 
Carchesium,  ....  93 
Carcinus,  ....  279 
Cardium,  ....  372 

Cardo, 295 

Caretta, 567 

Carina  of  Cirripedia,       .         .     274 
„       or  keel  of  birds,  .      597,  605 
Carinaria,        .         .         .         -357 
Carinatae,        .         .         .618,  620 
,,         and    Ratitse    con- 
trasted,        .         .618 
Carinella,        .         .         .         .177 
Carinoma,       .         .         .         .180 
Carmarina,      .         .         .         .156 
Carnassial  teeth,      .         .         .     713 
Carnivora,       .         .         .         .712 

Carp, 510 

Carpo-metacarpus,  .  .  .  597 
Carpus,  .....  665 
Cartilage,  .  .  38 

,,       -bones,      .         .     428,  660 
Caryophyllseus,        .         .  171 

Cassiopeia,  .  .  .  .148 
Cassowary,  .  .  .  .619 

Castor 711 

Casuarius,       .         .         .         .619 

Cat, 714 

Catallacta,  ....  99 
Catarrhini,  ....  722 
Caterpillar,  ....  322 
Caudata,  .  .  .  -556 
Cave  Fauna,  .  .  .  -757 

Cavia, 711 

Caviidse,  .         .         .         .711 

Cebidee, 725 

Cebus 725 

Cell  cycle  among  Protozoa,  .  97 
division  .  .  .  45-48 
,,  Rationale  of,  .  46 
Nucleus  of,  .  .  .  43 
Substance  of,  .  .  .  42 
Theory,  .  .  37,  41,  50,  69 
Wall  of,  .  .  .  44-45 


Cells,  .  .  .  .  .40 
,,  Forms  of,  .  .  .  42 
,,  History,  .  .  .41 
,,  Structure  of,  .  .  42 

Cellepora,  .  .  .  .  225 
Cellulose  in  Tunicates,  .  .  402 
Centetes,  .  .  .  .717 
Centipedes,  .  .  42,  285,  291 
„  and  Millipedes,  286,  292 
Central  corpuscles,  .  .  44 
Centrolecithal,  ...  65 
Centrosomes,  ...  44 

Cephalaspis,   .         .         .         .     509 
Cephalochorda,       ,         .         .410 
„     General  characters  of,  410 
Cephalodiscus,        .         .         .     399 
Cephalopoda,          .         .         .     374 
„  Classification  of,     385 

Cephalothorax  of  Crayfish,  .  254 
Eurypterina,  340 
Limulus,  .  338 
Scorpion,  .  329 
Spider,  .  332 
Trilobita,  .  341 
Cephalothrix,  .  .  .180 

Ceratiocaris,  ....  276 
Ceratites,  ....  385 
Ceratium,  .  .  .  .105 
Ceratodus,  .  .  .  .  511 
Ceratophrys,  ....  556 
Ceratospongise,  .  .  .118 
Cercarice,  .  .  .  .169 
Cerci  of  cockroach,  .  .  295 
Cercopithecinse,  .  .  .  725 
Cercopithecus,  .  .  .  725 
Cere  of  pigeon,  .  .  .  607 
Cerebellum,  ....  438 
Cerebral  hemispheres,  .  .  435 
Cerebratulus,  .  .  .  .180 
Cerianthus,  .  .  .  157 

Cervidse,  ....  697 
Cervus,  .....  697 
Cestoda,  .  .  .  .171 
Cestracion,  .  .  .  .  506 
Cestum  Veneris,  .  .  -157 
Cetacea,  ....  705 

Cetochilus,  .  .  .  .  272 
Chsetoderma,  ....  347 
Chcetognatha,  .  .  .  222 
Choetopoda,  .  .  .  187,  212 


788 


INDEX. 


Chcetopoda,  General  survey  of,    209 

Chsetopterus,  .         .         .         .211 

Chalicotherium,       .         .         .     702 

Chalina,  .         .         .         .125 

Chamseleo,      ....     578 

Chamaeleon,  the  American,     .     577 

Chamaeleonidse,       .         .         .578 

Charybdea,     .         .         .         .156 

Chelicene  of  Limulus,     .         .     338 

„  Scorpion,    .         .     329 

„  Spider,        .         .     332 

Chelifer,          .         .         .         .331 

Chelone,          ....     566 

Chelonia,        .         .         .      560,  563 

,,         Classification  of,     .     566 

„         General  characters  of,  563 

,,         Organs  of,       .         .     566 

Chelydidse,      ....     567 

Chelydra,        ....     567 

Chelys, 567 

Chemotaxis,  .  .  .  .107 
Chernes,  .  .  .  .  331 
Chevron  bones,  .  .  .  706 
Chevrotain,  ....  696 
Chiasma  of  optic  nerves,  .  484 
Chilognatha,  ....  292 
Chilopoda,  ....  292 
Chimsera,  ....  507 
Chimpanzee,  ....  726 
Chinchilla,  .  .  .  .711 
Chiromys,  ....  722 
Chiroptera,  .  .  .  .  719 
Chitin,  ....  254,  745 
Chiton,  .....  345 
Chlamydomyxa,  .  .  .  100 
Chlamydophorus,  .  .  .  690 
Chlamydosaurus,  .  .  -577 
Chlorophyll,  .  .  .  19 

Choeropus, 686 

Cholcepus,  ....  689 
Chondracanthus,  .  .  .  272 
Chondrocranium,  .  .  .  427 
Chondrosia,  .  .  .  .125 
Chordata  and  Non-chordata,  .  6 
Chordae  tendineoe,  .  .  .  673 
Chordotonal  organs,  .  ,  3 1 1 
Chorion,  .  .  .  .651 
Choroid  of  Eye,  .  .  .  446 
„  plexus,  .  .  .  436 
Chromatin,  ....  43 


Chromosomes,  ...  44 
Chrysalis,  ....  322 
Chrysaora,  .  .  .  .148 
Chrysochloris,  .  .  .718 
Chrysophrys,  ....  525 
Chrysothrix,  ....  725 
Chyle,  .  .  .  .  .24 

Chyme, 22 

Cicada,  .  .  .  .  .  305 
Cidaris,  ....  240 

Cilia, 106 

Ciliary  nerve,  .         .         .441 

„      processes  of  the  choroid,  447 

Ciliata, 105 

Cilium, 1 06 

Ciona,    .....     409 
Circulatory   system,    see    Vas- 
cular system. 

Cirri  of  Chsetopods,         .         .     209 

„       Crinoids,    .         .         .     244 

„       Lancelet,    .         .         .411 

Cirripedia,       ....     269 

Cirrus  sac,       .         .         .         .167 

Civet, 715 

Cladocera,       .         .         .         .271 

Claspers  of  Skate,  .         .         -478 

Classification  of  Animals,        .       13 

„  The  basis  of      .       10 

„  of  organs, .         .       36 

,,  grades  of,  .         .       12 

Clathrulina     .         .         .         .     100 

Clavellina,       .         .         .         .401 

Clavicle,          .         .         .      434,  664 

Clemmys,  "  Placenta  "  of,       .     591 

Clepsine,         .         .         .      208,  214 

Clidastes,        ....     593 

Clio, 358 

Clione, 125 

Clitellum  of  Earthworm,         .     189 

Clitoris, 678 

Cloaca  of  Vertebrates,     .         .461 

Clotho, 583 

Clupea, 503 

Clypeaster,  ....  240 
Cnidoblasts,  .  .  .  .136 
Cnidocil,  .  .  .  .136 

Cobra, 5^3 

Coccidium,  .  .  .  .104 
Coccus  insects,  .  .  .  3°5 
Cochlea,  ....  444 


INDEX. 


789 


Cockle, 372 

Cockroach,     .         .          293-98,  305 
Cocoon,  ....     322 

Cod, 510 

Codosiga,        .....         .104 

Ccelentera,      .         .         .         .129 

„     Classification  of,        129,  154-5 

.,     General  characters  of,    .     129 

„          „        life  of,       .         .134 

,,          „        scheme  of,         .     159 

„     History  of,     .         .  157 

„     Pedigree  of,   .         .         .158 

Cceliac  ganglia,        .         .         .671 

Ccelogenys,     .         .         .         .     711 

Coelomata  and  Coelentera,       8,  130 

„          ,  Worms  the  first,  .     161 

Ccelome,  see  Body  cavity,        .     453 

Cceloplana,     .         .         .         .164 

Ccenurus,         .         .         .  173 

Coleoptera,     ....     304 

Collagen,         ....     745 

Collembola,     ....     305 

Collozoum,      ....     103 

Colobus,          ....     726 

Colon,    .....     670 

Coloration,     ....     758 

Colour  Sense,  Theory  of,         .       28 

Colouring  matters  of  Animals,     746 

„     Protective,  among  Insects,  317 

Colubriformes,         .         .         .     583 

„  venenosi,          .     583 

Columba,        ....     599 

livia,        .         .        83,  599 

Columella  of  Vertebrate  ear,     .     445 

or  epi-pterygoid,  573,  586 

„          in  cotals,         .  154 

„  snail,  .         .     349 

Comatula,        ....     244 

Commensalism    among    Crus- 
tacea, ....     283 

„  „  Fishes,     527 

Commissures   of   Mammalian 

brain,  .         .         .     636,  668 
Complemental    males    among 

Cirripedia,    .         .         .     280 

„         Myzostomata,          .     212 

Compsognathus,      .         .         .     593 

Conchifera,  see  Bivalves,          .     362 

Conchiolin,     ....     745 

Condylarthra,          ...         .     704 


PAGE 

Cone  shells,    .... 

357 

Conjugation,  Dimorphic, 

112 

„            Multiple,    . 

112 

„             of  Paramoecium, 

92 

„             of  Protozoa, 

III 

„            of  Vorticella, 

94 

Conjunctiva,  .         . 

446 

Connective  tissue,  .         .         21 

,38 

Contractile  Vacuoles, 

86 

Contraction  of  muscle,    . 

39 

Conus,    .         .         .         . 

357 

Conus  arteriosus,    . 

454 

Convergence, 

688 

Convoluta,      ..... 

162 

Copelata  —  Larvacea, 

400 

Copepoda,       .... 

269 

Coracoid,        .         .         .     434, 

664 

Corallium,                .         .      153, 

T57 

Corals,    .                  .130,  133, 

157 

Cordylophora          .               155, 

1  60 

Corium,  .                  ... 

426 

Cornea,  .                  ... 

446 

Coronary,        .         .      431,  586, 

605 

Coronella,        .... 

583 

Corpora  adiposa,     . 

548 

Corpus  callosum,     . 

668 

,,       geniculatum, 

668 

,,       striatum, 

668 

Corrodentia,   .... 

305 

Corticata,         .... 

Q7 

Coryphodon,  .... 

.?/ 
704 

Costal  plates, 

564 

Cotylophora, 

696 

Couguar,         .... 

71"^ 

Courtship  of  Spiders, 

/    j 
334 

Cowper's  glands,     . 

678 

Coxa,      ..... 

295 

Coxal  glands  of  Limulus, 

339 

„             ,,        Peripatus, 

285 

„             ,,        Scorpion, 

330 

,,             ,,        Spider,  . 

332 

Coxopodite  of  Crayfish,  . 

255 

Coypu,    

711 

Crab,       

279 

279 

Crania,   ..... 

226 

Cranial  nerves,        .         .      441, 

483 

Craniata,         .         .         . 

425 

Craspedon  of  Swimming  bell, 

140 

Craspedota,     .         .         . 

140 

790 


INDEX. 


PAGE 

Craspedota  and  Acraspeda,          148 

i 
Cuspidaria,     .... 

>AGE 

372 

Crayfish,  The  Fresh  water,    253-269 

Cuticle,  

254 

The  Sea,  .         .         .253 

Cutis,      ..... 

426 

Creodonta,      .         .         .         •     717 

Cuttlebone,     .... 

377 

Cribrella,         ....     235 

Cuttlefish,       .... 

375 

Cribriform  plate,     .         .         .     663 

„          Characteristics  of,  . 

375 

Cricetus,          .         .         .         .711 

Cuvierian  organs,    . 

241 

Cricket,           ....     305 

Cyamidae,        .... 

277 

Crinoidea,       ....     243 

Cyanea,  ....      148, 

156 

,,         Classification  of,     .     246 

Cyanosoma,    .... 

240 

Cristatella,      .         .         .         .     225 

Cyclas,    ..... 

372 

Crocodiles,     Alligators,     and 

Cyclodus,        .... 

578 

Gavials,        .         .      585,  589 

„         "  Placenta  "  of, 

j  i 
59i 

Crocodilia,      ....     583 

Cycloid  scale, 

521 

„         Classification  of,      .     588 

Cycloporus,     .... 

163 

„         General  characters  of,  583 

Cyclops,          .... 

269 

„         History  of,      .         .     589 

Cyclostomata, 

465 

Crocodilus,      ....     588 

„        General  characters  of, 

465 

Crop  of  Earthworm,        .         .192 

Cycloturus,     .... 

690 

„     of  Leech,        .         .         .218 

1^7 

„     of  Pigeon,       .         .         .     608 

Cymbulia,       .... 

j  i 

358 

„     of  Snail,          .         .         .     352 

Cymothoa,       .... 

277 

Crossopterygii,         .         .         .     509 

Cymothoid0e,  .... 

277 

Crotalus,          ....     583 

Cynocephalus, 

725 

Crumb  of  Bread  Sponge,         .     125 

Cynoidea,        .... 

715 

Crura  cerebri,          .         .         .     439 

Cynomorph  monkeys, 

725 

Crustacea  — 

Cynomys,         .... 

711 

General  characters  of,    .     253 

Cynopterus,    .... 

721 

Habits  and  Habitats  of,      283 

Cypnea,           .... 

372 

History  of,     .         .         .     279 

Cypridina,       .... 

269 

Life  History  of,      .         .281 

Cyprina,          .... 

372 

Systematic  survey  of,     269-79 

Cypris,    .         .         . 

269 

Cryptobranchus,      .         .         .     556 

Cysticercus,    .... 

175 

Cryptoniscidse,         .         .         .     277 

Cystoidea,       .... 

246 

Cryptophialus,         .         .         .     275 

Cysts  of  Protozoa,  . 

no 

Crystalline  style,     .         .         .     367 

Cytoplasm,      .         .         . 

42 

Ctenidia,         ....     362 

Cteniza,  336 

Daphnia,         .... 

269 

Ctenodus,        .         .         .         .     511 

Dart  sac  of  Snail,    . 

356 

Ctenoid  scale,          .         .         .521 

Dasornis,         .... 

619 

Ctenophora,    .         .         .      134,  157 

Dasypodidte,  .... 

690 

Ctenoplana,    .         .         .         .164 

Dasyprocta,     .... 

711 

Cubomedusse,          .         .         .156 

Dasyproctidae, 

711 

Cucullanus,     .         .         .         .183 

Dasypus,          .... 

690 

Cucumaria,      ....     243 

Dasyuridse,      .... 

686 

Cucumerina,    .         .         .               175 

Dasyurus,         .... 

686 

Cuma,     269 

Dead  man's  fingers, 

157 

Cumacea,         ....     276 

Decapoda  (Crustacea),    . 

276 

Cunina,  .         .         .         .         .156 

,,         (Cephalopods), 

386 

Cuscus,   .         .         ...         .     687 

Decidua,          .... 

657 

INDEX. 


791 


Decidua  reflexa, 

PAGE 

.     655 

Dei 

Deer,      .... 

.     697 

> 

Degeneration, 

•      35 

Delamination, 

.      67 

, 

Delphinapterus, 
Delphinoidea, 

..     709 
.     709 

' 

Delphinus, 

.     709 

, 

Demodex, 

336 

Demospongke, 

.     116 

( 

Dendrerpetum, 

•     557 

, 

Dendrobates,  . 

•    555 

5 

Dendrocoelida, 

.     163 

Dendrocoelum, 

•     163 

j 

Dendrosoma,  .         . 

.     105 

, 

Dentalium, 

.    362 

? 

Dentary,          .         .      431, 

586,  605 

Dentition  of  Mammals,  . 

•    643 

j 

Dermal  denticles,    . 

.    478 

, 

Dermaptera,   . 

•    305 

Dia 

Dermatobranchia,   . 

•     357 

Dia 

Dermis  of  Vertebrates,    . 

.    426 

Dia 

Dermochelys, 

.     566 

Dit 

Dermoptera,    . 

.     718 

Die 

Descent,  Doctrine  of, 

81,  765 

,,       of  man,    . 

•     655 

Desmodus, 

.     721 

Die 

Desmognathge, 

.     620 

Die 

Desmognathus, 

.     556 

Die 

Desmosticha,  . 

.     240 

Die 

Desor,  Larva  of,     ... 

.     1  80 

Did 

Deutoplasm,  ... 

•      65 

Did 

Development  of  — 

Did 

„     Amphioxus,   . 

.    417 

Difl 

„     Annelids, 

.     208 

Difl 

,,     Anodonta, 

•     370 

Dig 

,,     Ascidia, 

.     407 

Dig 

,,     Balanoglossus, 
,     Chsetognatha, 

•     396 

.       222 

3  J 
»3 

,     Chick,    . 

.613-7 

Dig 

,     Clepsine, 

.     220     Din 

,     Crayfish, 

.     266     Din 

,     Crustacea, 

.     280     Din 

,     Earthworm,  . 

199    Din 

,     Echinoderma, 

247-50     Din 

,     Eye,       ... 

.     445     Din 

,     Feather, 

.     600    Die 

,     Frog,     . 

.     549    Dip 

,     Haddock, 

.     502     Dip 

,     Hair,      . 

.     641     Dip 

,     Herring, 

.     505     Dip 

138 
319 

643 
647 

183 

289 
472 
647 
206 

589 
330 
152 

493 
429 

122 
407 
421 


Development  of — 

Hydra, . 

Insects, 

Mammalian  teeth, 

Mammals, 

Nemerteans,  . 

Peripatus, 

Petromyzon, . 

Placenta, 

Polychaeta,     . 

Reptilia, 

Scorpion, 

Sea  anemone, 

Skate,   .         . 

Skull,    . 

Sponges, 

Tunicata, 

Vertebrata,     . 

Diadema,        ....     240 
>is,     .  541 

jm,    .         .         .         .674 
Dibranchiata,          .         .      385,  386 
Dichogamy  in  Cymothoidas,    .     277 
Myxine,  .         .     468 
Tunicata,         .     407 

.     3> 695 

Dicyema,        .         .         .         .127 

"ilse,  .  .  .  .  127 
Dn,  .  .  .  .  592 
a,  .  .  .  .684 
ddse,  .  .  .  686 

/s, 686 

Differentiation,        .         .         .       32 

_ 99 

Digenetic  Trematodes,  .  .170 
estion,  .  .  .22-23,  737 
Intra-cellular  in  Protozoa,  86 

,,  in  Sponges, 

jitigrade,    .... 
Dimorphism  of  sexes, 
Dinoflagellata, 
Dinophilus,    .         . 
Dinornis,         .         .         .         .619 
ia,     ....     593 
mm,  .         .         .     704 

Diodon,  .         .         .         .     511 

Diphycercal,  .         .         .         .518 
.     156 

Diphyodont  dentition,    .         .     644 
stic,  .         .         .         .116 


121 
7H 
50 
105 
223 


792 


INDEX. 


Diplopoda  =  Millipedes,  .  292 
Diplotrophoblast,  .  .  .651 
Diplozoon,  .  .  .  .170 
Dipneumones,  .  .  333,  336 
Dipnoi,  .  .  .  .  -5ii 
Dipodidae,  .  .  .  .711 
Diprotodon,  .  .  .  .688 
Diptera,  ....  304 
Dipterus,  .  .  .  .511 

Dipus, 711 

Direct  division,       ...       45 

Discoidal  segmentation,  .       65 

Discomedusae,         .         .         .156 

Discophora,    .         .         .         .213 

Distomum,      .         .         .         .164 

Distribution,  Geographical,    .     753 

,,  of  Lemurs,         .     722 

,,  of  Marsupials,    .     684 

,,  of  Tapirs,  .         .     700 

„  Geological,         .       74 

Division  of  Labour,         .         .15-16 

Dochmius,      .         .         .         .183 

Doctrine  of  Descent,       .        81,765 

Dodo, 618 

Dog, 715 

Dogfish,  ....  506 
Dogwhelk,  .  .  .  •  357 
Dolabella,  .  .  .  '357 
Doliolum,  .  .  .401,  409 
Dolphin,  ....  709 
Donkey,  .  .  .  .701 
Dorcatherium,  .  .  .  696 
Doris,  .....  357 
Dormouse,  .  .  .  .  711 
Dorsal  lamina  of  Tunicate,  .  404 
Draco,  .....  577 
Dracunculus,  .  .  .  .184 
Dragonet,  ....  520 
Dragon-flies,  ....  305 
Dreissena,  ....  372 
Drepanidium,  .  .  .104 
Drepanophorus,  .  .  .176 
Dromaeognathse,  .  .  .  620 
Dromseus,  .  .  .  .619 
Dromatherium,  .  .  .  640 
Dromia,  ....  279 

Duckmole,  .  .  .  68 1, 682 
Ductus  endolymphaticus,  .  444 
Dugong,  .  .  .  .691 
Duodenum,  .  .  .  .  670 


Duplicidentata, 
Dura  mater,    . 

Ear  of  Arenicola,    . 
, ,      Crayfish, 
,,      Myxine, 
,,      Vertebrates, 

Earthworm, 


PAGE 

.     711 

•     439 

.  203 
.  260 
.  467 
.  444 
.  187 

Development  of,  196-201 
Structure  of,  187-196 
Earwigs,  ....  305 
Ecardines,  ....  226 
Ecaudata,  ....  555 
Ecdysis  or  moulting  of  Ar- 
thropods, .  .  .  254 
Echidna,  .  .  .  .681 
Echinochrome,  .  .  .  238 
Echinococcus,  .  .  .173 
Echinoderma,  .  .  7, 227 
,,  Contrast  between  the 

classes  of,     .         .251 

,,       Development  of,        .     247 

,,       General  characters  of,    227 

,,       Larvae  of,  .         .         .     250 

,,       Relationships  of,        .     250 

Echinoidea,    ....     236 

,,          Classification  of,  .     240 

Echinorhynchus,     .         .         .185 

Echinus,  .         .         .     236 

Echiuridae,  .         .         .211 

Ectoderm,  .         .          36, 68 

Ectoplasm,  .         .         .no 

Ectoprocta,  .         .         .     225, 

Edentata,  .         .         .688 

,,         Families  of,      .         .     689 

Edriophthalmata,   .         .         .     276 

Eel, 510 

Egg  case  of  Skate,  .         .     493 

Egg  cases  of  Buccinum,  lan- 

thina,  Purpura,        .         .     356 

Eggs,  see  Ova,         ...       56 

,,      of  Birds,        .         .         .     627 

Eimeria,          .         .         .         .104 

Elaps, 583 

Elasipoda,  ....  243 
Elasmobranchii,  .  .  477, 505 
Electric  organ  of  Skate,  .  478 
,,  organs  of  Teleostei,  .  478 
Eledone,  ....  386 
Elephant's  tooth  shells,  .  .  362 


INDEX. 


793 


PAGE 

PAGE 

Elephas, 

•     703 

Epithelial  Tissue,    . 

37 

Elimination,   . 

.     769 

Equidae,           .         .         .         . 

700 

Elk  

.     698 

Equus,    .         .         .         .         . 

700 

Elpidia, 

.     243 

Erinaceus,       . 

718 

Elytra  of  Cockroach, 

•     295 

Eruciform  larva, 

323 

Embole, 

67 

Estheria,          . 

270 

Embryology,  . 

.       56 

Ethiopian  region,    . 

764 

„            Physiological, 

.      68 

Eucyrtidium   .         .         .         . 

103 

Emu,      .... 

.     619 

Eudrilini,        . 

209 

Emys,     .... 

.     567 

105 

Enchytraeidse, 

.    209 

Eulamellibranchia, 

372 

Encystation,   . 

87,88 

Eulima.           . 

360 

Endoderm, 

36,68 

Eumeces  

578 

Endolymph,   . 

.     444 

Eunice,  

2IO 

Endoplasm,    . 

.     no 

Eupagurus,     .         .         .         . 

279 

Endopodite  (crayfish),     . 

•     255 

Euphausia,      . 

278 

Endoprocta,    . 

.      200 

Euplectella,    . 

125 

Endostyle  of  Tunicates,  . 

.      404 

Eupomatus,     . 

206 

Enoplidae, 

.       183 

236 

Entalium,       . 

362 

Eurypharynx, 

511 

Enterochlorophyll, 

232,  352 

Eurypterina,  . 

340 

Enteroccele,    . 

.      472 

Eurypterus,     . 

340 

Enteron  =  gut, 

.      448 

Euscorpius,     .         .         .         . 

331 

Enteropneusta, 
Entoconcha,   .         ... 

•       391 
.       360 

Euspongia,      . 
Eustachian  tube,     . 

118 
449 

Entomostraca, 

.      269 

Eustrongylus, 

185 

Entoprocta,    . 

.       225 

Eutheria,         . 

688 

Entosternjte  of  Limulus, 

•     339 

Euthyneura,    .         .         .         . 

357 

,,             Scorpion, 

•     329 

"  Evolution," 

49 

Spider,  . 

•     332 

Evolution  of  Animals,     . 

81 

Environment,  influence  of, 

.     768 

Evidences  of, 

82 

Eolis,      . 

•     357 

Factors  in,  . 

765 

Epeira,  . 

334,  336 

Summary  of  theories  of, 

771 

Ephemeridse, 

•     305 

of  Man, 

729 

Ephyra, 

.     147 

of  Sex, 

52-53 

Epiblast, 

36,67 

Excretion  in  Animals,     . 

25-26 

Epibole, 

.      67 

Excretory  system  of  — 

Epicrium, 

•    557 

„         Amphioxus,     . 

416 

Epidermis  of  Insects, 

.    310 

„         Anodonta, 

369 

,,            Vertebrates, 

.    426 

„         Arenicola, 

206 

Epididymis  of  Skate, 

•     493 

,         Balanoglossus, 

395 

„             Rabbit,    . 

.     676 

Bee, 

306 

Epigenesis, 

•      5° 

,         Cockroach, 

297 

Epimeron  (crayfish), 

.    257 

,         Crayfish, 

264 

Epiphragm  of  snail, 

•     349 

,         Crocodilia, 

588 

Epiphyses, 

.     660 

,         Crustacea, 

280 

Epiphysis  =  Pineal  Body, 

•     437 

,         Earthworm,    . 

194 

Epipodite  of  Crayfish,     . 

.     264 

Frog,      . 

547 

Epipubic  bones,      .      680, 

682,  684 

,         Haddock, 

502 

Epistoma  (crayfish), 

.     257 

Helix,     . 

353 

794 

INDEX. 

PAGE 

PAGE 

Excretory  System  of  — 

Fertilisation,  .         .         .          54,  6  1 

,         Hirudo, 

219 

Fiber,      .         .         .         .         .711 

Insects,  . 

317 

Fibula,   434 

,         Lizards, 

576 

Fierasfer.         ....     527 

,         Mammalia, 

675 

Filaria,   .         .         .         .         .184 

,         Myxine, 

468 

Filibranchia,  ....     372 

,         Nematoda, 

182 

Fins,       .         .         .      476,  518,  520 

,         Nemerteans,    . 

179 

Fishes,  Abyssal,      .         .         .     527 

,         Peripatus, 

287 

,,       and  Amphibians  com- 

,        Petromyzon,   . 

472 

pared,     .         .         -53° 

,         Pigeon,  . 

611 

,        Colour  of,  .         .         .517 

Rabbit,  . 

675 

Commensalism  in,       .     527 

,         Scorpion, 

330 

Contrasts  between,      .     518 

,         Sea  urchin, 

239 

Fins  of,       .         .517,  520 

,         Sepia,      . 

38i 

Flat,  ....     522 

,         Skate,     . 

490 

Food  of,     .         .         .     520 

Starfish, 

234 

General  characters  of,      476 

,         Tunicates, 

406 

Hermaphrodite,  .         .     525 

,         Vertebrates,    . 

458 

Orders  of,  .           4,  475,  505 

Exopodite  (crayfish), 

255 

Parental  care  among,  .     526 

Exoskeleton  of  Vertebrates,    . 

426 

Relationships  of,          .     527 

Extinction  of  Animals,    . 

77 

Reproduction  of,          .     525 

Eyes  of  Caecilia, 

556 

Senses  of,   .         .         .     525 

Crayfish,     . 

260 

Viviparous,          .         .     526 

Cuttlefish,  . 

376 

Fission,  52 

Helix, 

35° 

Fissipedia,       .         .         .         .     713 

Insects, 

in 

Flagella,          .         .         .         .106 

Lamprey,  . 

3 

4.7O 

Flagellata,       .         .         .         .104 

Myxine, 

t/ 
467 

Flagellum  of  snail,           .         .     354 

Proteus, 

445 

Fleas,     304 

Tunicata,    . 

445 

Flies,      304 

Vertebrata, 

445 

Flight  of  Birds,       .         .         .625 

,,             Develop- 

Floridine in  Sponges,     .         .     121 

ment  of, 

447 

Floscularia,     ....     223 

Flounder,        .         .         .         .510 

Facial  nerve,  . 

441 

Flustra,  .         .         .         .              225 

Faeces,             . 

23 

Flying  Foxes,          .         .         .721 

Fallopian  tube, 

678 

Lemur,         .         .         .     637 

Fangs  of  Ophidia,  . 

580 

,       Mammals,    .         .         .     637 

Fasciola,          . 

164 

,       Phalangers,           .         .     687 

Fat  body  of  Insects, 

3J3 

,       Squirrels,     .         .         .     711 

Frog,  . 

548 

Fcetal  membranes  of  Birds,     .     616 

Feathers  of  Pigeon, 

599 

,                ,,                Mammals,  653 

,,        Development  of, 

600 

,               ,,               Reptiles,     590 

Feather  stars,           .         ; 

243 

Follicle  cells,           .         .         .     463 

Felidse,  .         .         . 

7H 

Fontanelles   in    Skull    of   the 

Felis,      

7H 

Skate,  .         .         .         -477 

Femur,    . 

434 

Food  vacuoles,        ...       86 

Fenestra  ovalis, 

445 

Foot  of  Anodonta,           .         .     363 

Ferments, 

22,  28 

„       Cephalopoda,     .         .     376 

INDEX. 


795 


PAGE 

Foot  of  Dibranchiata,      .         .     385 
„       Gasteropoda,      .         .     348 
Molluscs,   .         .         -343 
„       Scaphopoda,       .         .     362 
„       Tetrabranchiata,          ,     385 
Foramen  of  Munro,         .         .     668 
,,          Panizzae,       .         .     587 
Foraminifera,           .         .         .100 
Fore  brain  =  Prosencephalon,       435 
„             Roof  of,  in  Ver- 
tebrates,        .     436 
Fore  gut  =  Stomatodaeum,        .     449 
Fornix,   .....     636 
Fossils,   .....       75 
Fovea  centralis,      .         .         .     448 
Fox,        715 

PAGE 

Gasteropoda,  Life  history  of,  .     360 
Mode  of  life  of,.     359 
„             Parasitic,  .         .     360 
„             Past  history  of,       361 
„             Symmetrical,     .     345 
„             Torsion  of,         .     359 
Gasterosteus,  ....     520 
Gastornis,        .         .         .         .619 
Gastrsea,          .         .         .         31,  67 
Theory,    ...       69 
Gastric  Juice,          ...       22 
Mill  of  Crayfish,          .     261 
„       Mill  of  Crustacea,        .     280 
Gastroliths  of  Crayfish,  .         .261 
Gastro-  vascular  canals,   .         .144 
Gastrula,         .         .         .         31,  67 

Fresh-water  Fauna,         .         .     756 
„           Mussel,       .         .     363 
Fritillaria,       ....     408 
Frog,       530 
Fuligo,   .....       98 
Functions,  Change  of,     .         .       34 
,,          of  Animals,    .         .       15 
„           Secondary,  of  organs,  34 
Fungia,  ....      154,  157 

Gad  fly,           .         .    '     .         .     304 

,,         of  Vertebrates,          .     464 
Gavials.           ....     589 
Gazella,           ....     698 
Gecarcinus,     ....     279 
Geckos,           ....     577 
Gelasimus,     ....     279 
Gemmation  =  Budding,   .         .       52 
Gemmules  in  pangenesis,        .       71 
,,         of  Spongilla,  .         .     121 
Genealogical  tree,  .         .         .11 
Genetta,          .         .         .         .     715 

Gadus,    495 
Galago,  722 
Galea,     ....     295,  308 
Galeodes,        .         .         .         .     331 

Geodesmus,    .         .         .         .163 
Geodia,           .         .         .         .125 
Geographical  areas,         .         .     762 
,,            Distribution,      .     753 

Galeopithecus,         .         .         .718 
Galesaurus,     ....     592 
Galithea,         ....     279 

Geological  Record,          .         .       74 
Geomys,          .         .         .         .711 
Geophilus,      .         .         .         .291 

Gall  bladder,  ....     670 
Gall  fly  304 
Galls  on  Plants,      .         .         .     336 
Gamasus,        ....     336 
Gammarus,     ....     278 
Ganglion,        ....       40 

Geoscolecini,  ....     209 
Geotria,           .         .         .         .473 
Gephyrea,       ....     224 
Germ  cells,     .         .         .         .51 
,,     plasm,   .         .         72,  132,  156 
Germinal  variations,         .         .     768 

Ganodichthyidae,     .         .         .     528 
Ganoid  scales,         ,         .      518,  521 
Ganoidei,        ....     507 

,,        vesicle,    ...       59 
Geryonia,        .         .         .      132,  156 
Gharials  —  Gavials,           .         .     589 

Garden  Spider,        .         .         .     336 
Gaste  opoda,  .         .         .      345,  347 
Asymmetrical,  .         .     357 

"  Giant  fibres,"       .         .         .191 
Gibbon,           ....     726 
Gill  clefts,       .         .         .         -449 

Classification  of,        .     357 
Food  of,  .         .         .     360 
General  characters  of,  347 
,,        notes  on,     .     358 

,,  slits  of  Balanoglossus,      .     395 
Gills  of  Amphibians,       .         .     557 
„         Anodonta,           .         .     369 
„         Crayfish,    .         .         .     263 

796 


INDEX. 


PAGE 

Gills  of  Crustacea,  .         .     280 

„  Fishes,  .  .  .  476 
„  Gasteropoda,  .  .  357 
„  Polycha3ta,  .  .  205 
„  Sepia,  .  .  .381 

Giraffe, 698 

Gizzard  of  Cockroach,     .         .     296 

,,         Crayfish,         .         .261 

,,         Earthworm,   .         .192 

,,         Pigeon,  .         .     608 

Glandiceps,     ....     392 

Glands  of  Crocodiles,      .         .     587 

„         Dipnoi,  .         .         -515 

„         Insects    .         .         -313 

„         Mammals,       .      635,  642 

„         Myxine,  .         .     466 

Glass  rope  sponge, .         .         .125 

,,     snake,  ....     577 

Globe  fishes,  .         .         .         .511 

Globicephalus.         .         .         .     709 

Globigerina,   ....     101 

Glochidium  of  Anodonta,        .     371 

Glossopharyngeal  nerve,          .     441 

Glossophora,  ....     388 

Glycogen,        .         .         .       24,  255 

Glyptodon,      ....     690 

Gnathia,          ....     277 

Gnathobdellidse,     .         .         .221 

Gnathostomata,       .         .         .     465 

Gnats,     .         .         .         .         .     304 

Gonads  =  Reproductive  organs, 

36,  171 

Gonapophyses  of  Cockroach, .  295 
Goniaster,  .  .  .  229, 235 
Gonophores,  .  .  .  142,  155 
Gordiidce,  .  .  .  .185 
Gordius,  .  .  '«-  .  .185 
Gorgonia,  .  .  .  157 

Gorgonocephalus,  .  .  .  236 
Gorilla,  .....  726 
Graafian  follicle,  .  .  .  463 
Graafilla,  .  .  .  .162 
Grampus,  ....  709 
Grantia,  .  .  .  .125 

Graptolites,  .  .  .  .158 
Green  gland  of  Crayfish,  .  264 
Gregarina,  ....  85 
Grey  matter  of  brain,  .  .  439 
Gromia, .....  101 
Guinea  pig,  .  .  .  .711 


Gunda,  . 

Gymnoblastic, 

Gymnomyxa, 

Gymnophiona, 

Gymnosomata, 

Gymnotus, 

Gynsecophorus, 

Gyrodactylus, 

Haddock,        . 
Hsemadipsa,    . 
Hsematin, 
Hsemerythrin , 
Hsemocoele,    . 
Haemocyanin, 
Haemoglobin  of  blood, 
Hciemopsis, 


PAGE 

.         •     163 

•  155 
.       97 

.         .  556 

•         •  358 

.  478 

.  170 

.  170 

•  495 
.  213 

•  747 

•  747 
.  262 

.  280,  747 
25,  26,  747 
214 


Hagfish,          .         .         .         .465 
Haimea,          .         .         .  153 

Hair, 641 

,,  worms, ....  180 
Halichoerus,  .  .  .  •  7J7 
Halichondria,  .  .  .125 
Halicore,  .  .  .  .691 
Halicryptus,  ....  224 
Haliotis,  ....  357 
Haliphysema,  .  .  .  102 
Halisarca,  .  .  .  .125 
Halitherium,  ....  692 
Halobates,  .  .  .  325,  753 
Hamadryad,  .  .  .  .  583 
Hamster,  .  .  .  .  711 

Hapale, 725 

Hapalidre 724 

Harderian  gland,    .         .         .     448 

Hare, 711 

Harelip,  .         .         .     477,  659 

Harvest  bugs,          .         .         .     331 
men,  .         .         -331 

,,       mites,         .         .         .     336 
Hastigerina,    .         .         .         .101 
Hatteria,          ....     568 
Heart  of  Vertebrates,      .         .     454 
Hectocotylus  of  Cuttlefish,      .     383 
Hedgehog,  Dentition  of,          .     718 
„          Development  of,  .     650 
„          Placenta  of,  .      714,  718 
Heliopora,       .         .         .         .153 


Heliozoa, 
Helix, 


100 

348-57 


INDEX. 


797 


349 
348 
349 
348 
577 
340 
240 

577 

322 

305 

262 

354 

54 

173 


•Helix,  External  appearance  of, 
„       Mode  of  life  of, 
„       Shell  of, 
„       Structure  of, 
Heloderma, 
Hemiaspis, 
Hemiaster, 
Hemichorda, 
Hernidactylus 
Hemimetabolic  Insects, 
Hemiptera, 
Hepatopancreas, 
Heredity,         .         .         . 
Hermaphrodite  duct  of  Snail, 
Hermaphroditism,  .. 
ofCestodes, 

Chaetognatha,  222 
Cirripedia,  .  280 
Crustacea,  .  280 
Cymothoidae,  277 
Earthworm,  194 
Frog  tadpole,  555 
Leech,  .  219 
Myxine,  .  460 
Myzostomata,  212 
Tardigrada,  337 
Trematoda,  1 70 
Tunicata,  .  407 
Turbellaria,  162 
Invertebrates,  463 
Hermione,  .  .  .  .210 
Herpestes,  .  .  .  .  715 
Herring,  .  .  .  503,  510 
Hesperornis,  .  .  .  .619 
Heteroceras,  ....  385 
Heterocercal,  .  .  .518 

Heterocoela,  .  .  .  .125 
Heterodera,  .  .  .  .185 
Heterodont  dentition,  .  .  645 
Heteromya,  ....  372 
Heterophrys,  .  .  .  .100 
Heteropods,  ....  357 
Heterotricha,  ...  105 

Hexacoralla,  .         .         .  157 

Hexactinellida,  .  .  .116 
Hexapoda  =  Insects,  .  -313 
Hind  gut  =  Proctodseum,  .  449 
Hippocampus,  .  .  .511 
„  of  brain,  .  .  668 

Hippolyte,      ....     279 


Hippopotamus,  .  .  .  693 
Hippotherium,  .  .  .701 
Hippurites,  ....  374 
Hirudinea,  .  .  .  .213 
„  Classification  of,  .  221 
Hirudo,  .  .  .  .  .214 
„  Structure  of,  .  214-21 
Histology,  ...  30,  37 
Histriodrilus,  .  .  .  .212 
Hoatzin  — Opisthocomus,  .  618 
Holoblastic  segmentation,  .  65 
Holocephali,  ....  507 
Holophytic,  ....  95 
Holopneustic,  .  •  .  .314 
Holopus,  ....  244 
Holothuria,  ....  243 
Holothuroidea,  .  .  .  240 
„  Classification  of,  243 

Holotricha,  ....  90 
Homarus,  .  .  .  253,  278 
Homo,  ....  728-30 
Homocercal,  .  .  .  .518 
Homocoela,  .  .  .  -125 
Homodont  dentition,  .  .  645 
Homology  of  organs,  .  .  33 
Homoplastic,  33 

Honeycomb  bag,  .  .  .  697 
Hoplonemertea,  .  .  .180 

Horns, 641 

Horse, 700 

Horse-shoe  Crab,  .  .  -337 
House  fly,  ....  304 
Howling  monkeys,  .  .  725 
Huanaco,  ....  696 
Humerus,  ....  434 

Hyaena, 715 

Hyaenodon,     .         .         .         .717 

Hyalea, 358 

Hyalonema,    .         .         .         .125 

Hydatina,        ....     223 

Hydra,    .         .  32,  3^,  131,  134 

„       Budding  of, .         .         .       51 

,,       Development  of,  .         .138 

„       General  life  of,     .         .134 

„  ,,       structure  of,     .     135 

„       Minute  „  .     136 

„       Muscle  system  of,          .       38 

,,       Physiology  of,       .         .       15 

„       Reproduction  of,  .       49,  138 

Hydrachna,     ....     336 


798 


INDEX. 


p 

AGE 

PAGE 

Hydractinia,    .         .         .      131, 

155 

Incus  of  Ear,  ....     445 

Hydra-tuba  of  Aurelia,    . 

147 

Indirect  development,     .         .     228 

Hydrochoerus, 

711 

„        division,     ...       45 

Hydrocorallinse, 

156 

Indris,     .....     722 

Hydroid  colonies,    . 

130 

Infundibulum,          .         .         .     436 

Hydromedusae,        .       129,  140, 

155 

Infusoria,         ....       85 

Hydrophis,      .... 

583 

„         Ciliary  movement  in,    106 

Hydrophora,  .... 

155 

Ink  bag  of  Cephalopods,          .     378 

Hydropotes,    .... 

698 

Innominate  Bone,    .         .         .     666 

Hydrozoa,       .         .                133, 

155 

Insecta,  ....      285,  292 

„          Types  of,        .      140, 

155 

,,       Classification  of,            304-5 

Hydrozoon    and    Scyphozoon 

,,       General  characters,     .     293 

contrasted,   . 

148 

,,             ,,        life  of,   .         .     324 

Hyla,       

556 

Insectivora,     .         .         .         .     717 

Hylobates,       .... 

726 

Insects  and  flowers,         .         .     325 

Hylodes,          .... 

556 

„       History  of,           .         .     326 

Hymenaster,   .... 

235 

Injurious,    .         .         .     325 

Hymenocaris, 

276 

„       Pedigree  of,         .         .     326 

Hymenoptera,          .         .      298, 

3°4 

Instinct,           ....     629 

Hyoid  arch  of  Vertebrates, 

43i 

Integration  of  individual,         .       32 

Plyo-mandibular  arch,     . 

431 

Integripallia,  ....     372 

Hyostylic,        .... 

534 

Intestine,         ....     448 

Hypapophysis, 

66  1 

Intracellular  digestion,     22,  86,  121 

Hyperia,          .... 

278 

Invagination,  ....       67 

Hyperolius,     .... 

555 

Invertebrata,  Classes  of,           .    6-10 

Hypoblast,      ...          36 

,67 

,,            and  Vertebrata,  .         6 

Hypodermis  =  Epidermis, 

254 

Iris,         446 

Hypophysis,    .... 

436 

Ischium,           ....     434 

Hypostome,     .... 

135 

Isis,                                                   ic  7 

Hypotricha,     .... 

105 

Isolation,         ....     770 

Hypural  bone, 

497 

Isomya,  372 

Hyracoidea,    .... 

702 

Isopleura,         ....     345 

Hyracotherium, 

701 

Isopoda,           ....     269 

Hyrax,    ..... 

702 

Isotrypsin,       ....     739 

Hystricomorpha, 

711 

Iter,         438 

Hystrix,  ..... 

711 

Ixodes,    .....     336 

lanthina,          .         ." 

357 

Jackal,    715 

Ichneumon,     .... 

7i5 

Jacobson's  organ,    .         .         .     669 

Ichthyomyzon, 

473 

Jaguar,    715 

Ichthyopsida,  Sauropsida  and 

Jellyfish,          .         .         .130,  142 

Mammalia  contrasted, 

56i 

Jerboa,    711 

Ichthyopterygia, 

593 

Jugal,      .         .         .  '        .     429,  662 

Ichthyopterygium  of  Fishes,    . 

520 

Julus,      292 

Ichthyosaurus, 

593 

Idant,      ..... 

60 

Kangaroo,       ....     687 

Idotea,    ..... 

277 

Karyokinesis,           ...       45 

Iguana,    ..... 

577 

Katabolism,    ....       28 

Iguanodon,      .... 

593 

Keber  s  organ,         .         .         .     368 

Ilium,      .         ... 

434 

Keratin,          ....     745 

INDEX. 


799 


PAGE 

PAGE 

Kidney,  Functions  of, 

25 

Larva  of  Sea  urchin, 

240 

,,       of  Vertebrates,  . 

.     458 

,,         Star  fish,  . 

234 

King  crab, 

•     337 

Larvacea,        .         .         .     400, 

408 

Kiwi,      .... 

.     619 

Larynx,  .         .         .      428,  534, 

675 

Kolga,    .... 

•     243 

Latax,    .         .         .         .         . 

716 

Kowalevskia, 

.     408 

Lateral  line  system,         .     495, 

525 

Laurer-Stieda  canal, 

167 

Labial  Cartilages  of  Skull, 

.     482 

Layers   of  Ccelomata  and 

„       Palp  of  Cockroach, 

•     295 

Hydra  contrasted, 

138 

„           „         Insects,    . 

.     308 

,,     The  germinal, 

36 

Labium  of  Insects, 

•     308 

Leech,    .         . 

213 

Labrum  of  Insects, 

.     295 

Lemming,       .... 

711 

Labyrinth  of  Ear,   . 

.     444 

Lemur,  ..... 

/  *  * 

721 

Labyrinthodontia,  . 

•     557 

Lemuridse,      . 

721 

Labyrinthula, 

IOO 

Lemuroidea,  .... 

721 

Labyrinthulidea, 

.      99 

Lens  of  Eye,  .... 

446 

Lacerta, 

^7o 

Leopard,          .... 

714. 

Lacertidse, 

j  i 

.     578 

Lepas,  .          .... 

/    t 
269 

Lacertilia, 

560,  569 

Lepidoptera,  .... 

304 

Lace-winged  flies,  . 

•     304 

Lepidosiren,  .         .         .      511, 

5l6 

Lachrymal  gland,   . 

.     448 

Lepidosteus,  .... 

508 

Lacinia,  .... 

.     308 

Lepisma,         .... 

^oc; 

,,         of  Cockroach,    . 

•     295 

Leporidse,       .... 

j^j 
711 

Lacteals, 

-       23 

Leptocardii  =  Cephalochorda, 

410 

Lactic  acid,     . 

21 

Leptodera,      .... 

183 

Lagena,  .         .         .         . 

IOI 

Leptodiscus,   .... 

105 

,,       =  Cochlea, 

.     444 

Leptodora,      .... 

271 

Lagomys, 

.     711 

Leptomedusse, 

i55 

Lambdotherium,     . 

.     702 

Leptoplana,    .... 

163 

Lamellibranchiata, 

.     362 

Leptostraca,   .... 

269 

Lamprey, 

•     469 

Lepus,    ....     657, 

711 

Lamp  shells,  . 

•     225 

Lernrea,  ..... 

272 

Lancelet,         . 

410 

Leucocytes  of  blood,       .      453, 

7^1 

Langda,  .... 

.     176 

Leucon  type  of  sponge,  . 

/  j* 
118 

Language, 

.     626 

Leucones,        .... 

125 

Lanice,  .... 

.       211 

Leucosolenia, 

I25 

Larva  of  Amphibians, 

.       551 

Lice,       ..... 

w; 

,         Anodonta, 

•       371 

Life  History  of  Amoeba, 

O^J 

87 

,         Antedon, 

.       246 

,,             Anodonta, 

370 

Ascidian, 

.      408 

„             Aurelia, 

146 

Aurelia,    . 

146 

„             Cestodes,     171, 

175 

Chsetopods, 

.       208 

,,             Crustaceans,   . 

282 

Crustaceans,     . 

281-2 

,,             Distomum, 

167 

Holothuria, 

•       243 

Frog,       . 

549 

Insects,     . 

.       322 

„             Gregarina, 

88 

Lamprey, 

-     473 

Hydra,    . 

138 

Molluscs, 

•     345 

,,             Insects,  . 

321 

Nemerteans,     . 

.     1  80 

„             Monocystis,     . 

90 

Ophiuroids, 

.     236 

„             Nematodes 

183 

Polygordius, 

.     208 

„             Paramcecium, 

92 

8oo 


INDEX. 


PAGE 

PAGE 

Life  History  of  Spongilla, 

122 

Loligo,    .                           .         .     386 

„             Teenia,    . 

171 

Lophiodontidae,                .         .     702 

,,             Trichina, 

184 

Lophobdella,  .                  .         .214 

„             Tunicates, 

407 

Lophobranchii,                 .         .     511 

„             Vorticella, 

94 

Lophogaster,  .                  .         .     278 

„             Volvox,  . 

95 

Lophophore,  .                   .         .     226 

Ligula,    .         .         .         .171, 

295 

Lophopus,       .                  .         .     225 

Lima,      ..... 

372 

Loxosoma,       .                  .         .     225 

Limax,   ..... 

358 

Lucernaria,     .                  .      148,  156 

Limbs  and  Girdles  of  — 

Lucifer,  .         .                  .              278 

Chelonia, 

565 

Lucina,  .         .                  .              372 

Crocodilia, 

586 

Luidia,    .                           .              235 

Frog  

534 

Lumbricini,     .                  .         .     209 

Haddock, 

499 

Lumbricus,      .                  -187,  209 

Lizards,   .... 

573 

Lung-books  of  Scorpion,      330,  333 

Pigeon,    .... 

605 

Lungs,    450 

Rabbit,    .... 

664 

,,      and  Air-bladder,  .         .     450 

Skate,      .... 

482 

,,      Function  of,         .         .       25 

Theories  as  to  origin  of,     . 

52i 

Lutra,     716 

Limnadia,       .... 

270 

Lycaon,           .         .         .         .715 

Limnoeus,        .         .         .169, 

358 

Lycosa,  336 

„           Movement  of, 

359 

Lymnaeus  =  Limnaeus,     .         .     358 

Limnocodium, 

1  60 

Lymph,  .         .         .                 23,  741 

Limnoria,        .... 

277 

,,      hearts,            .      .         -545 

Limpet,  ..... 

357 

Lymphatic  system  of  Frog,     .     545 

Limulus,          .... 

337 

,,                 „     Rabbit,       .     674 

Lineus,    ..... 

176 

„                 „     Vertebrates,    457 

Linguatulida,  .... 

336 

Lingula,          .         . 

226 

Macacus,         ....     726 

Liodon,  ..... 

593 

Machairodus,           .         .         -715 

Lion,      ..... 

7H 

Macrauchenia,         .         .         .     702 

556 

Macrobdella,  .         .         .         .213 

Lipocephala,  .... 

362 

Macroclemmys,       .         .         .     567 

Lipochrome  pigment,      .      121, 

748 

Macromere,    .         .         .         •     417 

Lithobius,       .... 

292 

Macronucleus          .         .         .112 

Lithodes,         .... 

279 

Macropodidae          .         .         .     687 

Lithodomus,   .... 

372 

Macropus,                .         .         .     687 

Littoral  life,    .... 

755 

Macrorhinus,           .         .         .     717 

Littorina,         .... 

357 

Macroscelides          .         .         .718 

Liver,  Functions  of,        .          23 

,  24 

Madrepora,               .         .               157 

„       of  Vertebrates, 

45i 

Madreporaria,          .         .      153,  157 

Liver  Fluke,  .... 

164 

Madreporic  plate  of  Brittle  star,  235 

Lizards,            .... 

569 

,,             ,,      Sea  urchin,  238 

„        Classification  of, 

577 

,,             ,,      Starfish,        232 

„        General  characters  of, 

569 

Mseandrina,    .         .         .         .     157 

Llama,    ..... 

696 

Magosphaera,           ...       99 

Lobosa,  .         .         .         .          86 

,99 

Maia,      279 

Lobster,           .... 

253 

Malacobdella,          .         .         .180 

Lobworm,       .... 

201 

Malacostraca,          .         .         .     269 

Locust,   ..... 

305 

Malapterurus,  Electric  organ  of,  478 

INDEX. 


801 


PAGE 

Malar  or  Jugal,       .         .      429,  662 
Malleus  of  Ear,       .         .         .     445 
Malpighian   body    of    kidney 
tubule,          .         .         .     459 
,,     tubules  of  Cockroach,    297 
,,           ,,          Scorpion,  .     330 
Spider,       .     332 
Mammalia,     ....     632 
Development  of,    .         .     647 
General  characters  of,   .     634 
,,       classification  of,    634 
„       life  of,      .         .     637 
,,       survey  of,          .     632 
History  of,    .         .         .     640 

PAGE 

Maxillae  of  Insects,          .         .     308 
,,           Myriopods,   .         .291 
Maxillipedes  of  Crayfish,         .     256 
May-flies,        ....     305 
Meckel's  cartilage,          ,         .431 
Medulla,         .         .         .         .438 
Medullary  canal,    .         .         .     434 
groove,  .         .         .434 
Medusae,          .         .         .         .147 
Medusoids,     .         .         .      130,  140 
Medusomes,    .         .         .         .156 
Megachiroptera,      .         .         .721 
Megalobatrachus,   .         .         .     556 
Megalopa,       .         .  '                .     282 

Orders  of,      .         .         .     679 
Sub-classes  of,        2,  679,  680 
Mammary  Glands,           .     642,  68  1 
Mammoth,      ....     704 
Manatee  (Manatus),        .         .691 
,,         Vertebrae  of,    .         .     692 
Mandible  of  Crayfish,     .         .201 
,,           Insects,       .         .     307 
,,           Myriopods,          .     292 
Mandibular  arch  of  Vertebrates,  428 
Mandril,          ....     725 
Manidae,          ....     690 

Megalosaurus,         .         .         .     594 
Megaptera,     ....     709 
Megascolides,          .         .         .     209 
Megatheriidae,         .         .         .     690 
Megatherium,          .         .     640,  690 
Meibomian  gland,  .         .         .     448 
Meles,    716 
Melicerta,       ....     223 
Mellivora,       .         .         .         .716 
Membrane  bones,  .         .         .     662 
Membranipora,       .         .         ,     225 
Menobranchus,        .         .              556 

Manis,    .         .         .         .              690 
,,      Vertebrae  of,         .         -635 
Mantle  of  Molluscs,         .         .     343 
Manubrium  of  Swimming  bell,    140 
„             Sternum,          .     66  1 
Many-plies,    ....     697 
Margelis,         .         .         .         .     155 
Marmosets,     ....     724 
Marmot,          .         .         .         .710 
Marsipobranchii,   see    Cyclos- 
tomata,         .         .         .     465 
Marsupial  bones,    .         .      682,  684 
Marsupials,     ....     684 
„         Families  of,     .         .     685 
„         General  characters  of,  684 
Marsupites,    ....     246 
Marten,           .         .         .         .716 
Mastigamceba,        .         .         .105 
Mastodon,       ....     704 

Menognatha,            .         .      304,  305 
Menorhyncha,         .         .         .     305 
Mento-meckelian,  .         .         .     534 
Mentum  of  Cockroach,  .         .     295 
,,          Insects,         .         .     308 
Mephitis,        .         .         .         .716 
Mermaid's  gloves,  .         .         -125 
purse,    .         .         -493 
Mermis,          .         .         .         .183 
Meroblastic  segmentation,       .       65 
Merostomata,          .         .         .     340 
Mesencephalon,      .         .         .     438 
Mesenchyme  cells,           .          38,  67 
Mesenteries  of  sea  anemone,  .     150 
Mesenteric  filaments,      .         .     151 
Mesenteron,   ....     448 
Mesentery,      .         .         .     487,  670 
Mesoderm  (or  Mesoblast),       36,  67 
„          segments,       .         .     430 

Mastodonsaurus,     .         .         .     557 
Mastoid  process,     .         .         .     663 
Maxilla  of  Vertebrate  Skull,   .     431 
Maxillae  of  Cockroach,    .         .     295 
,,          Crayfish,        .         .     256 
51 

Mesoglcea  of  Ccelentera,         .     130 
Sponges,    .         .116 
Mesonephros,          .         .         .     459 
„             in  the  different 
Vertebrate  groups,        .     461 

802 


INDEX. 


Mesosoma  (of  Scorpion),  .  320 
Mesozoa,  ..  .  .  .126 
Metabola,  ....  304 
Metabolism,  ....  14 
,,  products  of,  .  743 

Metacarpals,  ....  434 
Metagenesis,  or  Alternation  of 

Generations,  .  .  55 
Metagnatha,  .  .  .  304,  305 
Metakinesis,  ....  46 
Metamere,  ....  419 
Metamorphosis  of — 

Anodonta,        .         .         .     371 

Crustacea,        .         .         .281 

Echinoderma,  .         .     250 

Frog,       ....     552 

Insects,    .         .         .         321-24 

Lamprey,         .         .         -473 

Tunicata,          .         .         .401 

Metanephros,  .         .         .     459 

,,  in  the  different 

Vertebrate  groups,        .     461 

Metapleural  fold,    .         .         .411 

Metasoma  (Scorpion),     .         .     329 

Metatarsals,    ....     434 

Metatheria,     ....     684 

Metazoa,         ....         9 

„         and  Protozoa,  .  115 

Metencephalon,      .         .         .     438 

Microchiroptera,     .         .         .721 

Microgromia,  .         .         .     101 

Microhydra,    .         .         .      140,  160 

Micromere,     .         .         .         .417 

Micronucleus,          .         .         .112 

Micropyle,      ....     298 

Microstoma,   .         .         .         .162 

Microzooids,   ....       94 

Midas,    .....     725 

Midge, 304 

Mid  gut  =  Mesenteron,  .  .  448 
Miliolina,  ....  101 
Milk,  .,  ...  642 
Millepora,  .  .  .  .156 
Millipedes,  .  .  .  285, 291 

Milt, 526 

Mites, 336 

Mitosis,  ....       45 

Moa,  .....  619 
Modiola,  ....  372 
Moina,  .....  269 


PAGE 

Molars,  .....     646 

Mole, 718 

Mole  Cricket,  .  .  .  305 
Mollusca,  ....  343. 
Classification  of,  .  388 
General  characters  of,  343 
,,  Notes  on,  .  387 
Life  history  of,  .  .  390 
Shell  of,  .  .  .  389 
Molluscoidea,  .  .  .  224 
Moloch,  ....  577 
Monachus,  .  .  .  .  717 
Monaxonida,  .  .  .  .116 
Monera,  ....  109 
Moniligastres,  .  .  .  209 
Monitor,  .  .  .  .  578 
Monkeys,  .  .  .  .  725 
Monobia,  ....  98 
Monocystis,  ...  88,  104 
Monocyttaria,  .  .  .103 
Monodon,  ....  709 
Monogenetic  Trematodes,  .  170 
Monomeniscous  eyes,  .  .  311 
Monomya,  ....  372 
Monostomum,  .  .  .170 
Monotremata,  .  .  .681 
Monoxenia,  .  .  .  153,  157 
Moose,  ....  698 

Morphology,  .         .         .  3O-31 

Morse,  .         .         .         .716 

Morula,  ....       65 

Mosasauria,  ....  593 
Moschus,  ....  698 
Mother  of  pearl,  .  .  .  389 

Moths, 304 

Moulting  of  cuticle  of  crayfish,    255 

Mouse,   .         .         .         .         -7ii 

Mouth,  Origin  of  Vertebrate,       449 

, ,       of  Lopadorhynchus,    .     449 

Mucous  canals  of  Skate,          .     487 

,,      glands  of  Myxine,         .     466 

Snail,    .      349,  356 

Mud  Fishes,  .         .         .         .     511 

Miillerian  duct,       .         .         .     462 

,,  „         in   different 

Vertebrate  groups,  .  461 
Multiple  conjugation,  .  .112 
Multituberculata,  .  .  .  688 
Mungoose,  ....  592 
Murex, 357 


INDEX. 

8o3 

PAGE 

PAGE 

Muridse, 

711 

Mygale,           .... 

333 

Mus,       .... 

.        711 

Myodes,          .... 

711 

Muscle,  Contraction  of,  . 

39 

Myogale,         .... 

/ 

717 

,,       Fibres, 

21,39 

Myomeres  or  Myotomes, 

i  *•  § 

413 

,,       markings, 

•       39 

Myomorpha,  .... 

711 

,,       Smooth, 

21,  39 

Myopotamus, 

711 

,,       Striped,      . 

21,  39 

Myotomes,      .... 

413 

Muscular  system  of  — 

Myoxidae,        .... 

711 

Amphioxus, 

.     413 

Myriopoda,     .         .         .     285, 

291 

Anodonta, 

•     365 

Myrmecobius, 

686 

Arenicola, 

.      202 

Myrmecophagidse,  . 

690 

Ascidian, 

.      402 

Myrmecophagus,     . 

690 

Aurelia,  . 

143 

Mysis,     ....     269, 

282 

Balanoglossus, 

•       392 

Mystacoceti,   .         . 

709 

Birds,       . 

.      600 

Mytilus,           .... 

372 

Cockroach, 

.       294 

Myxidium,      .... 

104 

Crayfish, 

•      257 

Myxine,           .... 

465 

Crocodilia, 

•       587 

,,        and  Petromyzon  con- 

Earthworm,    . 

.       189 

trasted, 

473 

Frog;, 

^36 

Myxospongise, 

Haddock, 

.     496 

Myzostomata, 

212 

Helix,      .... 

•     350 

Hirudo,  .         '. 

.     216 

Nais,       

209 

Insects,    . 

.     310 

Naja,       ..... 

583 

Mammals, 

.     659 

Narcomedusse, 

I56 

Myxine,  . 

.     466 

Narwhal,         .... 

709 

Peripatus, 

.     287 

Nasal  Capsule, 

431 

Petromyzon,     . 

•     47o 

Nasalis,            .... 

726 

Pigeon,    . 

.     600 

Naso-buccal  groove, 

487 

Rabbit,    . 

•    659 

Naso-palatine  canal, 

669 

Sepia, 

376 

Natural  Selection,  . 

769 

Skate, 

.     478 

Nauplius,         .... 

269 

Snakes,    . 

-    579 

Nautilus,         .         .         .     383, 

385 

Starfish,  . 

.    230 

Nearctic  region, 

764 

Vertebrates,     . 

.    426 

Nebalia,          .... 

269 

Muscular  tissue, 

38 

2O9 

,,         activity  in  animals, 

21 

Necturus,        . 

556 

Musk  deer,     . 

.      698 

Nemathelminthes,  . 

1  86 

,,      glands, 

.      698 

Nematocysts, 

136 

,,      rat, 

•       711 

Nematoda,      .         .         . 

1  80 

Mussel,  Edible, 

•       372 

Nemertea  or  Nemertines, 

175 

„       Horse, 

•      372 

,,         Affinities  of,  to 

Mustela, 

.       716 

Vertebrates,    180, 

425 

Mustelidse, 

.       716 

,,         Classification  of, 

1  80 

Mustelus, 

.       506 

Nemertes,       . 

175 

„         "  Placenta  "  of, 

Neo-crinoidea, 

246 

Mya,       .... 

.       372 

Neomenia,       .... 

347 

Mycetes, 

•       725 

Neornithes,     .... 

2 
617 

Mycetozoa, 

.         98 

Neotropical  region, 

764 

Myelencephalon,     . 

•    439 

Nephelis,         .... 

214 

INDEX. 


PAGE 

PAGE 

Nephridioblast, 

.     199 

Nervous  System  of  — 

Nephridia  of  Crayfish,     . 

.     264 

Myxine,  .... 

466 

Earthworm, 

.     194 

Nematoda, 

181 

Gasteropoda, 

345, 

Nemerteans,    . 

176 

354,  357 

Peripatus, 

287 

Leech, 

.     219 

Petromyzon,    . 

470 

Mussel, 

•     369 

Pigeon,    .... 

606 

Peripatus, 

.     288 

Rabbit,    .... 

667 

Vertebrates, 

.     458 

Rotifers,  .... 

223 

253 

Scorpion, 

329 

Nephrostoma  of  kidney,  . 

•    459 

Sea-urchin, 

237 

Nephthys, 

2IO 

Sepia,      .... 

378 

Nereis,    .... 

187,  210 

Skate,      .... 

j/ 

483 

Nerve  Cells,    . 

.         40 

Spider.    .... 

332 

„      Fatigue, 

21 

Starfish,  .... 

231 

,,      Fibres, 

.         40 

Vertebrates,     . 

434 

,,      Tissue, 

•      39 

Nervures,        .... 

308 

Nerves,  Cranial, 

.    441 

Nesodon,         .... 

705 

,,       Morphology  of, 

441,  442 

Neurapophysis  =  Neural 

/   j 

,,       Origin  of, 

•      39 

spine,  .         .         -433 

,661 

,,       Structure  of, 

.      40 

Neurilemma,  .... 

40 

Nervous   Activities   in 

Neuroblast,     .... 

199 

Animals, 

20,  731 

Neurochord,    .... 

191 

Nervous  System  of  — 

Neuro-muscular  cells,     . 

39 

Amphioxus, 

-     413 

Neuro-nephroblast, 

220 

Anodonta, 

.     366 

Neuropodium, 

209 

Arenicola, 

.     203 

Neuroptera,    .... 

304 

Ascidian, 

.     402 

Newts,    ..... 

556 

Aurelia,  . 

•     H3 

Nictitating  membrane  of  Eye, 

658 

Balanoglossus, 

•     393 

Noah's  ark-shell,     . 

372 

Bee, 

.     301 

Noctiluca,       .... 

105 

Birds, 

606 

Nose,  The,      .... 

443 

Cephalopoda,  . 

•     378 

,,     of  Myxine,    . 

466 

Chsetognatha,  . 

.       222 

Nothosaurus, 

592 

Cockroach, 

•       294 

Notochord  of  Balanoglossus,  . 

432 

Crayfish, 

•       259 

of  Fishes,      . 

432 

Crinoid, 

•       244 

,,          Origin  of, 

432 

Distomum, 

.       I67 

,,          Sheath  of,     . 

432 

Earthworm,     . 

.       190 

Notommata,   .... 

223 

Frog, 

536 

Notopodium,  .... 

209 

Haddock, 

•       500 

Notopteris,     .... 

721 

Helix,      . 

•       350 

Notoryctes,     .... 

688 

Herring,  . 

•     5°4 

Nototrema,     .... 

556 

Hirudo,  . 

.     216 

Nucleus,          .... 

43 

Holothurian,   . 

.     241 

,,       Division  of, 

43 

Hydra,    . 

.     136 

Nucula,           .... 

372 

Insects,   . 

.     310 

Nudibranchs, 

357 

Limulus, 

•     339 

Nummulites,  .... 

102 

Lizards,  . 

•     573 

Nycticebus,     .... 

722 

Mammals, 

.     667 

Nyctipithecus, 

725 

INDEX. 


805 


Nymph, . 
Nymphon, 


Obelia,   .... 
Ocellatae,        .         .         .         .141 
Ocelli,    .... 
Octacnemus,  . 
Octocoralla,    . 
Octodontidse, 
/Dctopoda, 
Vpctopus, 
Oculomotor  nerve, . 
Odonata, 
Odontoceti,    . 
Odontoid  process,  . 
Odontoloe,     . 
Odontophora,    see    Glosso- 

phora, 
Odontophore  of  Cephalopoda,     375 

,,  „  Snail,    . 

CEsophagus  of  Vertebrates, 
Oikapleura,     . 
Olfactory  lobes, 
„        nerves,    . 
„        tracts,     . 
Oligochaeta,    . 
Oligosoma, 

Ollulanus,       .         .         .         .183 
Omentum, 
Ommastrephes, 
Oniscus, 
Ontogeny, 
Ooze,  Atlantic, 
Opalina, 
Opalinopsis,    . 
Operculum  of  Gasteropods, 

„  Limulus,  . 

„  Scorpion, 

„  Teleostei, 

Opheosaurus, 
Ophidia, 

„         Classification  of, 
Ophiocoma,    . 
Ophiophagus, 
Ophiopholis,  . 
Ophiothrix,     . 
Ophiuroidea,  . 
Ophryodendron, 
Ophryotrocha, 
Ophthalmosaurus,  . 


PAGE 

PAGE 

322 

Opisthobranchs, 

357 

342 

Opisthocoelous, 

508 

Opisthocomus, 

631 

155 

Opossums,      . 

686 

141 

Optic  chiasma, 

484 

311 

,     foramen, 

663 

409 

,     lobes,    .... 

436 

157 

,     nerves,  .... 

441 

711 

,     thalami, 

441 

386 

,            ,,        Structures  con- 

386 

nected  with, 

436 

441 

Orang,    

726 

305 

Orca,      .         .         .         . 

709 

7O9 

Orchestia,       .... 

278 

660 

Oreocephalus, 

577 

6l9 

Organs,  

32 

,,       Analogous, 

33 

388 

,,        Classification  of, 

36 

375 

„        Correlation  of,  . 

33 

35i 

„        Homologous,     . 

33 

45° 

„        Origin  of  ,  . 

68 

408 

„        Rudimentary,     . 

35 

436 

„        Substitution  of, 

34 

441 

Organ  Pipe  Coral,  . 

157 

667 

Oriental  region, 

764 

187 

Ornithodelphia, 

68  1 

578 

Ornithorhynchus,    . 

68  1 

183 

Ornithosauria,  .... 

594 

611 

Orthagoriscus, 

5ii 

386 

Orthoceras,     .... 

385 

277 

Orthonectidoe, 

127 

70 

Orthoptera,     .... 

305 

101 

Orycteropus,  .... 

690 

ICK 

119 

j 
IO9 

117 

357 

Osphradium  of  Molluscs, 

367 

340 

Ossicles  of  Ear, 

445 

330 

Osteoblasts,    .... 

38 

^10 

Ostracion,        .... 

^n 

j 

577 

Ostracoda,       .... 

-> 
269 

,  578 

Ostrea,    

372 

583 

Ostriches,        .... 

618 

236 

Otaria,    ..... 

716 

583 

Otocyon,          .... 

715 

235 

Otocysts,  see  Otoliths,     .      351 

,367 

236 

Otoliths  of  Anodonta,     . 

367 

235 

,,          Aurelia, 

H3 

105 

,,          Crayfish, 

260 

212 

,,          Haddock, 

500 

593 

,,          Helix,  . 

35i 

8o6 


INDEX. 


PAGE 

PAGE 

Otoliths  of  Sepia,    . 

382 

Palate  of  Mammals, 

.     669 

,,          Skate,   . 

487 

Palato-pterygo-quadrate 

car- 

„          Vertebrates,  . 

444 

tilage, 

•     431 

Otter,     

716 

Palinurus, 

253>  278 

Ova  of— 

Palisade  Worm, 

.     185 

Amphibia, 

549 

Pallialline,     . 

•     364 

Anodonta, 

^70 

Pallium, 

34^ 

Birds,       . 

9 
613 

Palmipes, 

.      229 

Cockroach, 

298 

Palp,       . 

.      308 

Crayfish, 

265 

Paludicella,     . 

.      225 

Earthworm,     . 

195 

Paludina, 

.       76 

Echinoderms,  . 

247 

Pancreatic  juice,     . 

•      23 

Fishes,     . 

5i9 

Panda,    . 

.     716 

Fluke,      . 

167 

Pandalus, 

•    279 

Hydra,     . 

138 

Pangenesis,     . 

-      7i 

Monotremes,    . 

682 

Pangolin, 

.     690 

Myxine    . 

469 

Panmixia, 

.     758 

Peripatus, 

288 

Panniculus  adiposus, 

•     659 

Placental  Mammals, 

678 

,,         carnosus, 

.     660 

Reptilia,  .         .         .         . 

589 

Panorpata, 

•     304 

Vertebrates,     . 

463 

Pantopoda,     . 

•     342 

Oviducal  gland  of  Skate, 

493 

Parachordals  of  Skull,     . 

•    43i 

Oviduct  of  Vertebrates,  . 

462 

Paraglossse,     . 

•     308 

Oviparous  Vertebrates,    . 

464 

„          of  Cockroach 

,       •     295 

Ovis,       

698 

Paramoecium, 

90,  105 

Ovists,  The,    . 

49 

Paranucleus,   . 

.     105 

Ovo-testis  of  Snail, 

354 

Parapodia, 

187,  210 

Ovo-viviparous  Vertebrates,    . 

464 

Parasphenoid, 

•     533 

Ovum,  The,    .... 

56 

Parasuchus,     .         . 

^80 

„       Maturation  of  the, 

59-60 

Parasitic  Fauna, 

y^y 

.      758 

„       Membranes        around 

Parasitism  of  — 

the  Vertebrate, 

466 

Acarina, 

•      336 

,.       Theory,       . 

69 

Cestoda, 

.      171 

Oxyuris,           . 

182 

Cirripedia, 

271; 

Oyster,   

372 

Copepoda, 

/  J 
.      271 

Crustacea, 

.      279 

Paca,       

711 

Cyamidoe, 

.      277 

Pachymatisma, 

120 

Cymothoidse,  . 

.      277 

Pagurus,           . 

27Q 

Gasteropods,    . 

^60 

Pakearctic  region,  . 

/  s 

763 

Insects,    . 

o 

•      325 

Pakeichthyes, 

528 

Nematoda, 

.     181 

Pakemon,        .... 

279 

Nemertea, 

.     184 

Palseo-Crinoidea,     . 

246 

Pentastomum, 

•     336 

,,      Echinoidea, 

240 

Trematoda, 

.     164 

Pakeonemertea, 

1  80 

Parenchymula, 

.     124 

Palceontological  series,    . 

76 

Parental  care  in  — 

Palaeontology, 
Paleeospondylus,      .         .         5 

74 
,  474 

Amphibians,    . 
Asteroids, 

•     558 
•     235 

Palaeostraca,    .         .         .327 

>  337 

Birds,       . 

.626-7 

Palseotherium, 

702 

Clepsine, 

.     220 

INDEX. 


8o; 


Parental  care  in — 

Crayfish,          .         .         .     269 

Crocodiles,       .         .         .588 

Echinoids,        .    -     .         .     240 

Fishes,    ....     526 

Mammals,        .         .         .     639 

Spiders,  .         .         .         -334 

Pariasaurus,    ....     592 

Paroccipital  process,        .         .     663 

Parovarium  =  Wolffian  body  in 

female,  .  .  .461 
Parthenogenesis,  .  .  49,  55 
in  Apus,  .  .  270 
Artemia,  .  270 
Crustacea,  .  280 
Insects,  .  318 
Limnadia,  .  270 
Rotifers,  .  223 
Patella,  .  .  .  -357 

Pathetic  nerve,  .  .  .441 
Pathology,  Comparative,  .  749 
Paunch,  .  .  .  697 

Pauropoda,  .         .         .     292 

Pauropus,  .         .         .     292 

Peccaries,  .         .         .     695 

Pecora,  .  ...     696 

Pecten,  .  ...     372 

„  of  Eye  of  Birds,  &c.,  .  447 
Pectines  of  Scorpion,  .  .  329 
Pectoral  girdle,  .  .  .  434 
Pedal  ganglion,  .  342,  250,  366 
Pedalion,  ....  223 
Pedata,  ....  243 

Pedicellarise  of  sea-urchin,       .     237 
,,  starfish,  .         .     230 

Pedicellina,  ....  225 
Pedipalpi,  .  .  .  .  331 
Pedipalps  of  Scorpion,  .  .  329 
Spider,  .  .  332 
Pelagia,  .  .  133,  148,  156 

Pelagic  Life,  .  .  .  .  753 
Pelagonemertes,  .  .  .176 
Pelagothuria,  .  .  .  240 

Pelecypoda,  see  Bivalves,         .     362 

Pelias, 583 

Pelican  fish,  .  .  .  .  5 1 1 
Pelobates,  ....  556 
Pelomyxa,  ....  99 
Pelvic  girdle, ....  434 
Penseus,  .  '  .  .  .278 


Penella,  ....     272 

Penis  of  Mammals,  .  .  678 
Pennatula,  .  .  .  153,  1 57 
Pentacrinus,  ....  244 
Pentastomum,  .  .  .  336 
Pepsin,  ....  22 

Peptic  digestion,  .  .  .  740 
Peragale,  .  .  .  .686 
Peramelidse,  ....  686 

Perch, 510 

Perching  of  Birds,  .  .  .  601 
Perennichordata,  see  Larvacea,  408 
Peribranchial  cavity,  .  .  406,  416 
Pericardium,  ....  454 
Pericolpa,  .  .  .  .156 
Peridinium,  ....  105 
Perigenesis,  .  .  .  .71 
Perilymph,  ....  444 
Perineal  glands,  .  642,  670,  678 
Peripatus,  ....  285 
,,  Detailed  account  of,  286 
,,  and  Annelids,  .  285,  291 
Peripheral  segmentation,  .  65 
Periplaneta,  ....  293 
Periptychus,  ....  704 
Perissodactyla,  .  .  .  698 
Peristaltic  action,  .  .  .23 
Peritoneum,  ....  670 
Peritricha,  93 

Periwinkle,     .         .         .         >     357 

Perla, 305 

Peromedusse,  .         .         .156 

Petalosticha,  .         .         .     240 

Petaurus,  ....  687 
Petromyzqn,  ....  469 
, ,  and  Myxine  contrasted ,  47 3 
Phacochcerus,  .  .  .  695 
Phacops,  .  .  .  .341 
Phagocytes,  ....  749 
Phagocytosis,  .  .  .  749 

Phalanger,  ....  687 
Phalangeridae,  .  .  .  687 
Phalanges,  ....  434 
Phalangidae,  .  .  .  .  331 
Phalangium,  .  .  .  331 

Phallusia,  ....  409 
Pharyngobranchii,  .  .410 

Pharyngognathi,  .  .  .510 
Pharynx,  ....  449 
Phascogale,  ....  686 


8o8 


INDEX. 


Phascolarctos, 

Phascolomyidse, 

Phascolomys, 

Phascolosoma, 

Phenacodus,  . 

Phoca,    . 

Phocsena, 


687 
686 
687 
224 
701 
717 
709 


Phocidoe,  .  .  .  .716 

Pholas, 372 

Phoronidea,  ....  224 
Phoronis,  ....  225 
Phronima,  .  .  .  .278 
Phrynosoma,  .  .  .  -577 
Phrynus,  .  .  .  .  331 
Phyllopoda,  ....  269 
Phyllopteryx,  .  .  .  511 

Phyllosoma,  ....  278 
Phylloxera,  ....  305 
Phylogeny,  ....  70 
Physalia,  .  .  .  .156 
Physeter,  ....  709 
Physiology,  .  .  .  14,  731 

,,  History  of,  .  16-17 

Physoclisti,  ....  495 
Physostomi,  ....  503 
Phytoptus,  ....  336 
Phytozoa,  .  .  .  .129 
Pia  mater,  ....  438 

Pica, 711 

Pigs, 694 

Pigeon,  .....  599 
Pigeon's  milk,  .  .  .  608 
Pigments,  ....  746 

Pilema, 148 

Pilidium  larva,         .         .         .176 

Pineal  body,   .         .         .         -437 

,,  in  Hatteria,,        .     437 

.,,  in  Iguana,  .  .  437 

Pinnipedia,  .  .  .  .716 
Pinnotheres,  .  .  .  279 

P|Pa> 556 

Pipe  fishes,  .  .  .  .  511 
Pipistrelle,  .  .  .  .721 
Pisces,  see  Fishes,  .  .  4,  475 
Pisiformis,  .  .  .  175 

Pithecia,          ....     725 
Pituitary  body,        .         .         .     436 
,,     Hypotheses  regarding,  .     436 
Placenta,   Hints  of  a,  before 

Mammalia,        .         •     591 


PAGE 

Placenta  of  Mammals,  .  .  648 
Placentation,  Classification  of,  655 
Placoid  scales,  .  .  .  478 
Plagiaulax,  ....  640 
Plagiostomata,  .  .  .  505 
Plakina,  .  .  .  .125 

Planaria,  .  .  .  162,  163 
Planorbis,  ...  76,  358 
Plantigrade,  .  .  .  .  714 
Plants  and  Animals,  .  17-19 

Planula  larva,  .  .  .141 
Plasmodium,  .  .  .  98,  112 
Plasticity  of  Organisms,  .  83 

Plastidules,  .  .  .  .71 
Plastron,  .  .  .  563,  565 
Platanista,  ....  709 
Plathelminthes,  .  . '  .162 
Platydactylus,  .  .  -577 
Platyrrhini,  ....  725 
Plecoptera,  ....  305 
Plectognathi,  .  .  .  .511 
Plesiosaurus,  ....  592 
Pleura  of  Crayfish,  .  .  257 

Pleural  membrane,  .         .     674 

Pleuracanthus,  .  .  .  506 
Pleurobranchs  of  Crayfish,  .  264 
Pleurodont  teeth,  .  .  .  570 
Pleuronectidse,  .  .  .  522 
Pliosaurus,  ....  592 
Ploughshare  bone,  .  .  .601 
Plumularia,  .  .  .  .  155 
Pluteus  larva,  .  .  236,  249 
Pluvianus,  ....  588 
Pneumatic  bones,  .  .  .  623 
Pneumoderma,  .  .  .  358 
Pneumodichthydae,  .  .528 
Pneumogastric  nerve,  .  .441 
Podical  plate  of  Cockroach,  .  295 
Podobranchs  of  Crayfish,  .  264 
Podophthalmata,  .  .  .  278 

Podura, 305 

Poison  gland  of  Snakes,  .     582 

Polar  globules,         ...       60 

„  in  Earthworm,      197 

,,  in  Vertebrates,     463 

Pole  cat,          .         .         .         .716 

Polia, 1 80 

Polian  Vesicle  (Holothuria),    .     242 

Polychaeta,      .         .187,  201,  209 

„  Development  of,  .     206 


Polycladida,    . 
Polyclinum,    . 
Polycyttaria,   .         .         . 
Polygordius,    . 
Polymastodon, 
Polymeniscous  eyes, 
Polynoe, 

IND 

PAGE 

•        I63 
.       409 
.        103 
208,  212 
.      640 

•      311 
.       2IO 

EX.                                                809 

PAGE 

Proglottis,       .         .         .         .17-1 
Proneomenia,           .         .         .     347 
Pronephros,    ....     458 
Pronephros    in    the    different 
Vertebrate  groups,         .     461 
Prongbuck,     .         .         .         .698 
Proscolex,       .         .         .               171 
Prbsencephalon,      .         .         .     439 
Prosopygii,      ....     224 
Prostate  glands,       .         .     219,  678 
Protandrous,  .         .         .      463,  468 
Protective  colouring  of  Insects,     324 
Proteleidoe,     .         .         .         .715 
Proteles,          .         .         .         .     715 

Polyodon, 

508 

Polyphemus,  . 
Polyplacophora, 
Polypodium,  . 
Polyprotodontia, 
Polypterus, 
Polyspermy,    . 
Polystomum,  . 
Polyzoa, 
Pondsnail, 
Pons  Varolii, 
Pontobdella,   . 
Porcellana, 
Porcellenaster, 
Porcellio, 

.       269 

-     345 
140,  1  60 
.     685 
.     508 

•      63 
.     170 
224,  225 

•    357 
.     668 
.     214 
.     279 
•     235 
.     277 

Proteolepas,    ....     275 
Proteomyxa,  ....       98 
Proterosaurus,         .         .         .     5^9 
Proterospongia,       .         .         .105 
Proteus,           .         .         .         .556 
Proteus  animalcule,  see  Amceba. 
Protobathybius,       ...       98 
Protobranchia          .         .         .     372 
Protocercal,             .         .         .521 
Protodrilus,              .         .         .212 
Protoelastin,            ...       87 
Protogenes,              ...       98 
Protogynous,            .         .        96,  463 
Protohippus,            .         .         .     7O1 
Protohydra,              .         .         .140 
Protomyxa,              ...       98 
Protoplasm,             .         .  26,  42,  47 
Protopodite  of  Crayfish,          .     255 
Protopterus,    .         .         .     511,  513 
Protospongia,           .         .         .125 
Prototheria     .         .         .         .681 
Protovertebrse  =  Mesoblastic 
segments,     .         .     426,  430 
Protozoa,         .         .         .         85-115 
Classification  of,        97-98 
Functions  in  the,     .     106 
General  interest  of,       114 
,,       Notes  on  the,   106 
History  of  the,            105-6 
Immortality  of,         .      115 
and  Metazoa,  .               115 
Reproduction  in,      .     1  10 
Structure  of,    .         .109 
Survey  of  the,          .       98 
Protracheata,  ....     285 
Proviverra,      .         .         .         .     717 

Porcupine, 
Porifera, 

.     711 
.     116 

Poromya, 

.     372 

Porpites, 

156 

Porpoise, 

7OQ 

Portal  veins  of  Mammals, 
Portuguese  Man-of-war,. 
Portunus, 

.   674 
132,  156 

.     279 

Post-anal  gut, 
Post-pubic  processes, 
Potamogale,    . 

.  452 

.    605 

.  717 

.    628 

Prairie  dog,     . 
Prawn,   .... 
Preen  gland  of  Pigeon,    . 
Preformation  theory, 
Priapulus, 
Primary  vesicles  of  brain, 
Primates, 
Primitive  streak, 
Pristis,    .... 
Proboscidea,  . 
Procavia, 

.   711 
.  279 

-     599 
.       49 
.     224 

•     435 
.     679 
.     614 
•     506 
-     703 
.     702 

Procoelous, 
Proctodseum,  . 
Procyon, 
Procyonidoe,    . 
Proechidna,    . 

•     532 
.     449 
.     716 
.     716 
.     68  1 

8io 


INDEX. 


PAGE 

P 

AGE 

Psalterium,     .... 

697 

Radius,  .         .         .... 

434 

Pseudobranchus,     . 

556 

Radula  of  Cuttlefish. 

375 

Pseudogastrula, 

123 

„         Gasteropods,  . 

Pseudolamellibranchia,    . 

372 

,,         Snail, 

351 

Pseudonavicellse,     . 

90 

Raja,       ....     477, 

506 

Pseudopodia, 

86 

Ran  a,  see  Frog, 

530 

Pseudopus,      .... 

578 

Rangifer,         .... 

698 

Pseudoscorpionidoe, 

33i 

Raphidiophrys, 

100 

Psolus,    ..... 

247 

Rat,         .                  ... 

71  1 

Pteranodon,    .... 

•  C) 

594 

Ratel,      

/  A  x 
7l6 

Pteraster,        .... 

235 

Ratitse,   

618 

Pterichthys,     .... 

509 

Rattlesnake,    .... 

583 

Pterodactyl,    .... 

Ray,        

477 

Pterodon,        .... 

717 

Razor  shell,     .... 

372 

Pteromys,        .... 

711 

Recapitulation    of    Ancestral 

Pteropods,       .... 

358 

History,      .... 

69 

Pteropus,         .... 

721 

Rectal  gland, 

487 

Pterotrachea,  .... 

357 

,,       respiration, 

280 

Pterygota,       .... 

304 

Red  Coral,      .... 

157 

Ptyalin,  

22 

Redia,     ..... 

169 

Ptychodera,    .... 

39^ 

Reducing  division,  . 

60 

Ptychozoon,    .... 

577 

Reed  

697 

Puff  Adder,     . 

583 

Reindeer,        .... 

608 

Pulmonary  sacs  of  Arachnids, 

JO 

327 

Relative  age  of  Animals,          79 

g~ 

,80 

„                 „     Scorpion,   . 

330 

Reproduction,          .         .         15 

i  49 

•      „     Spider, 

333 

of  Amphibia,  . 

558 

Pulmonata,     .... 

357 

Aurelia, 

145 

Puma,     ..... 

Crustacea, 

280 

Pupa,      

322 

Earthworm,    . 

194 

Pupil  of  Eye, 

446 

Echinoderms, 

247 

Purpura,          .... 

357 

Fishes,    . 

525 

Pycnogonidse, 

342 

Hydra,    . 

138 

Pycnogonum, 

342 

Insects,  .         .     317, 

Pycnopodia,    .... 

235 

Mammals, 

647 

Pygostyle,        .... 

601 

Modes  of, 

52 

Pylangium,     .... 

541 

Rotifers, 

223 

Pyloric  cseca  of  Insects,  &c.;    . 

313 

Sponges, 

121 

Pyrosoma,       .         .         .401 

,  409 

Taenia,    . 

172 

Python,            .         . 

583 

Reproductive  system  of  — 

Amphioxus, 

417 

Quadrate,        .... 

428 

Anodonta, 

370 

Quadrumana, 

729 

Arenicola, 

2O6 

Quagga  

702 

Ascidian, 

407 

Aurelia,   .... 

145 

Rabbit,  

657 

Balanoglossus, 

395 

Raccoon,         .... 

716 

Bee,         .... 

306 

Rachiodon,     .... 

1  j.  VJ 

500 

Choetognatha, 

JW 

222 

Radial  symmetry,    . 

Cockroach, 

297 

Radials  of  a  fin, 

521 

Crayfish, 

264 

Radiolaria,      .... 

102 

Crinoid,  .... 

246 

INDEX. 

811 

PAGE 

PAGE 

Reproductive  System  of  — 

Respiratory  System  of  — 

Crocodilia, 

.       588 

Frog,       . 

.       546 

Distomum, 

•        153 

Haddock, 

.        500 

Earthworm,     . 

.        194 

Helix,      . 

-     353 

Frog, 

-     549 

Herring, 

•     504 

Haddock, 

.     502 

Holothurian,   . 

.    241 

Helix,      . 

•     354 

Insects,    . 

•     3H 

Herring,  . 

•     5°4 

Limulus, 

•     340 

Hirudo,  . 

.     219 

Lizards,  . 

-     575 

Holothurian,   . 

•     243 

Mammals, 

.    674 

Insects,    . 

•     317 

Myxine, 

.    467 

Limulus, 

•     340 

Peripatus, 

.    287 

Lizards,  . 

•     576 

Petromyzon,    . 

.    471 

Mammals, 

.     676 

Pigeon,    .       '  . 

.     611 

Myxine,  . 

.    468 

Rabbit,    . 

.     674 

Nematoda, 

.     182 

Scorpion, 

•     330 

Nemerteans,    . 

.     179 

Sea  urchin, 

-     239 

Ophiuroids, 

Sepia, 

•  3!S 

Peripatus. 

.     288 

Skate,      . 

.  488 

Petromyzon,    . 

.    472 

Spider,    . 

•     333 

Pigeon,    . 

.     612 

Starfish,  . 

•     234 

Rabbit,    . 

.     676 

Tracheata, 

•     3H 

Scorpion, 

•     330 

Vertebrates,     . 

.     458 

Sea  urchin, 

.     240 

Retia  mirabilia  of  Cetacea, 

.     705 

Sepia,          .     . 

•     382 

,,                Sirenia, 

.     691 

Skate,          .     . 

492 

„               Sloths, 

.     689 

Spider,     . 

•     330 

Reticulum, 

•     445 

Starfish,  . 

2  7  A 

697 

Tsenia,     . 

•^J*t 
172 

83 

Vertebrates,     . 

*  /  •" 
.      462 

Rhabdites,      . 

.     162 

Reptilia, 

3>  56o 

Rhabdoccelida, 

.     162 

„       and  Birds, 

.     562 

Rhabdonema, 

.     183 

„       Development  of, 

.     589 

Rhabdopleura, 

•     399 

„       Extinct,   . 

-     592 

Rhacophorus, 

•    555 

,,       Pedigree  of, 

•     594 

Rhamphorhynchus, 

•     594 

Respiration  in  Animals, 

24,  25 

Rhea,      .... 

.     619 

Respiratory  System  of  — 

Rhinatrema,  . 

•    557 

Acarina, 

•     336 

Rhinoceros,    . 

.    702 

Amphioxus,     . 

.     416 

Rhinoderma,  . 

.    558 

Anodonta, 

•     369 

Rhinolophus, 

.    721 

Arenicola, 

.     205 

Rhizocrinus,   . 

.    246 

Ascidian, 

•     405 

Rhizopoda, 

•      85 

Balanoglossus, 

•     395 

Rhizostoma,   . 

148,  156 

Bee, 

.     306 

Rhodeus, 

.    527 

Cephalopoda,  . 

•     381 

Rhomboidal  sinus,  . 

.    606 

Cockroach, 

.     297 

Rhopalia, 

•     H3 

Crayfish, 

•     263 

Rhopalodina, 

•    243 

Crinoid, 

•     245 

Rhopalura, 

.     127 

Crocodilia, 

.     588 

Rhynchobdellidse,  . 

.      221 

Crustacea, 

.     263     Rhynchocephalia,  . 

.     568 

812 

Rhynchoflagellata, 
Rhynchota,     .  V,-J°-         •     •    • 
Rhytina,          .... 
Ribs  of  Vertebrates, 
Rock-dove,     .... 
Rodentia,        .... 
Roe  deer,        .... 

IND 

PAGE 

105 
305 
691 

433 
599 
710 
698 
709 
257 
223 
35 
374 
'57 
697 
696 
697 

209 
716 

212 

275 

444 
670 
,660 

222 
725 
556 
127 
22 
690 
296 
627 
352 

218 

313 

670 
670 

510 
,  409 

272 
104 
41 
21 
336 
155 
620 

56l 
592 
6l7 

EX. 

Saw  flies, 
Saxicava, 
Scales  of  Birds, 
Fishes,      . 
„        Mammals, 
„        Reptiles,  562,  563, 
Scallop,  .... 
Scalpellum,     . 
Scaphirhynchus, 
Scaphognathite  of  Crayfish, 
Scaphopoda,   . 
Scapula, 
Schizocardium, 
Schizocoele,     . 
Schizogenes,  . 
Schizognathse, 
Schizonemertea, 
Schizopoda,    . 

PAGE 

•     3°4 
•     372 
.     600 
.     521 
.     641 
579,  583 
•     372 
.     274 
.     508 
.     263 
.     362 
•     434 
•     392 
176,  472 
.       98 
.     620 
.     1  80 
.     276 
•     578 

Rorqual,          .... 

Rostrum  of  Crayfish, 
Rotat^ria  =  Rotifers, 
Rudimentary  organs, 
Rudistae,         .... 

Rugosa,           .... 

Rumen,           .... 
Ruminants,     .... 
Rumination,   .... 

Sabella, 
Sable,      
Saccocirrus,     .... 
Sacculina,        .... 
Sacculus  of  ear, 
,,         rotundus, 
Sacrum,          .         .         .      601 
Sagitta,  
Sakis,      ..... 
Salamander,    .... 
Salinella,         .... 
Saliva.     ..... 
Salivary  glands  of  Ant-eaters,  . 
„                        Cockroach, 
Collocalia,  . 
Helix, 
Hirudo, 
Insects, 
Mammals,   . 
Rabbit, 
Salmon,           .... 

Scincus, 

578 

Sciuromorpha, 
Sciuropterus,  . 
Sciurus,  .... 
Sclerobasic,     . 
Sclerodermic, 
Sclerotic, 
,,        ossicles,    . 
Scolex,    .... 
Scolopendra,  . 
Scolopendrella, 
Scorpion, 
flies, 
Scrotum, 
Scuta,     .... 
Scutes,    .... 

.     711 
.     711 
.     711 

.     154 
.     154 
.     446 
.     608 

•     173 
.     292 
.     291 
•     327 
•     3°4 
•     677 
.     272 
.     426 
.     506 

Scyphistoma,  . 
Scyphomedusse, 
Scyphozoa, 
Sea  anemone, 
„    butterflies, 
„    cows, 
,    cucumbers, 
,    horse, 

.     147 
142,  156 
142,  156 
149 
•     358 
..   691 
.     240 
.     511 

Salpa,     .         .         .         .401 
Sapphirina,     .... 
Sarcocystis,     .... 
Sarcode,          .... 

Sarcolemma,  .... 
Sarcoptes,       .... 
Sarsia,     ..... 
Saurognathse, 
Sauropsida,  Ichthyopsida,,  and 
Mammalia,  . 
Sauropterygia, 
Saururse,          .... 

.  716 

22^ 

,    mouse, 
,    otter, 

.      2IO 

*        7l6 
.       157 

t      ' 
,    snakes, 

•       583 

INDEX.                                                813 

PAGE 

PAGE 

Sea  squirts,  vide  Tunicata,       .     401 

Sense  organs  of  — 

,,    urchin,     .         .         .         236-40 

Amphioxus,      .         .         .414 

Seals,      716 

Anodonta,        .         .         .     367 

Sebaceous  glands,  .         .         .     642 

Ascidian,          .         .         .     402 

Sedentaria,      .         .         .         .210 

Aurelia,  ....     143 

Segmental  duct  of  Vertebrates,     462 

Crayfish,           .         .         .     259 

„         Organs,          .         .194 

Crocodilia,       .         .         .     587 

,,         Papillae,          .         .215 

Crustacea,        .         .         .     280 

Segmentation  of  Ovum,  .         .       63 

Frog,       ....     539 

„     in  Amphioxus,    .         .417 

Haddock,         .         .         .     500 

„         Anodonta,       .         .     370 

Hagfish,  ....     467 

„         Ascidian,         .         .     407 

Helix,      ....     350 

,         Balanoglossus,         .     396 

Hirudo,  .         .         .         .216 

,         Birds,      .         .         .     613 

Holothurian,    .         .         .     241 

,         Chaetognatha,          .     222 

Insects,    .         .         .         .311 

,         Crustacea,       .         .281 

Lamprey,         .         .         .     470 

,         Dipnoi,  .         .         .     513 

Limulus,           .         .         .     295 

,         Earthworm,    .         .194 

Pigeon,    ....     607 

,         Echinoderms,          .     247 

Rabbit,    ....     669 

,         Fowl,      .         .         .613 

Scorpion,         .         .         .     329  « 

Frog,      .         .         -549 

Sea-urchin,       .         .     •    .     237 

,         Ganoids,          .         .     508 

Sepia,       ....     379 

;,         Gasteropoda,  .         .     356 

Skate,      .         .         .           486-7 

,         Haddock,        .         .     503 

Spider,    ....     332 

Hydra,    .         .         .139 

Starfish,  .         .         .         .231 

,         Insects,  .         .         -319 

Vertebrates,     .         .         .     443 

,         Lamprey,        .         .     472 

Sepia,     ....     375,  386 

,         Monotremata,          .     649 

Sepiostaire,     ....     375 

,         Placental  Mammals,    648 

Seps,       578 

,         Reptilia,          .         .561 

Septibranchia,         .         .         .     372 

,         Scorpion,         .         .     330 

Sergestes  278 

Skate,     .         .         .493 

Serpents,         ....     578 

Spider,   .         .         .333 

Serpula,           .         .         .         .211 

„         Tcleosteans,    .         .     503 

Serranus,         ....     525 

„         Vertebrata,      .         .     463 

Sertularians,    .         .         .               155 

Seison,    .....     223 

Sesamoid  bones,      .         .         .     660 

Selachii  =  Elasmobranchs,        .     505 

Setse,       189 

Selachodichthyidae,          .         .     528 

Sex,        .         .         .         .           52-53 

Selachoidei,    ....     506 

Sexual  selection,     .         .         .     770 

Selection,        ....     769 

„      selection  among  Birds,     626 

Selenodont,     ....     693 

„            „         among  Spiders,  334 

Self-  fertilisation  in  Hydra,      .     138 

,,      reproduction,       .                50 

,,               in  Serranus,  .     463 

„       Divergent    modes    of 

,,               in  Tapeworms,  171 

sexual  reproduction,   54-56 

,,               in  Trematoda,    164 

Sharks,          ....       506 

Sella  turcica,  ....     663 

Shell  of— 

Semicircular  canals  of  Ear,      .     444 

Anodonta,        .         .         .     364 

Semirrl  vesicle,       .         .219,  677 

Argonauta,       .         .         .     386 

Semnopithecinae,     .         .         .     726 
Semnopithecus,       .         .         .     726 

Cephalopoda,  .         .         .     375 
Chiton,    ....     346 

814 

INDEX. 

PAGE 

PAGE 

Shell  of— 

Skeleton  of  — 

Helix,      . 

•     349 

Mammals, 

•        635 

Molluscs, 

•     343 

Myxine, 

.        466 

Nautilus, 

•     384 

Petromyzon,     . 

.        470 

Planorbis, 

•     384 

Pigeon,    . 

.     601 

Scaphopoda,    . 

•     362 

Rabbit,    . 

.     660 

Spirula,    . 

.     386 

Sea  urchin, 

.     236 

Shell  gland  or  sac,  .     361, 

371,  387 

Sepia, 

•     377 

Shepheardella, 

IOI 

Skate,      . 

.    478 

Shrews, 

.     718 

Snakes,   . 

•     579 

Shrimp, 

.    279 

Spiders,  . 

•    332 

Sida,       .... 

271 

Starfish,  . 

23O 

Silicispongiae, 

.     125 

Vertebrates,     . 

**3\j 

.      427 

Simia,     .... 

.     726 

Skin  of  — 

Simiidae, 

.     726 

Amphioxus,     . 

.      412 

Simplicidentata, 

.     710 

Anodonta, 

•            -       365 

Sinews,  .... 

21 

Balauoglossus, 

.       392 

Sinupallia, 

•       372 

Birds, 

.      596 

Sinus  venosus, 

•     454 

Cockroach, 

.       293 

Siphon  of  Cephalopods,  . 

•     374 

Crayfish, 

•      254 

„     .    Gasteropods,  . 

•    357 

Crocodilia, 

•        583,  584 

„         Lamellibranchs, 

•    373 

Earthworm,     . 

.       I89 

Siphonaptera, 

•    304 

Fishes,    . 

.      476 

Siphonoglyphes, 

i">i 

Frog, 

r?I 

Siphonophorae, 

-> 
.     156 

Haddock, 

3tji 

.      496 

Siphonops, 

•     556 

Helix,      . 

•      350 

Siphuncle  of  Nautilus,     . 

-     384 

Insects,    . 

.      310 

Sipunculoidea, 

.     224 

Leech,     . 

•      215 

Sipunculus,     . 

.    224 

Lizards,  . 

•      571 

Siredon, 

W7 

Mammals, 

64.0 

Siren,      .... 

•     556 

Myxine,  . 

.            WVfV 

.466 

Sirenia,  .... 

.     691 

Peripatus, 

.     .  287 

Skate,     .... 

•    477 

Petromyzon.    . 

.    470 

„      Development  of, 

•     493 

Rabbit,    . 

.  659 

„      Structure  of, 

477,  493 

Sepia, 

.     .  376 

Skeletal  tissues  of  animals, 

•     743 

Skate,      . 

.  478 

Skeleton  of  — 

Snakes,    . 

.     .  578 

Amphioxus, 

.     412 

Tunicate, 

.    402 

Balanoglossus, 

•     393 

Vertebrates,     . 

.   425 

Birds,      . 

.     622 

Skull,     . 

427,  429,  431 

Chelonia,          .         .    • 

•     563 

Skull  of— 

Crayfish, 

•     257 

Crocodilia, 

.    584,  586 

Crinoids, 

244 

Frog, 

$11 

Crocodilia, 

•     584 

Haddock, 

497 

Frog,       . 

-     532 

Hatteria, 

568 

Haddock, 

.    496 

Lizards,  . 

572 

Hatteria, 

.     568 

Mammals, 

635 

Holothurian,    . 

.     241 

Pigeon,    . 

602 

Limulus, 

•     338 

Rabbit,    . 

661 

Lizards,  . 

•     57i 

Skate,      . 

480 

INDEX.                                                815 

PAGE 

PAGE 

Skull  of  Snakes,      .         .         .580 

Spiracles  of  Skate,           .         -477 

Skunk,  716 

Spiracular  cartilage,         .         .     482 

Sloth  animalcules,  .         .         .     337 

Spiral  valve,   ....     487 

Sloths,    690 

Spirula,  386 

,,       Vertebrae  of,         .         .     689 

Splanchnopleure,    .         .     415,  616 

Slow  worm,    ....     577 

Spleen,  457 

Slug,       358 

„       of  Frog,      .         .         .     546 

Smynthurus,  ....     305 

„       of  Pigeon,  .         .         .611 

Snails,    358 

„       of  Rabbit,  .         .         .671 

Snail,  see  Helix,     .         .      348-357 

„       of  Skate,    .         .         .     490 

Snakes,           ....     578 

Splenial,          .         .         .     586,  605 

Solaster,          .         .         .      229,  235 

Sponge,  Development  of  a,     .     122 

Solen,     372 

„        Structure  of  a,   .       117-120 

Solenomya,     ....     372 

Spongelia,       .         .         .         .124 

Solenostoma,           .         .         .     526 

Sponges,         .         .         .       116-128 

Solpuga,          .                  .         .     331 

Spongicola,     .         .         .         .148 

Solpugidse  or  Solifugse,  .         .     33* 

Spongilla,       .         .         .         .122 

Somatic  cells,          .         .         .       97 

Somatopleure,                  .     415,  61  6 

Sporozoa,        ....     103 

Somites  =  segments,      415,  430,  426 

Spring-tails,    .         .         .         .     305 

Sorex,     7i8 

Squalus,           ....     506 

Sousliks,         .         .         .         .711 

Squamosal,     .         .431,  533,  662 

Spadella,         .         <         .         .222 

Squilla,  .....     269 

Spalacotherium,      .         .         .     640 

Squirrels,         .         .         .         .     711 

Spatangus,      ....     240 

Stagonolepis,  ....     589 

Spatularia,      ....     508 

Stapes  of  ear,          .         .         .     445 

Species,           .         .         .12,  766 

Starfish,           .         .         .     229,  235 

Spermatheoe,          .       187,  196,  295 

Statoblasts,     .         .         .         .121 

Spermatophores,     .         .         .219     Stegocephala,          .         .         -557 

Spermatozoa,           ...       58     Stegosaurus,    ....     594 

Spermophilus,         .         .         .     711     Steller's  Sea  Cow,  .         .         .     692 

Sphseridia,      ....     237     Stentor,  .         .         .         .         .105 

Sphaerophrya,          .         .         .105     Stephanoceros,        .         .         .     223 

Sphserozoum,           .         .         .     103     Sterno-tracheal  muscles,          .     601 

Sphserularia,  .         .         .         .182 

Sternum  of  Amphibia,    .         .     535 

Sphargidse,     ....     566 

Crustacean  segment,  257 

Sphargis,         ....     566 

Lizards,        .         .     573 

Sphenethmoid,        .         .         .     533 

Monotremes,         .     68  1 

Sphenodon,    ....     568 

Pigeon,         .         .     605 

Sphenotic,       ....     497 

Rabbit,         .         .     664 

Sphincter  muscles,           .         .149 

Stickleback,    ....     520 

Spicules  (of  Sponge),      .         .120 

Stigmata,         ....     295 

Spiders,                    .         .         .     331 

Sting  of  scorpion,   .         .         .     328 

,,        Classification  of,         .     336 

Stinging  Animals,  .         .       129-160 

Spider  monkeys,     .         .         .     725 

,,        cells,         .         .      130,  136 

Spinning  glands  of  Insects,     .     318 

Stipes,    295 

Spiders,     .     333 

Stoat,      716 

Spinal  cord,    ....     439 

Stomatodseum,        .         .         .     449 

,,       ganglia,       .         .      440,  443 

Stomato-gastric  nerves,  .         .     259 

,,       nerves,         .         .      440,  442 

Stomatopoda,          .         .         .     276 

8i6 


INDEX. 


PAGE 

PAGE 

Stone  canal  of  starfish,    . 

232 

Tamandua, 

.     690 

Strepsiptera,   .... 

309 

Tanais,  .... 

•     277 

Streptoneura,  .... 

357 

Tapeworms,    . 

•     I7i 

618 

Tapir,     .... 

.     700 

Strobila,           .         .         .14?, 

171 

Tarantula, 

•     336 

Strongylocentrotus, 

236 

Tardigrada,     . 

•     337 

Strongylus,     . 

183 

Tarsipes, 

687 

Struggle  for  existence,    . 

769 

Tarsius,  .... 

.     722 

Struthio,         .... 

618 

Tarsus,   .... 

434,  666 

Sturgeon,        .... 

508 

Tarso-metatarsus,    . 

.     598 

Stylaster,         .... 

i">6 

Tasmanian  Wolf,    . 

.    686 

Stylifer,  

2 

360 

Tatusia,  .... 

.     690 

Stylonichia,     .         .         .      105, 

"3 

Tealia,    .... 

149,  156 

Suberites,        .... 

125 

Tectibranchia, 

•    357 

Submentum,   .... 

295 

Teeth  of  Crocodilia, 

.    586 

Sub-neural  gland  of  Ascidian, 

402 

Extinct  birds,  . 

.    619 

Substitution  of  organs,    . 

34 

Gasteropods,     . 

•     344 

Subzonal  membrane, 

651 

Leech, 

.     218 

Sudorific  glands,     . 

642 

Lizards,    . 

•     573 

Suidae,    ..... 

694 

Mammals, 

•    643 

Suina,     ..... 

693 

Myxine,    . 

.    467 

Sun  animalcules,     . 

100 

Ornithorhynchus, 

.    68  1 

„     fish,          .    >     . 

5ii 

Skate,       . 

.    487 

Supra-renal  bodies,          .     462, 

676 

Snakes,     . 

.     580 

Surangular,     .         .         .     586, 

605 

Tegenaria, 

-     336 

Sus,         

694 

Teiidse,  .... 

.     578 

Suspensorium, 

431 

Telegony, 

63 

Sycandra,        .... 

123 

Teleostei,        .         . 

4.OC.  COQ 

Sycon  type  (of  sponge),  . 

118 

Telolecithal,   . 

^•yjj  yy 
.       65 

Sycones,          .... 

125 

Telphusa, 

.     279 

Syllids,   .         . 

210 

Telson, 

•     257 

Symbiosis,       .         .         .     103, 

126 

Temperature  of  Birds,     . 

.     598 

Symmetry  of  Animals,    . 

31 

„               Mammals, 

.     680 

Sympathetic  nervous  system,  . 

434 

Temporal,  see  Squamosal, 

.     662 

Symphyla,       .... 

294 

Tendons, 

21 

Symplectic,     .... 

498 

Tenrec, 

.       7l8 

Synangium,    .... 

54i 

Tentaculocysts, 

•       143 

Synapta,          .... 

243 

Tentacles  of  Hydra, 

134 

Syn-cerebrum, 

to 

271 

Medusa, 

AOt 

.        142 

Syncoryne,      .... 

155 

Medusoid,  . 

I4O 

Syngamus,      .... 

185 

Polychaeta,  . 

.      210 

Syngnathus,    .... 

511 

Sea  anemone, 

•    J51 

Syrinx,   ..... 

611 

Snail, 

.  348 

Systemodon,  .... 

701 

Terebella, 

.      210 

Terebratula,  . 

.      226 

Tadpole  of  Frog,    . 

550 

Teredo, 

•      372 

Taenia,    ..... 

171 

Tergum, 

257,  274 

Tseniolce,         .... 

147 

Termites, 

•      305 

Talitrus,          .... 

278 

Terrestrial  Fauna,  . 

.      758 

Talpa,    

718 

Tesselata, 

.      246 

INDEX. 


817 


PAGE 

PAGE 

Test  of  ascidian, 

4O2 

^06 

Testicardines, 

.       226 

,,         Electric  organ  of,     . 

478 

Testudinidse,  . 

.        567 

Tortoises,        .... 

563 

Testudo, 

.        567 

Toxodcn,        .... 

705 

Tetrabranchiata, 

-       385 

Trabeculge  of  Skull, 

431 

Tetracoralla  =  Rugosa,    . 

•        157 

Tracheae,         .... 

3H 

Tetractinellida, 

.     116 

„         of  Arachnoidea, 

327 

Tetrapneumones,    . 

•     336 

„             Mites,  . 

336 

Tetrarhyncus, 

3ii 

„              Peripatus,      . 

285 

Tetrodon, 

„             Spider, 

333 

Tetronerythrin, 

.'     263 

Tracheal  gills, 

Thalamencephalon, 

•     439 

Tracheata,      .... 

285 

Thalassema, 

.      211 

Trachomedusae, 

156 

Thalassicola, 

.       103 

Trachydosaurus,  "Placenta"  of, 

Thaliacea, 

•      409 

TrachymedusDe,  •     . 

156 

Theca,             .         .         . 

154 

Tragulina,       .... 

696 

Thecophora,  . 

.      566 

Tragulus, 

696 

Thecosomata, 

•      358 

Trematoda,     .... 

165 

Thelyphonus, 

•      313 

,,          Classification  of,  . 

170 

Thoracic  duct, 

.      674 

Treptoplax,     .... 

127 

Thoracostraca, 

.      269 

Triceratops,    .... 

594 

Thornback, 

•    477 

Trichechidae,  .... 

7.16 

Thread  cells  =  stinging  cells, 

.     136 

Trichechus,    .... 

716 

Threadworms, 

.     1  80 

Trichina,         .         . 

184 

Thrips,  .         .         .         . 

•     305 

Trichocephalus, 

184 

Thylacinus,     . 

.     686 

Trichocysts,   .... 

91 

Thylacoleo,     .         .         . 

.     687 

Trichodes,       .... 

183 

Thymus, 

•     450 

Trichodina,     .         . 

105 

,  ,         of  Frog,  . 

-     546 

Trichoplax,     .... 

127 

,  ,         of  Rabbit, 

.     671 

Trichoptera,  .... 

3°4 

of  Skate, 

.     490 

Tricladida,      .... 

163 

Thyroid, 

•    450 

Triconodon,  .... 

640 

„         of  Frog,    . 

^46 

Tridacna,        .... 

372 

„         of  Rabbit, 

2 
.     675 

Trigeminal,    .... 

o  / 

441 

of  Skate, 

.     490 

Trilobites,       .... 

341 

Thysanoptera, 

•     305 

Trionychidae, 

567 

Thysanozoon, 

.     163 

Trionyx,          .... 

567 

Thysanura, 

j 
•     305 

Tristomum,     .... 

170 

Tibia,     .... 

•     434 

Triton,   .         . 

556 

Tibio-tarsus,   . 

•     598 

Tritylodon,     .... 

640 

Ticks,     .... 

•     336 

Trivium,         .... 

230 

Tiedemann's  bodies, 

•     233 

Trochanter,     .         .         .      308, 

666 

Tiger,     .... 

.     714 

Trochoceras,  .... 

385 

Tillotherium, 

•     705 

Trochlear  nerve, 

441 

Tinamou, 

.     620 

Trochosphere, 

208 

Tissues, 

37-40 

Troglodytes,  .... 

726 

,,       Nutrition  of, 

.     741 

Trombidium, 

336 

Titanotherium, 

.     702 

Trophoblast,  .... 

651 

Toads,    .... 

•    555 

Trophospongia, 

653 

Tornaria, 

•    396 

Tropidonotus, 

583 

52 

8i8 


INDEX. 


PAGE 

PAGE 

Truncus  arteriosus, 

•   541 

Utricles, 

.       41 

Trunk  fishes, 

.   511 

Utriculus  of  Ear,     . 

•     444 

Trypsin, 

22 

Tuber  cinereum, 

•     537 

Vacuoles, 

92,  108 

Tubercle  of  rib, 

.     66  1 

„         Contractile, 

86,  101 

Tube  feet  of  Brittle  star, 

•     235 

Food,      . 

86,  no 

„           Sea  urchin, 

•     239 

Vagina, 

.     678 

,.           Starfish,       . 

.     229 

Vagus  nerve, 

.     441 

Tubifex, 

174,209 

Valves  of  Mammalian 

heart,   .     671 

Tubipora, 

153^  157 

Vampire  bat, 

.     721 

Tubularia, 

140,  155 

Vampyrella,    . 

.         .       98 

Tunicata, 

.    400 

Vampyrus, 

.     721 

,,         Classification  of, 

.    408 

Varanidse, 

.     577 

Tunicin, 

•     744 

Variation, 

.     766 

Tupaia,  .... 

•     717 

Vas  deferens, 

.    462 

Turbellaria,     . 

.     162 

Vascular  System  of— 

,,           Classification 

of,  .     162 

Amphioxus, 

.    416 

Turrilites, 

•     385 

Anodonta, 

.    .  367 

Turtles, 

-•     563 

Arenicola, 

.     204 

Tylenchus, 

.     183 

Ascidian, 

.    406 

Tylopoda, 

•     695 

Balanoglossus, 

•        •     395 

Tympanic  bulla, 

•     663 

Bee, 

.     306 

Tympanum,    . 

•     445 

Cockroach, 

.     297 

Typhlopidse,  . 

•     583 

Crayfish, 

.    263 

Typhlops, 

•     583 

Crinoid,  . 

-     245 

Typhlosole,     . 

.     192 

Crocodilia, 

•     584,  587 

Typotherium, 

•     705 

Dipnoi,    . 

.        .     516 

Tyroglyphus, 

•     336 

Earthworm, 

.     192 

Frog,       . 

•     54i 

Haddock, 

.     500 

Uintatherium, 

.     704 

Helix,      . 

-    353 

Ulna,      .... 

•     434 

Hirudo,  . 

.    218 

Umbilical  Cord, 

495>  651 

Insects,   . 

•    3i5 

Vesicle,  . 

.     651 

Limulus, 

•     339 

Umbo,    .... 

•     364 

Lizards,  . 

•     574 

Umbrella, 

•     357 

Mammalia, 

.        .    671 

Uncinate  processes, 

.    602 

Myxine, 

.    468 

Ungulata, 

•    693 

Nemerteans,    . 

.     179 

Unio,      .... 

•     372 

Peripatus, 

.        .     287 

Ureters, 

.    461 

Petromyzon,    . 

•    472 

Urethra, 

678 

Pigeon,    . 

.    609 

Urinogenital  ducts, 

.    461 

Rabbit,    . 

.        .    671 

Urnatella, 

•    225 

Scorpion, 

•     330 

Urochorda,     . 

.    400 

Sea  urchin. 

-     239 

U  rode  la, 

.  556 

Sepia,      . 

.     380 

Urostyle, 

$12 

Skate,      . 

.    488 

Ursidae, 

.       7I6 

Spider,    . 

•    332 

Ursus,     . 

.       7l6 

Starfish,  . 

•    234 

Uterus,  . 

.      678 

Vertebrates,     . 

•     453 

,,       masculinus, 

.      677 

Velarium, 

.     142 

INDEX. 


819 


PAGE 

PAGE 

Velella,            .         .         .         .156 

Visceral  arches, 

.     427 

Veliger,           .         .      317,  345,  361 

clefts, 

.     449 

Velum,    140 

„        nerves. 

.     191 

Ventricles  of  brain,          .         .     436 

„        membrane  of  ovum, 

57.613 

„            heart,          .         .     454 

Vitreous  humour,    . 

•     446 

Venus,    .....     372 

Viverra, 

•     715 

Venus'  flower  basket,      .         .125 

Viviparous  Fishes,  . 

.     526 

„       girdle,          .         .         .157 

,,          Insects, 

.     318 

Vermiform  appendix,      .         .     670 

,,          Lizards, 

•  >577 

Vertebra,  Parts  of  a,        .      433,661 

,,          Vertebrates,  . 

.    464 

Vertebral  column,            .         .     432 

Voice  of  Birds, 

.    625 

Vertebral  column  of  Crocodile,     584 

Vole,       .... 

.    711 

Frog,        .     532 

Volvox,  .... 

69,94 

Haddock,     496 

Vortex,  .... 

.     162 

Lizard,    .     571 

Vorticella,       .    •     . 

93>  105 

Pigeon,    .     60  1 

Rabbit,    .     660 

Waldheimia,  . 

.     226 

Skate,      .     479 

Walking-stick  insect, 

•     305 

theory  of  Skull,        .     429 

,,      leaf, 

^cx 

Vertebrata,     .         .         .         .421 

Wallaby, 

.     687 

Affinities  of  Annelids  with,  424 

Walrus,  .... 

.     716 

,,        of  Nemerteans 

Wasps,   .... 

•     304 

with,    .         .     425 

Water  bears,  . 

•     337 

Ancestry  of,    .         .         .     424 

Scorpion, 

•     305 

Development  of,    .      425-464 

Vascular  System,  of- 

General  characters  of,     .     421 

Crinoid,    . 

•     245 

„        classification  of,     423 

Holothurian,     . 

.     241 

Gill  clefts  of,  .         .         .     449 

Ophiuroid, 

•     235 

Heart  of,         ...     454 

Sea  urchin, 

•     239 

and  Invertebrata,    .         .         5 

Starfish.    . 

.     232 

Nervous  system  of,          .     434 

Weasel,  .... 

.     716 

Notochord  of,          .         .     432 

Web  of  Spiders, 

•     334 

Origin  of,        .         .         .421 

Whales,  .... 

.     709 

Segmental  symmetry  of,      421 

Wheel  animalcule,  . 

.     223 

Structure  of,  .         .     425,  464 

Whelk,  .... 

•     357 

Vesiculse  seminales  of  Earth- 

Whip scorpions, 

•     33i 

worm,         114 

White  matter  of  Brain,    . 

•     439 

,,     of  Frog,      .     547 

,,               Spinal  Con 

1,      440 

,,           ,,     of  Leech,    .     219 

White  of  eyes, 

.     613 

,,           ,,     of  Mammals,  677 

Wing  of  bat,  . 

.     719 

,,           ,,     of  Skate,     .     492 

,,       birds, 

.     624 

Vesiculatoe,     .         .         .         .141 

,,       insect, 

.     308 

Vespertilio,     .         .         .         .721 

,,       pterodactyl, 

•     594 

Vesperugo,      .         .                  .721 

Wolf,       .... 

•     7i5 

Vestigial  structures,         .         .       36 

Wolffian  duct, 

.     461 

Vibrissae,         ....     669 

Wombat, 

.     686 

Vicugna,          ....     696 

Worms,  .         .         .       7,  8, 

161-226 

Villi  23 

Wrass,    . 

.     510 

Vipera,   583 

Viperiformes,  ....     583 

Xantharpyia,  . 

.     721 

820 


INDEX. 


Xenopus, 
Xiphisternum 
Xiphosura, 
Xylophaga, 

Yapock, 

PAGE 
.        556 

.     66  1 

•     337 
•     374 

686 

Yellow  cells, 
Yolk,       . 

,,     sac, 
„       „    plaa 
„     spherule 

Zebra,     . 

mta, 
s  (in 

4< 
Hydr 

.     108 

•      57 
)5,  617,  651 
•     679 
a),      .     138 

.     702 

Zoantharia,     . 

Zooea, 

Zona  radiata  of  ovum, 

Zoochlorellse,  . 

Zoo-Geographical  regions, 

Zoonerythrin, 

Zoophytes, 


. 

€»* 


Zootoca, 
Zygeena, 
Zygapophyses, 
Zygobranchs, 
Zygomatic  arch, 

If  $]$£***•   ^ 
1  14^^  jj 

A    /  ^      LfJL't^"' 

U^U    tw>- 


PAGE 

153,    157 
282 

463 

108 
763 

121 
129 
578 
506 

661 

357 
662 


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